Search

More than 150 years ago, in 1866, Ernst Haeckel published a book in two volumes called Generelle Morphologie der Organismen (General Morphology of Organisms) in the first volume of which he formulated his biogenetic law, famously stating that ontogeny recapitulates phylogeny. Here, we describe Haeckel's original idea as first formulated in the Generelle Morphologie der Organismen and later further developed in other publications until the present situation in which molecular data are used to test the "hourglass model," which can be seen as a modern version of the biogenetic law. We also tell the story about his discovery, while traveling in Norway, of an unknown organism, Magosphaera planula, that was important in that it helped to precipitate his ideas into what was to become the Gastraea theory. We also follow further development and reformulations of the Gastraea theory by other scientists, notably the Russian school. Elias Metchnikoff developed the Phagocytella hypothesis for the origin of metazoans based on studies of a colonial flagellate. Alexey Zakhvatin focused on deducing the ancestral life cycle and the cell types of the last common ancestor of all metazoans, and Kirill V. Mikhailov recently pursued this line of research further.

The vertebrate head as a major novelty is directly linked to the evolutionary success of the vertebrates. Sequential information on the embryonic pattern of cartilaginous head development are scarce, but important for the understanding of its evolution. In this study, we use the oriental fire bellied toad, Bombina orientalis, a basal anuran to investigate the sequence and timing of larval cartilaginous development of the head skeleton from the appearance of mesenchymal Anlagen in post-neurulation stages until the premetamorphic larvae. We use different methodological approaches like classic histology, clearing and staining, and antibody staining to examine the larval skeletal morphology. Our results show that in contrast to other vertebrates, the ceratohyals are the first centers of chondrification. They are followed by the palatoquadrate and the basihyal. The latter later fuses to the ceratohyal and the branchial basket. Anterior elements like Meckel's cartilage and the rostralia are delayed in development and alter the ancestral anterior posterior pattern observed in other vertebrates. The ceratobranchials I-IV, components of the branchial basket, follow this strict anterior-posterior pattern of chondrification as reported in other amphibians. Chondrification of different skeletal elements follows a distinct pattern and the larval skeleton is nearly fully developed at Gosner Stage 28. We provide baseline data on the pattern and timing of early cartilage development in a basal anuran species, which may serve as guidance for further experimental studies in this species as well as an important basis for the understanding of the evolutionary changes in head development among amphibians and vertebrates.

The acquisition of a movable jaw and a jaw joint are key events in gnathostome evolution. Jaws are derived from the neural crest derived pharyngeal skeleton and the transition from jawless to jawed vertebrates consists of major morphological changes, which must have a genetic foundation. Recent studies on the effects of bapx1 knockdown in fish and chicken indicate that bapx1 has acquired such a role in primary jaw joint development during vertebrate evolution, but evidence from amphibians is missing so far. In the present study, we use Ambystoma mexicanum, Bombina orientalis, and Xenopus laevis to investigate the effects of bapx1 knockdown on the development of these three different amphibians. Using morpholinos we downregulated the expression of bapx1 and obtain morphants with altered mandibular arch morphology. In the absence of bapx1 Meckeĺs cartilage and the palatoquadrate jaw joint initially develop separately but during further development the joint cavity between both fills with chondrocytes. This results in the fusion of both cartilages and the loss of the jaw joint. Despite this the jaw itself remains usable for feeding and breathing. We show that bapx1 plays a role in jaw joint maintenance during development and that the morphants morphology possibly mirrors the morphology of the jawless ancestors of the gnathostomes.

The vertebrate head/trunk interface is the region of the body where the different developmental programs of the head and trunk come in contact. Many anatomical structures that develop in this transition zone differ from similar structures in the head or the trunk. This is best exemplified by the cucullaris/trapezius muscle, spanning the head/trunk interface by connecting the head to the pectoral girdle. The source of this muscle has been claimed to be either the unsegmented head mesoderm or the somites of the trunk. However most recent data on the development of the cucullaris muscle are derived from tetrapods and information from actinopterygian taxa is scarce. We used classical histology in combination with fluorescent whole-mount antibody staining and micro-computed tomography to investigate the developmental pattern of the cucullaris and the branchial muscles in a basal actinopterygian, the Longnose gar (Lepisosteus osseus). Our results show (1) that the cucullaris has been misidentified in earlier studies on its development in Lepisosteus. (2) Cucullaris development is delayed compared to other head and trunk muscles. (3) This developmental pattern of the cucullaris is similar to that reported from some tetrapod taxa. (4) That the retractor dorsalis muscle of L. osseus shows a delayed developmental pattern similar to the cucullaris. Our data are in agreement with an explanatory scenario for the cucullaris development in tetrapods, suggesting that these mechanisms are conserved throughout the Osteichthyes. Furthermore the developmental pattern of the retractor dorsalis, also spanning the head/trunk interface, seems to be controlled by similar mechanisms.

Xenopus laevis is widely used as a model organism in biological research. Morphological descriptions of the larval cartilaginous skeleton are more than half a century old and comprehensive studies of early cartilage differentiation and development are missing. A proper understanding of early cranial skeletal development in X. laevis requires a detailed description that can function as a baseline for experimental studies. This basis makes it possible to evaluate skeletal defects produced by experiments on gene interactions, such as gain- or loss-of function experiments. In this study, we provide a detailed description of the pattern and timing of early cartilage differentiation and development in the larval head of X. laevis. Methods used include antibody staining, confocal laser scanning microscopy and 3D-reconstruction. Results were than compared to earlier studies based on classical histological approaches and clearing-and-staining. The first cartilage to chondrify is, in contrast to other vertebrates investigated so far, the ceratohyal. The components of the branchial basket chondrify in anterior-to-posterior direction as reported for other amphibians. Chondrification of different cartilages begins at different stages and the majority of cartilages are fully developed at Ziermann and Olsson stage 17. Our baseline data on the pattern and timing of early cartilaginous development in X. laevis is useful for evaluation of experiments which alter head skeletal development as well as for identifying heterochronic shifts in head development in other amphibians.

150 years ago, in 1866, Ernst Haeckel published a book in two volumes called "Generelle Morphologie der Organismen" (General Morphology of Organisms) in which he formulated his biogenetic law, famously stating that ontogeny recapitulates phylogeny. Here we describe Haeckel's original idea and follow its development in the thinking of two scientists inspired by Haeckel, Alexei Sewertzoff and Adolf Naef. Sewertzoff and Naef initially approached the problem of reformulating Haeckel's law in similar ways, and formulated comparable hypotheses at a purely descriptive level. But their theoretical viewpoints were crucially different. While Sewertzoff laid the foundations for a Darwinian evolutionary morphology and is regarded as a forerunner of the Modern Synthesis, Naef was one of the most important figures in 'idealistic morphology', usually seen as a type of anti-Darwinism. Both Naef and Sewertzoff aimed to revise Haeckel's biogenetic law and came to comparable conclusions at the empirical level. We end our review with a brief look at the present situation in which molecular data are used to test the "hour-glass model", which can be seen as a modern version of the biogenetic law.

The head is considered the major novelty of the vertebrates and directly linked to their evolutionary success. Its form and development as well as its function, for example in feeding, is of major interest for evolutionary biologists. In this study, we describe the skeletal development of the cranium and pectoral girdle in Siberian (Acipenser baerii) and Russian sturgeon (A. gueldenstaedtii), two species that are commonly farmed in aquaculture and increasingly important in developmental studies. This study comprises the development of the neuro-, viscero- and dermatocranium and the dermal and chondral components of the pectoral girdle, from first condensation of chondrocytes in prehatchlings to the early juvenile stage and reveals a clear pattern in formation. The otic capsules, the parachordal cartilages, and the trabeculae cranii are the first centers of chondrification, at 8.4mm TL. These are followed by the mandibular, then the hyoid, and later the branchial arches. Teeth form early on the dentary, dermopalatine, and palatopterygoid, and then appear later in the buccal cavity as dorsal and ventral toothplates. With ongoing chondrification in the neurocranium a capsule around the brain and a strong rostrum are formed. Dermal ossifications start to form before closure of the dorsal neurocranial fenestrae. Perichondral ossification of cartilage bones occurs much later in ontogeny. Our results contribute data bearing on the homology of elements such as the lateral rostral canal bone that we regard homologous to the antorbital of other actinopterygians based on its sequence of formation, position and form. We further raise doubts on the homology of the posterior ceratobranchial among Actinopteri based on the formation of the hyoid arch elements. We also investigate the basibranchials and the closely associated unidentified gill-arch elements and show that they are not homologous. J. Morphol. 278:418-442, 2017. © 2017 Wiley Periodicals, Inc.

Background Fox genes are a large family of transcription factors that play diverse roles in the immune system, metabolism, cancer, cell cycle, and animal development. It has been shown that FoxN3 is indispensable for normal craniofacial development in the mouse and the African clawed frog, Xenopus laevis. Morpholino-mediated knockdown of FoxN3 in X. laevis delays overall development of early tadpole stages and causes eye defects, the absence of some cranial nerve branches, and malformations of the cranial skeleton and some cranial muscles, while the skeleton, nerves and muscles of the trunk are unaffected. Results We report a delay in heart morphogenesis, the absence of the interatrial septum, and a reduction and compaction of the ventricular trabeculation after knockdown of FoxN3 in X. laevis. Furthermore, we found malformations of the cucullaris and diaphragmatico-branchialis muscles, two head muscles that develop in the head/trunk interface of X. laevis. Conclusions FoxN3 is necessary for the development of the interatrial septum and trabeculae in the frog heart, as well as the cranial muscles developing in the head/trunk interface. This gives the first evidence for a dependence on the head myogenic program of the cucullaris muscle in an anuran species.

As part of an ongoing comparative study of pigment patterns and their formation in embryos and larvae of ambystomatid salamanders, Ambystoma maculatum from two differnt populations, one in the northern (New York) and one in the central (Tennessee) United States, were investigated. Scanning electron microscopy was used to study early neural crest development. Light microscopy in combination with markers for the two pigment cell types (xanthophores and melanophores) made it possible to follow pigment cell migration before the pigment cells were fully differentiated. A bilateral pigment pattern consisting of two horizontal melanophore stripes surrounding an interstripe area populated by xanthophores formed in the larvae. In both populations, some variation was present in the form of a continuum ranging from clear horizontal stripes to extreme cases with a random pattern. Unlike the other ambystomatids that have been investigated, the neural crest cells in A. maculatum do not form aggregates and no vertical bars are formed. Instead, both the pattern and its formation are very similar to what has been reported for salamandrids. If pattern formation mechanisms can act as developmental constraints we would expect the A. maculatum pattern to be the primitive condition in the Ambystomatidae, using the Salamandridae as the outgroup. There is no strong support for this when aggregate formation is used as a character and mapped onto phylogenies for the group. The aggregate formation mechanism, and the pigment pattern that it leads to, have most likely been secondarily lost in A. maculatum. © 1993 Wiley-Liss, Inc.

The pigment pattern formation in embryos and larvae of three ambystomatid salamanders was investigated in an evolutionary context. Early neural crest development was studied with scanning electron microscopy. Pigment cell migration and pattern formation were investigated at the light microscopy level with markers that labelled the two pigment cell types specifically before they were fully differentiated. In all three species, the pigment pattern formation started when xanthophores that had first formed aggregates in the crest migrated ventrally. As previously observed in other species, vertical bars always form by a mechanism involving earlier onset of migration in melanophores than in xanthophores and aggregate formation in the crest. In Ambystoma talpoideum and A. annulatum, a pattern of vertical chromatophore bars formed, which was superimposed on a pattern of horizontal stripes. In Ambystoma barbouri, the tendency to form this pattern was obscured by the high density of melanophores. It is suggested that variation among species may be due to differences in the chromatophore density and in the melanophore/xanthophore ratio. Mapping of the evolution of vertical bars onto existing phylogenies for the group was confounded by controversies about how to interpret the phylogenetic data. On the phylogeny that takes all the available evidence into account, there are two equally parsimonious mappings. Vertical bars have either evolved only once and been lost twice, or evolved twice and been lost once. This rather conservative pattern can be explained both as an effect of stabilizing selection and as a result of developmental constraints. © 1994 Wiley-Liss, Inc.

This is the first in a planned series of studies in which we examine the cranial muscle ontogeny of exemplar taxa of actinopterygian clades to obtain a better understanding of the evolution of the cranial musculoskeletal system within the Actinopterygii. The Longnose Gar, Lepisosteus osseus, is a member of the basal actinopterygian family Lepisosteidae. Juvenile and adult gars are highly derived and anatomical characters can easily be misinterpreted, which makes a comparison with other taxa difficult. Highly complex adult structures such as the cranial skeleton and musculature are organized more simply early in development, making comparisons and homology assessments easier. Established methods such as clearing and double staining are widely used to study skeletal structures. However, methods to analyze the ontogeny of soft tissues are scarce. To study the development of the cranial musculature of L. osseus, we used a combination of 3D-reconstruction of soft tissue μCT scans and whole-mount antibody stain...

Today the Mexican axolotl is critically endangered in its natural habitat in lakes around Mexico City, but thrives in research laboratories around the world, where it is used for research on development, regeneration, and evolution. Here, we concentrate on the early history of the axolotl as a laboratory animal to celebrate that the first living axolotls arrived in Paris in 1864, 150 years ago. Maybe surprisingly, at first the axolotl was distributed across Europe without being tied to specific research questions, and amateurs engaged in acclimatization and aquarium movements played an important role for the rapid proliferation of the axolotl across the continent. But the aquarium also became an important part of the newly established laboratory, where more and more biological and medical research now took place. Early scientific interest focused on the anatomical peculiarities of the axolotl, its rare metamorphosis, and whether it was a larva or an adult. Later, axolotl data was used to argue both for (by August Weismann and others) and against (by e.g., Albert von Kolliker) Darwinism, and the axolotl even had a brief history as a laboratory animal used in a failed attempt to prove Lysenkoism in Jena, Germany. Nowadays, technical developments such as transgenic lines, and the very strong interest in stem cell and regeneration research has again catapulted the axolotl into becoming an important laboratory animal.

Xenopus laevis is one of the most widely used model organism in neurobiology. It is therefore surprising, that no detailed and complete description of the cranial nerves exists for this species. Using classical histological sectioning in combination with fluorescent whole mount antibody staining and micro-computed tomography we prepared a detailed innervation map and a freely-rotatable three-dimensional (3D) model of the cranial nerves and anterior-most spinal nerves of early X. laevis tadpoles. Our results confirm earlier descriptions of the pre-otic cranial nerves and present the first detailed description of the post-otic cranial nerves. Tracing the innervation, we found two previously undescribed head muscles (the processo-articularis and diaphragmatico-branchialis muscles) in X. laevis. Data on the cranial nerve morphology of tadpoles are scarce, and only one other species (Discoglossus pictus) has been described in great detail. A comparison of Xenopus and Discoglossus reveals a relatively conserved pattern of the post-otic and a more variable morphology of the pre-otic cranial nerves. Furthermore, the innervation map and the 3D models presented here can serve as an easily accessible basis to identify alterations of the innervation produced by experimental studies such as genetic gain- and loss of function experiments.

Lungfishes are the extant sister group of tetrapods. As such, they are important for the study of evolutionary processes involved in the water to land transition of vertebrates. The evolution of a true neck, that is, the complete separation of the pectoral girdle from the cranium, is one of the most intriguing morphological transitions known among vertebrates. Other salient changes involve new adaptations for terrestrial feeding, which involves both the cranium and its associated musculature. Historically, the cranium has been extensively investigated, but the development of the cranial muscles much less so. Here, we present a detailed study of cephalic muscle development in the Australian lungfish, Neoceratodus forsteri, which is considered to be the sister taxon to all other extant lungfishes. Neoceratodus shows several developmental patterns previously described in other taxa; the tendency of muscles to develop from anterior to posterior, from their region of origin toward insertion, and from lateral to ventral/medial (outside-in), at least in the branchial arches. The m.protractor pectoralis appears to develop as an extension of the most posterior m.levatores arcuum branchialium, supporting the hypothesis that the m.cucullaris and its derivatives (protractor pectoralis, levatores arcuum branchialium) are branchial muscles. We present a new hypothesis regarding the homology of the ventral branchial arch muscles (subarcualis recti and obliqui, transversi ventrales) in lungfishes and amphibians. Moreover, the morphology and development of the cephalic muscles confirms that extant lungfishes are neotenic and have been strongly influenced via paedomorphosis during their evolutionary history.

The skeleton of the jaws and neurocranium of sturgeons (Acipenseridae) are connected only through the hyoid arch. This arrangement allows considerable protrusion and retraction of the jaws and is highly specialized among ray-finned fishes (Actinopterygii). To better understand the unique morphology and the evolution of the jaw apparatus in Acipenseridae, we investigated the development of the muscles of the mandibular and hyoid arches of the Siberian sturgeon, Acipenser baerii. We used a combination of antibody staining and formalin-induced fluorescence of tissues imaged with confocal microscopy and subsequent three-dimensional reconstruction. These data were analyzed to address the identity of previously controversial and newly discovered muscle portions. Our results indicate that the anlagen of the muscles in A. baerii develop similarly to those of other actinopterygians, although they differ by not differentiating into distinct muscles. This is exemplified by the subpartitioning of the m. adductor mandibulae as well as the massive m. protractor hyomandibulae, for which we found a previously undescribed portion in each. The importance of paedomorphosis for the evolution of Acipenseriformes has been discussed before and our results indicate that the muscles of the mandibular and the hyoid may be another example for heterochronic evolution.

Parsimov is a parsimony-based method for identifying the minimum number of heterochronic event-shifts on all branches of a given phylogenetic framework to explain the developmental sequences seen in the species investigated, and has been used to investigate the evolution of developmental sequences in various animal groups. However, the biological interpretation of the results is difficult not least because Parsimov uses non-independent data resulting from event-pairing as the basis for its analyses. To test the applicability of Parsimov to a large data set, larval cranial muscle development was studied in 15 anurans, three caudates and the Australian lungfish. We analyzed the developmental sequences with Parsimov to investigate: if there are (1) heterochronies on deep branches of a cladogram indicating changes in the ancestral sequences, (2) heterochronies that can be related to larval life-history, and (3) the sensitivity of the analysis to different underlying cladograms. We discovered general patterns of cranial muscle development, such as an anterior-to-posterior gradient, an outside-in pattern and a tendency for cranial muscles to develop from their region of origin toward their insertion. We found most heterochronies on terminal branches and only a few shifts on deep branches in the cladograms indicating changes in the ancestral sequences. No changes could be related to larval life-history. The underlying cladogram clearly influenced the outcome of the analysis. We propose that Parsimov has the potential, combined with other methods, to find evolutionary important changes and to aid the biological interpretation of these changes.

Our research on the evolution of the vertebrate head focuses on understanding the developmental origins of morphological novelties. Using a broad comparative approach in amphibians, and comparisons with the well-studied quail-chicken system, we investigate how evolutionarily conserved or variable different aspects of head development are. Here we review research on the often overlooked development of cranial muscles, and on its dependence on cranial cartilage development. In general, cranial muscle cell migration and the spatiotemporal pattern of cranial muscle formation appears to be very conserved among the few species of vertebrates that have been studied. However, fate-mapping of somites in the Mexican axolotl revealed differences in the specific formation of hypobranchial muscles (tongue muscles) in comparison to the chicken. The proper development of cranial muscles has been shown to be strongly dependent on the mostly neural crest-derived cartilage elements in the larval head of amphibians. For example, a morpholino-based knock-down of the transcription factor FoxN3 in Xenopus laevis has drastic indirect effects on cranial muscle patterning, although the direct function of the gene is mostly connected to neural crest development. Furthermore, extirpation of single migratory streams of cranial neural crest cells in combination with fate-mapping in a frog shows that individual cranial muscles and their neural crest-derived connective tissue attachments originate from the same visceral arch, even when the muscles attach to skeletal components that are derived from a different arch. The same pattern has also been found in the chicken embryo, the only other species that has been thoroughly investigated, and thus might be a conserved pattern in vertebrates that reflects the fundamental nature of a mechanism that keeps the segmental order of the head in place despite drastic changes in adult anatomy. There is a need for detailed comparative fate-mapping of pre-otic paraxial mesoderm in amphibians, to determine developmental causes underlying the complicated changes in cranial muscle development and architecture within amphibians, and in particular how the novel mouth apparatus in frog tadpoles evolved. This will also form a foundation for further research into the molecular mechanisms that regulate rostral head morphogenesis. Our empirical studies are discussed within a theoretical framework concerned with the evolutionary origin and developmental basis of novel anatomical structures in general. We argue that a common developmental origin is not a fool-proof guide to homology, and that a view that sees only structures without homologs as novel is too restricted, because novelties must be produced by changes in the same framework of developmental processes. At the level of developmental processes and mechanisms, novel structures are therefore likely to have homologs, and we need to develop a hierarchical concept of novelty that takes this into account.

Ziermann, J.M., Infante, C., Hanken, J. and Olsson, L. 2011. Morphology of the cranial skeleton and musculature in the obligate carnivorous tadpole of Lepidobatrachus laevis (Anura: Ceratophryidae). —Acta Zoologica (Stockholm) 00:1–12. Lepidobatrachus laevis (Ceratophryidae: Ceratophryinae) is a bizarre frog endemic to the Chacoan desert of central South America. Its tadpole is an obligate carnivore that can catch and consume live prey nearly its own size. Morphological adaptations associated with this unique feeding mode, including the larval skull anatomy and associated cranial musculature, have only been partly described. We studied the head of Stages 26–27 larvae using gross dissection, immunohistochemistry, and standard histology. Derived features of this tadpole compared to the microphagous, herbivorous larvae of most other anurans include simplified chondrocranial cartilages and very robust jaw muscles. The mm. suspensorio- et quadratoangularis do not take their origin from the processus muscularis of the palatoquadrate, as in most other tadpoles, but instead originate from the corpus of the palatoquadrate caudal to this process. The jaw levators are unusually large. The tadpole of Ceratophrys, another member of the ceratophryine clade, also consumes large animal prey, but its morphology is very different. It probably has evolved independently from a generalized, mainly herbivorous tadpole similar to the larva of Chacophrys, the third ceratophryine genus. Most specialized features of the larval head of Lepidobatrachus laevis are adaptations for ‘megalophagy’—ingestion of whole, very large animal prey.

The pectoral girdle is a unique skeletal element that underwent drastic morphological changes during its evolution, especially in association with the fin-to-limb transition. Comparative studies of its development are needed to gain a deeper understanding of its evolution. Transplantation experiments using the quail-chick chimeric system have revealed that not only lateral plate mesoderm but also somites contribute to the pectoral girdle in birds. Studies in mice and turtles also document somitic contributions to the pectoral girdle, but extirpation experiments in a salamander did not affect shoulder girdle development. Somitic contributions to the pectoral girdle therefore have been interpreted as a feature unique to amniotes. Here, we present a long-term fate map of single somites in the Mexican axolotl, based on transplantations of somites two to six from GFP-transgenic donors into wild-type hosts, as well as injections of fluorescein dextran into single somites. The results show a somitic derivation of the dorsal region of the suprascapula, demonstrating that somitic contributions to the pectoral girdle are not restricted to amniotes. Comparison with the few other species studied so far leads us to suggest a position-dependent origin of the pectoral girdle. We propose that embryonic origin is determined by the proximity of the developing pectoral girdle to the somites or to the lateral plate mesoderm, respectively. This position-dependent origin and the diversity of the anatomy of the pectoral girdle among vertebrates implies that the embryonic origin of the pectoral girdle is too variable to be useful for defining homologies or for phylogenetic analysis.

The origin of morphological novelties is a controversial topic in evolutionary developmental biology. The heads of anuran larvae have several unique structures, including the supra- and infrarostral cartilages, the specialised structure of the gill basket (used for filtration), and novel cranial muscle arrangements. FoxN3, a member of the forkhead/winged helix family of transcription factors, has been implicated as important for normal craniofacial development in the pipid anuran Xenopus laevis. We have investigated the effects of functional knockdown of FoxN3 (using antisense oligonucleotide morpholino) on the development of the larval head skeleton and the associated cranial muscles in X. laevis. Our data complement earlier studies and provide a more complete account of the requirement of FoxN3 in chondrocranium development. In addition, we analyse the effects of FoxN3 knockdown on cranial muscle development. We show that FoxN3 knockdown primarily affects the novel skeletal structures unique to anuran larvae, i.e. the rostralia or the fine structure of the gill apparatus. The articulation between the infrarostral and Meckel's cartilage is malformed and the filigreed processes of the gill basket do not develop. Because these features do not develop after FoxN3 knockdown, the head morphology resembles that in the less specialised larvae of salamanders. Furthermore, the development of all cartilages derived from the neural crest is delayed and cranial muscle fibre development incomplete. The cartilage precursors initially condense in their proper position but later differentiate incompletely; several visceral arch muscles start to differentiate at their origin but fail to extend toward their insertion. Our findings indicate that FoxN3 is essential for the development of novel cartilages such as the infrarostral and other cranial tissues derived from the neural crest and, indirectly, also for muscle morphogenesis.

Our research on the evolution of head development focuses on understanding the developmental origins of morphological innovations and involves asking questions like: How flexible (or conserved) are cell fates, patterns of cell migration or the timing of developmental events (heterochrony)? How do timing changes, or changes in life history affect head development and growth? Our ‘model system’ is a comparison between lungfishes and representatives from all three extant groups of amphibians. Within anuran amphibians, major changes in life history such as the repeated evolution of larval specializations (e.g. carnivory), or indeed the loss of a free-swimming larva, allows us to test for developmental constraints. Cell migration and cell fate are conserved in cranial neural crest cells in all vertebrates studied so far. Patterning and developmental anatomy of cranial neural crest and head mesoderm cells are conserved within amphibians and even between birds, mammals and amphibians. However, the specific formation of hypobranchial muscles from ventral somitic processes shows variation within tetrapods. The evolution of carnivorous larvae in terminal taxa is correlated with changes in both pattern and timing of head skeletal and muscle development. Sequence-heterochronic changes are correlated with feeding mode in terminal taxa and with phylogenetic relatedness in basal branches of the phylogeny. Eye muscles seem to form a developmental module that can evolve relatively independently from other head muscles, at least in terms of timing of muscle differentiation.

The number of decaying lymphocytes in the peripheral blood of the mouse was determined by means of the dye exclusion test. Under normal conditions the lymphocyte decay was very constant (1.1%). Treatment with pertussis vaccine, anti-lymphocyte serum, and hydrocortisone either alone or in combination greatly influenced the number of decaying blood lymphocytes. Treatment with heparin markedly increased the number of circulating non-viable lymphocytes, probably due to inhibition of the reticuloendothelial macrophage system. Methods for determination of the average period a decaying lymphocyte stays in blood were elaborated and used under normal and experimental conditions.

The importance of embryonic development for evolutionary biology has been an issue ever since Charles Darwin and Ernst Haeckel, however, Modern Synthesis approaches to evolution have often neglected certain aspects of developmental biology or treated development as a black box (Mayr and Provine 1980; Breidbach and Ghiselin 2007a, b). Although Wallace’s statement that ‘‘problems concerned with the orderly development of the individual are unrelated to those of the evolution of organisms through time’’ (Wallace 1986, p. 149) is extreme, midtwentieth century mainstream evolutionary biology did not feel much need for an integration of developmental biology into its theoretical foundations. The fact that evolutionary questions have been of interest to some developmental biologists between the era of Darwin and Haeckel and the 1980s, i.e., that modern evolutionary developmental biology, or Evo-devo, as the field is often called by its practitioners, in fact has a history, has received relatively little attention. It has even been claimed that ‘‘Following a quiescent period of almost a century, present-day Evo-devo erupted out of the discovery of the homeobox on the 1980s’’ (Arthur 2002, p. 757). Thus, there is a need for more research into the period ‘‘between Ernst Haeckel and the homeobox’’ in order to gain a more complete understanding of the roots of modern Evo-devo. We are helped by the recent upsurge in interest in the history of Evodevo, which has produced several edited volumes (e.g., Laubichler and Maienschein 2007) and thematic issues of journals. Also this journal has contributed to this increased publication activity on the pre-history and history of Evodevo, having had thematic issues (Theory in Biosciences 124:259–420; 126:115–175) on different aspects of the subject. The history of Evo-devo in the Anglo-American world has received renewed attention recently, as exemplified, e.g., by the work of Alan Love (e.g., Love 2003, 2006; Love and Raff 2003), whose scheme of the historical development of the relationship between evolution and development is here reproduced as Fig. 1. We have ourselves concentrated on the history of Evo-devo in the Germanand Russian-speaking countries (e.g., Hossfeld and Olsson 2003; Levit et al. 2004, 2006; Olsson et al. 2006; Breidbach and Ghiselin 2007a, b; Reis et al. 2007). In Love’s scheme (Fig. 1), he contrasts the ‘‘textbook version’’ (left) with an improved, updated version (right). In the left diagram, evolutionary biology is split from developmental biology, which was dominated by ‘‘Entwickelungsmechanik’’ (Developmental Mechanics) in the first third of the twentieth century. The developmental biologist Thomas H. Morgan (1866–1945) is seen as an example of the split between experimental embryology and genetics, which he helped to found and develop into molecular genetics. Another part of genetics, population genetics, became an important part of the Modern Synthesis of evolutionary biology (Mayr and Provine 1980; L. Olsson (&) Institut fur Spezielle Zoologie und Evolutionsbiologie mit Phyletischem Museum, Friedrich-Schiller-Universitat Jena, Erbertstr, 1, 07743 Jena, Germany e-mail: Lennart.Olsson@uni-jena.de

A multi-gene genealogy based on maximum parsimony and distance analyses of the exonic genes for actin (act) and translation elongation factor 1 alpha (tef), the nuclear genes for the small (18S) and large (28S) subunit ribosomal RNA (comprising 807, 1092, 1863, 389 characters, respectively) of all 50 genera of the Mucorales (Zygomycetes) suggests that the Choanephoraceae is a monophyletic group. The monotypic Gilbertellaceae appears in close phylogenetic relatedness to the Choanephoraceae. The monophyly of the Choanephoraceae has moderate to strong support (bootstrap proportions 67% and 96% in distance and maximum parsimony analyses, respectively), whereas the monophyly of the Choanephoraceae-Gilbertellaceae clade is supported by high bootstrap values (100% and 98%). This suggests that the two families can be joined into one family, which leads to the elimination of the Gilbertellaceae as a separate family. In order to test this hypothesis single-locus neighbor-joining analyses were performed on nuclear genes of the 18S, 5.8S, 28S and internal transcribed spacer (ITS) 1 ribosomal RNA and the translation elongation factor 1 alpha (tef) and beta tubulin (betatub) nucleotide sequences. The common monophyletic origin of the Choanephoraceae-Gilbertellaceae clade could be confirmed in all gene trees and by investigation of their ultrastructure. Sporangia with persistent, sutured walls splitting in half at maturity and ellipsoidal sporangiospores with striated ornamentations and polar ciliate appendages arising from spores in persistent sporangia and dehiscent sporangiola represent synapomorphic characters of this group. We discuss our data in the context of the historical development of their taxonomy and physiology and propose a reduction of the two families to one family, the Choanephoraceae sensu lato comprising species which are facultative plant pathogens and parasites, especially in subtropical to tropical regions.

The early development of skeletal structures of manus and pes was studied using whole-mount, alcian-blue-stained embryos at different stages in an ontogenetic series of Graptemys nigrinoda. Sequences of chondrification events are similar to those reported for other turtle species, with respect to both the primary axis and the digital arch. There is no evidence of anterior condensations in the region distal to the radius and the tibia, supporting the hypothesis that the radiale and tibiale are absent in turtles, except for some potential atavistic occurrences. The anlagen for the fibulare and intermedium were identified, and in some but not all specimens two centralia elements are also present. These elements of the proximal and medial tarsal row fuse into a single cartilaginous structure, which later becomes the astragalocalcaneum. Inconsistencies in the literature about the details of the chondrification sequence of autopodial elements are in part related to real intraspecific and interspecific variation across turtles. The patterns of connectivity, often cited in studies of the limb chondrification sequence of tetrapods, are difficult if not impossible to objectively detect using standard alcian-blue-stained specimens.

The cranial neural crest has been shown to give rise to a diversity of cells and tissues, including cartilage, bone and connective tissue, in a variety of tetrapods and in the zebrafish. It has been claimed, however, that in the Australian lungfish these tissues are not derived from the cranial neural crest, and even that no migrating cranial neural crest cells exist in this species. We have earlier documented that cranial neural crest cells do migrate, although they emerge late, in the Australian lungfish. Here, we have used the lipophilic fluorescent dye, DiI, to label premigratory cranial neural crest cells and follow their fate until stage 43, when several cranial skeletal elements have started to differentiate. The timing and extent of their migration was investigated, and formation of mandibular, hyoid and branchial streams documented. Cranial neural crest was shown to contribute cells to several parts of the head skeleton, including the trabecula cranii and derivatives of the mandibular arch (e.g., Meckel's cartilage, quadrate), the hyoid arch (e.g., the ceratohyal) and the branchial arches (ceratobranchials I–IV), as well as to the connective tissue surrounding the myofibers in cranial muscles. We conclude that cranial neural crest migration and fate in the Australian lungfish follow the stereotyped pattern documented in other vertebrates. J. Exp. Zool. (Mol. Dev. Evol.) 310B:345–354, 2008. © 2007 Wiley-Liss, Inc.

The fate of single somites has not been analyzed from a comparative perspective with modern cell-marking methods. Most of what we know is based on work using quail-chick chimeras. Consequently, to what degree cell fate has been conserved despite the anatomical differences among vertebrates is unknown. We have analyzed the cell fate of the cranialmost somites, with the focus on somite two, in the Mexican axolotl (Ambystoma mexicanum). Somite cells were marked by injection of dextran-fluorescein and detected using immunofluorescence after 2 months of development in paraffin sections. Our data confirm and extend earlier studies based on classical histology in salamanders. We show that somite two contributes to different muscles, skeletal elements, and connective tissues of the head and cranial trunk region. Cells from somites two and three migrate latero-ventrally and contribute to the hypobranchial muscles mm. geniohyoideus and rectus cervicis. We provide evidence that the specific formation of the hypobranchial musculature from ventral processes of the somites might be variable in different classes of vertebrates. We further demonstrate that mm. cucullaris and dilatator laryngis, which were earlier thought to have a branchial origin, arise from somitic material in a manner very similar to the findings in quail-chick chimeras. Our findings indicate that the pattern of somitic derivatives is highly conserved within tetrapods.

The African Clawed Toad, Xenopus laevis, has been a major vertebrate model organism for developmental studies for half a century. Because most studies have focused on the early stages of development, this has had the effect that many aspects of organogenesis and later development remain relatively poorly known in this species. In particular, little is known about cranial muscle development even at the level of morphology and histological differentiation of muscle anlagen and muscle fibers. In this study, we document the morphogenesis and histological differentiation of cranial muscles in X. laevis. We provide a detailed account of the timing of development for each of the cranial muscles, and also describe a new muscle, the m. transversus anterior. The cranial musculature of X. laevis larvae generally develops in a rostrocaudal sequence. The first muscles to differentiate are the extrinsic eye muscles. Muscles of the mandibular and hyoid arches develop almost simultaneously, and are followed by the muscles of the branchial arches and the larynx, and by the mm. geniohyoideus and rectus cervicis. Despite the fact that differentiation starts at different stages in the different muscles, most are fully developed at Stage 14. These baseline data on the timing of muscle differentiation in the X. laevis can serve as a foundation for comparative studies of heterochronic changes in cranial muscle development in frogs and other lissamphibians.

We describe the development of the cranial neural crest cell streams relative to embryonic events such as neural tube formation and somite appearance in two Eurasian frog species belonging to the Ranidae, Rana temporaria and Sylvirana nigrovittata, and demonstrate developmental heterochronies. The mandibular stream appeared well developed in R. temporaria at a time when the embryo was still spherical, the neural folds were elevated, and the neural plate was wide open, thus earlier than known from any frog species so far. The appearance of the second stream and its division into hyoid and branchial portions was clearly accelerated in R. temporaria relative to other embryonic events when compared to S. nigrovittata. For example, in R. temporaria, the hyoid and branchial portions of the cranial neural crest cell streams were separated before the neural folds had started to fuse, whereas in S. nigrovittata this event took place only after the neural folds had fused completely. Such ostentatious heterochronies related to the characters used herein have formerly only been reported from comparisons between species belonging to different higher taxa. Our results re-confirm that to understand the full dynamics of the evolution of development, studies need to implement comparative embryological approaches, and include phylogenetically relatively closely related taxa.

Coarse spatial resolution datasets are invaluable at the global scale, but they lack the thematic and spatial detail required for habitat assessments at the country level and for finer-resolution assessments such as vegetation species distribution or high-quality forest-change monitoring. Mapping, monitoring, and assessments at the national and subnational level are performed using moderate-resolution sensors such as Landsat, ASTER, SPOT HRV, and IRS with spatial resolutions from 15 to 60 m. Newer, high-resolution optical sensors (5 m or better) provide enough spatial and spectral detail to discriminate between individual trees and, in some cases, species, but high-resolution imagery is prohibitively costly (see Annex 7) for many national governments and research institutions (Strittholt and Steininger 2007).

Abstract The vertebrate cranium consists of three parts: neuro-, viscero- and dermatocranium, which differ in both developmental and phylogenetic origin. Traditionally, developmental origin has been used as a criterion for homology, but this becomes problematic when skull elements such as the parietal bone are now shown, by modern fate-mapping studies, to have different developmental origins in different groups of tetrapods. This indicates a flexibility of developmental programmes and regulatory pathways which has probably been very important in cranial evolution. The trabecula cranii is an intriguing cranial element in the anterior cranial base in vertebrates. It forms a viscerocranial part of the neurocranium and is believed to be neural crest-derived in gnathostomes, but a similarly named structure in lampreys has been shown to have a mesodermal origin. Topographically, this trabecula seems to be homologous to the gnathostome trabecula cranii, and might also have the same function: to form a border between adjacent morphogenetic domains, to constrain and redirect growth of both brain and stomodeum and thus to refine developmental schedules of both. We suggest that such a border zone can recruit cells from either the mesoderm (as in the lamprey) or from the neural crest (as in the gnathostomes investigated), and still retain its homology. In our view, the trabecula is an interface element that integrates the respective divergent morphogenetics programs of the preotic head into a balanced unit; we suggest that such a definition can be used to define "the sameness" of this element throughout vertebrates.

The homology concept has had a long and varied history, starting out as a geometrical term in ancient Greece. Here we describe briefly how a typological use of homology to designate organs and body parts in the same position anatomically in different organisms was changed by Darwin’s theory of evolution into a phylogenetic concept. We try to indicate the diversity of opinions on how to define and test for homology that has prevailed historically, before the important books by Hennig (1950. Grundzuge einer Theorie der Phylogenetischen Systematik. Deutscher Zentralverlag, Berlin) and Remane (1952. Die Grundlagen des Naturlichen Systems, der Vergleichenden Anatomie und der Phylogenetik. Geest & Portig, Leipzig) brought more rigor into both the debate on homology and into the usage of the term homology among systematists. Homology as a theme has recurred repeatedly throughout the history of the “Phylogenetisches Symposium” and we give a very brief overview of the different aspects of homology that have been discussed at specific symposia over the last 48 years. We also honour the fact that the 2004 symposium was held in Jena by pointing to the roles played by biologists active in Jena, such as Ernst Haeckel and Carl Gegenbaur, in starting the development towards a homology concept concordant with an evolutionary world view. As historians of biology, we emphasize the importance of major treatises on homology and its history that may be little read by systematists active today, and have sometimes also received less attention by historians of biology than they deserve. Prominent among these are the works of Dietrich Starck, who also happened to be both a student, and later a benefactor, of systematics at Jena University.

The development of the vertebrate head is a complex process involving interactions between a multitude of cell types and tissues. This thesis describes the development of the cranial neural crest and of the visceral arch muscles in the head of two species. One, the Mexican axolotl (Ambystoma mexicanum), is a basal tetrapod, whereas the other, the Australian lungfish (Neoceratodus forsteri), belongs to the Dipnoi, the extant sister group of the Tetrapoda. The migration of neural crest cells, which form most of the bones and connective tissues in the head, and the morphogenesis of the jaw, was determined in the Mexican axolotl. It was shown that both the upper and lower jaws form from ventral condensations of neural crest cells in the mandibular arch. The dorsal condensation, earlier considered to give rise to the upper jaw, was shown to form the trabecula cranii.The normal spatio-temporal development of visceral arch muscles was investigated in both the Mexican axolotl and the Australian lungfish. In axolotl, the muscles tended to start forming almost simultaneously in all visceral arches at their future origins and extend towards their future insertions at the onset of muscle fibre formation. In lungfish, fibres formed simultaneously throughout most of each muscle anlage in the first and second visceral arch, but were delayed in the branchial arches. The anlagen were first observed at their future insertion, from which they developed towards future origins. To test the ability of neural crest cells to pattern the visceral arch muscles, migrating crest cells were extirpated from axolotl embryos, which resulted in a wide range of muscle malformations. In most cases, the muscles appeared in the right position but were small and extended in abnormal directions. This shows that neural crest cells are responsible not for the position of the muscles but for their correct anatomical pattern. Fate mapping showed that connective tissue surrounding myofibers is, at least partly, neural crest derived.In conclusion, the work presented in this thesis shows that although early development may map out the patterns of later development, the differences between axolotl and lungfish head development are not seen until during morphogenesis. Further investigation of morphogenesis is needed to explain the great variation of head morphology seen in vertebrates today.

In the evolution of land-living vertebrates, the transition from spending the entire life cycle in the water to first a biphasic (adult on land, eggs and larvae in water) and later a terrestrial life-history mode was achieved by changes in developmental processes and regulatory mechanisms. Lungfishes, salamanders and frogs are studied as examples of species which span this transition. The migration and fate of the embryonic cells that form the head is studied, using experimental embryology (extirpation and transplantation of cells), molecular markers and novel microscopy techniques — such as confocal microscopy. Knowing the migratory routes and fates of the cells that form head structures is important for an elucidation of the changes that took place e.g. when gill arches transformed into head cartilages, and when the specialised larval mouth structures present in today’s frogs and toads arose as an evolutionary innovation. Results so far indicate that the early migration and pattern formation of neural crest cells in the head region is surprisingly conserved. Both the amphibians investigated and the Australian lungfish have the same number of migrating neural crest streams, and the identity of the streams is preserved. The major difference lies in the timing of migration, where there has been a heterochronic shift such that cell migration starts much later in the Australian lungfish than in the amphibians. The molecular mechanisms regulating the formation of streams of cranial neural crest cells seem, at least in part, to be differential expression of ephrins and ephrin receptors, which mediate cell sorting. Our understanding of the behaviour of migrating cells (primarily the more well characterised neural crest cells) could be enhanced by a modelling approach. I present preliminary ideas on how this could be achieved, inspired by recent work on Dictyostelium development and our own previous work on pigment cells and their pattern formation during salamander embryogenesis.

Comparative anatomy and zoology both have long academic traditions in Jena. At first, the two subjects developed in parallel and had many similarites in research topics. This development is covered in the first part of the paper. The close relationship between the two subjects started to break apart when Carl Gegenbaur and Ernst Haeckel were active at Jena University. In 1865 Haeckel became the first full professor of zoology in Jena, and zoology became more independent from comparative anatomy. In the second part of our paper, we follow the developments in comparative anatomy in Jena from Gegenbaur’s immediate students up until the end of Hans Boker’s tenure in Jena in the mid-1930s. Certain subjects are in focus throughout this period, for example vertebrate head morphology and development, (the “head problem”), the relationship between anatomy and biology, and evolutionary questions. Some of these subjects have remained important research topics in zoology and comparative anatomy in Jena until the present day.

Summary The timing and pattern of cranial neural crest cell emergence and migration in the Mexican axolotl, Ambystoma mexicanum, are assessed using scanning electron microscopy (SEM). Cranial neural crest cells emerge and begin to migrate at the time of neural fold closure and soon form three distinct streams. The most anterior (mandibular) stream emerges first, at the level of the mesencephalon. Cells in this stream migrate rostroventrally around the optic vesicle. The second (hyoid) and third (branchial) streams emerge in close succession at the level of the rhombencephalon and extend ventrolaterally. Cells forming the hyoid stream migrate rostral to the otic vesicle, whereas the branchial stream divides into two parallel streams, which migrate caudal to the otic vesicle. At later stages (stage 26 onwards) the cranial neural crest cells disperse into the adjacent mesoderm and can no longer be followed by dissection and SEM. The pattern of cranial neural crest emergence and migration, and division into migratory streams is similar to that in other amphibians and in the Australian lungfish ( Neoceratodus forsteri). Emergence of crest cells from the neural tube, relative to the time of neural tube closure, occurs relatively late in comparison to anurans, but much earlier than in the Australian lungfish. These results establish a morphological foundation for studies in progress on the further development and fate of cranial neural crest cells in the Mexican axolotl, as well as for studies of the role of cranial neural crest in cranial patterning.

Summary Direct development is a specialized reproductive mode that has evolved repeatedly in many different lineages of amphibians, especially anurans. A fully formed, albeit miniature adult hatches directly from the egg; there is no free-living larva. In many groups, the evolution of direct development has had profound consequences for cranial development and morphology, including many components that are derived from the embryonic neural crest. Yet, the developmental bases of these effects remain poorly known. In order to more fully characterize these changes, we used three molecular markers to analyze cranial neural crest-cell emergence and migration in the direct-developing frog, Eleutherodactylus coqui: HNK-1 immunoreactivity, Dlx protein expression, and cholinesterase activity. Our study validates and extends earlier results showing that the comprehensive changes in embryonic cranial patterning, differentiation, and developmental timing that are associated with direct development in Eleutherodactylus have not affected gross features of cranial neural crest biology: the relative timing of crest emergence and the number, configuration and identity of the principal migratory streams closely resemble those seen in metamorphic anurans. The three markers are variably expressed within and among neural crest-cell populations. This variation suggests that determination of cranial neural crest-cells may already have begun at or soon after the onset of migration, when the cells emerge from the neural tube. It is not known how or even if this variation correlates with differential cell lineage or fate. Finally, although HNK-1 expression is widely used to study neural crest migration in teleost fishes and amniotes, E. coqui is the only amphibian known in which it effectively labels migrating neural crest-cells. There are not enough comparative data to determine whether this feature is functionally associated with direct development or is instead unrelated to reproductive mode.

The segmental series of somites in the vertebrate embryo gives rise to the axial skeleton. In amniote models, single vertebrae are derived from the sclerotome of two adjacent somites. This process, known as resegmentation, is well-studied using the quail-chick chimeric system, but the presumed generality of resegmentation across vertebrates remains poorly evaluated. Resegmentation has been questioned in anamniotes, given that the sclerotome is much smaller and lacks obvious differentiation between cranial and caudal portions. Here, we provide the first experimental evidence that resegmentation does occur in a species of amphibian. Fate mapping of individual somites in the Mexican axolotl (Ambystoma mexicanum) revealed that individual vertebrae receive cells from two adjacent somites as in the chicken. These findings suggest that large size and segmentation of the sclerotome into distinct cranial and caudal portions are not requirements for resegmentation. Our results, in addition to those for zebrafish, indicate that resegmentation is a general process in building the vertebral column in vertebrates, although it may be achieved in different ways in different groups.

The contribution of cranial neural crest cells to the development and patterning of cranial muscles in amphibians was investigated in the phylogenetically basal and morphologically generalized frog, Bombina orientalis. Experimental methods included fluorescent marking of premigratory cranial neural crest and extirpation of individual migratory streams. Neural crest cells contributed to the connective tissue component, but not the myofibers, of many larval muscles within the first two branchial arches (mandibular and hyoid), and complex changes in muscle patterning followed neural crest extirpation. Connective tissue components of individual muscles of either arch originate from the particular crest migratory stream that is associated with that arch, and this relationship is maintained regardless of the segmental identity—or embryonic derivation—of associated skeletal components. These developmental relations define a pattern of segmentation in the head of larval anurans that is similar to that previously described in the domestic chicken, the only vertebrate that has been thoroughly investigated in this respect. The fundamental role of the neural crest in patterning skeleton and musculature may represent a primitive feature of cranial development in vertebrates. Moreover, the corresponding developmental processes and cell fates appear to be conserved even when major evolutionary innovations—such as the novel cartilages and muscles of anuran larvae—result in major differences in cranial form.

Any comprehensive theory of organismal form must relate the short-term processes of embryonic development to the long-term processes of evolutionary change—in what Mayr (1982, 1994, 1997) termed "internal" and "external" biology; see Mayr and Provine (1980) and Hall (1998) for overviews. Any integration of internal and external biology involves a synthesis, or at the very least a coming together, of developmental and evolutionary biology as Evolutionary Developmental Biology (Hall, 1998). Hence, a symposium on Developmental and Evolutionary Perspectives on Major Transformations in Body Organization, the selection of approaches and speakers, and the venue—the Society for Integrative and Comparative Biology. Although attempts to understand the processes and mechanisms underlying body organization have been made for millennia, two periods of particularly intense research activity stand out. The first, in the latter half of the 19th century, was characterized by the discovery that all animals are built from similar cells, tissues and organs; that these cells arise embryologically from the same germ layers; that embryos of very different organisms progress through surprisingly similar stages using similar developmental processes of cell division, migration, commitment, differentiation and morphogenesis (see Bowler, 1996; Gee, 1996 and Hall, 1998 for reviews). Still, embryos, larvae and adults of dif

The current theory of felid coat pattern evolution proposes that the primitive pattern is one of relatively large spots that break down into smaller spots (here denoted flecks) and rosettes while at the same time leading to various striped patterns as sidelines. We have coded the coat patterns of felids into uniform, flecks, rosettes, vertical stripes, small blotches and blotches and show by mapping these character states onto phylogenies of the family that the current theory is flawed. Instead, the primitive pattern appears to be flecks and it is from this type that nearly all other types have developed. The robustness of this hypothesis is shown by the fact that it remains unchanged regardless of which of several quite different, competing phylogenies of the family is used. The pattern of transformations reconstructed is not predicted by current theories of pattern formation and we suggest that modellers pay closer attention to the phylogenetic histories of the features that they model.

Certain peptides derived from the α1 domain of the major histocompatibility class I antigen complex (MHC-I) inhibit receptor internalization, increasing the steady-state number of active receptors on the cell surface and thereby enhancing the sensitivity to hormones and other agonists. These peptides self-assemble, and they also bind to MHC-I at the same site from which they are derived, suggesting that they could bind to receptor sites with significant sequence similarity. Receptors affected by MHC-I peptides do, indeed, have such sequence similarity, as illustrated here by insulin receptor (IR) and insulin-like growth factor-1 receptor. A synthetic peptide with sequence identical to a certain extracellular receptor domain binds to that receptor in a ligand-dependent manner and inhibits receptor internalization. Moreover, each such peptide is selective for its cognate receptor. An antibody to the IR peptide not only binds to IR and competes with the peptide but also inhibits insulin-dependent internalization of IR. These observations, and binding studies with deletion mutants of IR, indicate that the sequence QILKELEESSF encoded by exon 10 plays a key role in IR internalization. Our results illustrate a principle for identifying receptor-specific sites of importance for receptor internalization, and for enhancing sensitivity to hormones and other agonists.

Synopsis. The primitive, or ancestral reproductive mode for Recent am? phibians involves a complex, biphasic life history. Yet evolutionarily derived, alternate modes are seen in all three living orders and predominate in some clades. Analysis of the consequences and mechanistic bases of one such mode?direct development?can provide insights into the evolutionary opportunities and constraints conferred by the ancestral metamorphic ontog? eny. Direct development in the anuran genus Eleutherodactylus involves fundamental alterations to many features of embryonic and posthatching devel? opment. At hatching, young emerge as fully formed, albeit tiny versions of the adult; most larvai features are absent. Pervasive changes in ontogenetic timing, in particular the precocious (embryonic) formation of many adult structures, appear to be correlated with early development of the thyroid axis, although responsiveness to exogenous thyroid hormone is diminished or even lacking in at least some peripheral tissues. Changes in cranial patterning are likely mediated by the embryonic neural crest, although many gross features of crest biology are highly conserved. Laboratory-based analyses of direct de? velopment and other derived reproductive modes in amphibians, using contemporary methods developed for more standard, "model" organisms, may contribute important insights into life-history evolution that complement those derived from analyses of morphology, ecology and phylogeny.