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The understanding of the evolution of variable sex determination mechanisms across taxa requires comparative studies among closely related species. Following the fate of a known master sex-determining gene, we traced the evolution of sex determination in an entire teleost order (Esociformes). We discovered that the northern pike ( Esox lucius ) master sex-determining gene originated from a 65 to 90 million-year-old gene duplication event and that it remained sex linked on undifferentiated sex chromosomes for at least 56 million years in multiple species. We identified several independent species- or population-specific sex determination transitions, including a recent loss of a Y chromosome. These findings highlight the diversity of evolutionary fates of master sex-determining genes and the importance of population demographic history in sex determination studies. We hypothesize that occasional sex reversals and genetic bottlenecks provide a non-adaptive explanation for sex determination transitions., Competing Interests: QP, RF, EJ, HD, AH, BK, ER, FG, WL, LB, MT, SC, ES, GD, Fv, SC, JL, HV, KO, RG, CE, JL, CR, HH, RT, PD, KN, LJ, HP, CK, EI, VT, MS, JP, YG No competing interests declared, (© 2021, Pan et al.)

Florida manatees (Trichechus manatus latirostris) possess an unusual suite of adaptations to accommodate both a fully aquatic lifestyle and an herbivorous diet, including a low metabolic rate and a very limited thermoneutral zone. Their relatively high lower critical temperature of around 20 °C suggests strong sensitivity to cold, thereby limiting their distribution to tropical and subtropical waters. "Cold stress syndrome" affects and kills Florida manatees every year during intense or prolonged cold weather, posing one of the major threats to manatees. However, knowledge regarding manatee thermoregulation is sparse, but essential for effective conservation and management of this threatened species. We measured heat flux in two captive Florida manatees at multiple times of the year, at 41 sites distributed across the entire body surface of each manatee. Heat flux differed significantly between individuals, and among body sites and times of the year. The pectoral flippers and axillae were identified as areas with highest heat exchange. Despite exposure to constant water temperature throughout the year, the manatees in this study had significantly lower heat flux in winter than in summer. We used the measured heat flux values to calculate total heat dissipation in individual manatees. The values estimated this way correspond well with the low metabolic rates estimated in previous studies, confirming the reliability of our novel approach. Our method provides simple and useful options for enhancing manatee welfare by monitoring the animals' thermal state during potentially stressful activities such as during medical treatment, capture restraints and transportation.

A thirteen-laboratory comparative study tested the performance of four methods as alternatives to mouse bioassay for the determination of brevetoxins in shellfish. The methods were N2a neuroblastoma cell assay, two variations of the sodium channel receptor binding assay, competitive ELISA, and LC/MS. Three to five laboratories independently performed each method using centrally prepared spiked and naturally incurred test samples. Competitive ELISA and receptor binding (96-well format) compared most favorably with mouse bioassay. Between-laboratory relative standard deviations (RSDR) ranged from 10 to 20% for ELISA and 14 to 31% for receptor binding. Within-laboratory (RSDr) ranged from 6 to 15% for ELISA, and 5 to 31% for receptor binding. Cell assay was extremely sensitive but data variation rendered it unsuitable for statistical treatment. LC/MS performed as well as ELISA on spiked test samples but was inordinately affected by lack of toxin-metabolite standards, uniform instrumental parameters, or both, on incurred test samples. The ELISA and receptor binding assay are good alternatives to mouse bioassay for the determination of brevetoxins in shellfish.

During blooms of the dinoflagellate Karenia brevis, filter-feeders such as oysters and clams bioaccumulate brevetoxins, often to levels that are toxic to humans. In controlled aquarium experiments, we exposed live oysters to bloom levels of toxic K. brevis, followed by 10 weeks of exposure to non-toxic microalgae. Oysters were harvested weekly and analyzed for brevetoxins and brevetoxin metabolites to quantify toxin bioaccumulation and depuration. All of the PbTx-2 concentrated by oysters was immediately converted to a mixture of polar metabolites that were then slowly eliminated from the oysters. However, 90% of measured PbTx-3 was eliminated within two weeks of toxic exposure but without apparent biotransformation. Extracts of oysters containing high levels of PbTx-3 were toxic to mice by intraperitoneal (IP) injection. Extracts of oysters harvested after PbTx-3 had been eliminated were non-toxic despite high concentrations of PbTx-2 metabolites. Oysters collected in Florida during and after a bloom of K. brevis contained polar metabolites of PbTx-2 as well as PbTx-3, but no PbTx-2. Again, PbTx-3 concentration was a good predictor of mouse toxicity. One hundred percent conversion of PbTx-2 to polar metabolites was also accomplished in vitro by spiking oyster or clam homogenate with PbTx-2, followed by a brief incubation at room temperature. These PbTx-2 metabolites did not kill mice, either orally or by intraperitoneal injection, even at concentrations 30 times greater than toxic PbTx-3 levels.

Analysing the population genetic structures of sympatric species provides opportunities to compare patterns of population genetic structure and phylogeography in order to gain insight into the factors that influence the development of the observed patterns. In this study, we compared the population genetic structures and phylogeographies of brown shrimp (Farfantepenaeus aztecus) and white shrimp (Litopenaeus setiferus), two sympatric penaeid shrimp species that inhabit the waters of the eastern USA, using sequence analysis of the mitochondrial DNA control region. Brown shrimp showed no significant phylogenetic structure or population subdivision, and closely related haplotypes were geographically dispersed. Mismatch analysis indicated that brown shrimp experienced a late-Pleistocene era sudden population expansion. In contrast, white shrimp had a complex haplotype phylogeny consisting of two distinct lineages and two less well-defined sublineages, and the haplotypes and lineages were geographically structured. Mismatch analysis for white shrimp also showed evidence of sudden population expansion, albeit for each lineage separately and more recently than in the brown shrimp. These disparate patterns may have developed as a result of species-specific differences in physiological tolerances and habitat preferences that caused greater fluctuations in white shrimp population sizes and reductions in long-term effective population size relative to that of the brown shrimp, and thereby increased the susceptibility of the white shrimp populations to stochastic genetic change.

Although Florida manatees (Trichechus manatus latirostris) have relatively low basal metabolic rates for aquatic mammals of their size, they maintain normal mammalian core temperatures. We describe vascular structures in the manatee tail that permit countercurrent heat exchange (CCHE) to conserve thermal energy. Approximately 1000 arteries juxtaposed to 2000 veins are found at the cranial end of the caudal vascular bundle (CVB); these numbers decrease caudally, but the 1:2 ratio of arteries to veins persists. Arterial walls are relatively thin when compared to those previously described in vascular countercurrent heat exchangers in cetaceans. It is assumed that CCHE in the CVB helps manatees to maintain core temperatures. Activity in warm water, however, mandates a mechanism that prevents elevated core temperatures. The tail could transfer heat to the environment if arterial blood delivered to the skin were warmer than the surrounding water; unfortunately, CCHE prevents this heat transfer. We describe deep caudal veins that provide a collateral venous return from the tail. This return, which is physically outside the CVB, reduces the venous volume within the bundle and allows arterial expansion and increased arterial supply to the skin, and thus helps prevent elevated core temperatures.

Analyses of life-history data show that both the size-specific batch fecundities and the age-specific spawning frequencies differ for two halfbeak species, Hemiramphus brasiliensis, the ballyhoo, and H. balao, the balao. Halfbeak ages were determined from sectioned otoliths; histological data was used to describe oocyte development and estimate spawning frequency; and batch fecundity was measured from counts of whole oocytes in final maturation. Hemiramphus brasiliensis lived longer (4 versus 2 years) and had a higher survival rate (14.9% versus 7.5% annually) than H. balao did. Of the two species the larger and longer-lived congener, H. brasiliensis, reached sexual maturity at a larger size (fork length 198 versus 160 mm). The spawning period of age-0 females was strongly related to season, whereas spawning by older females occurred throughout the year. Reproduction by both species peaked during late spring or early summer, and all mature females were spawning daily during April (H. brasiliensis) or June (H. balao). This is the first demonstration of iteroparity for the family Hemiramphidae. H. brasiliensis had a lower batch fecundity (about 1164 versus 3743 hydrated oocytes for a 100-g female) than H. balao did. Such low batch fecundities are typical of the order Beloniformes, but quite different from those of other fishes that live in association with coral reef habitats. H. balao's higher batch fecundity is consistent with the life-history theory that predicts higher numbers of eggs for shorter-lived species; this is possible because H. balao produces smaller hydrated oocytes than H. brasiliensis (modal diameter about 1.6 versus 2.4 mm). The high spawning frequency of Hemiramphus species compensates for their low batch fecundity. The annual fecundity of both species is similar to that of other reef fish species, after adjusting for body size and spawning frequency. The lifetime fecundity of H. balao was very similar to that of H. brasiliensis, after accounting for the differences in survival for each species. This suggests a fine tuning of different reproductive traits over the entire life cycle that results in roughly equivalent lifetime fecundity for both species.

We examined the response of demographic, morphological, and chemical parameters of turtle grass (Thalassia testudinum), to much-higher-than-normal rainfall associated with an El Niño event in the winter of 1997-1998. Up to 20 inches of added rain fell between December 1997 and March 1998. triggering widespread and persistent phytoplankton blooms along the west coast of Florida. Water-column chlorophyll concentrations estimated from serial Sea WiFS imagery were much higher during the El Niño event than in the previous or following years, although the timing and magnitude of phytoplankton blooms varied among sites. Seagrass samples collected in 1997, 1998, and 1999 provided an excellent opportunity to test the responsiveness of Thalassia to decline and subsequent improvement of water quality and clarity in four estuaries. Using a scoring technique based on temporal responsiveness, spatial consistency, and statistical strength of indicators, we found that several morphological parameters (Thalassia shoot density, blade width, blade number, and shoot-specific leaf area) were responsive and consistent measures of light stress. Some morphological parameters, such as rhizome apex density, responded to declines and subsequent improvement in water clarity, but lacked the statistical discriminating power necessary to be useful indicators. However, rhizome sugar, starch, and total carbohydrate concentrations also exhibited spatially and temporally consistent variation as well as statistical strength. Because changes in shoot density, as well as water clarity, affect rhizome carbohydrate levels, a composite metric based on Thalassia shoot density and rhizome carbohydrate levels together is probably more useful than either parameter alone as an indicator of seagrass health.

International organizations and biodiversity scientists recognize three levels of biodiversity: genetic, species, and ecosystem. However, most studies with the goal of assessing biodiversity collect data at only a single level--that of the species. Even when multiple levels of biodiversity are considered, usually only ecosystem diversity is also evaluated. Genetic diversity is virtually never considered. Yet, genetic diversity is essential for the maintenance of populations and species over ecological and evolutionary time periods. Moreover, because components of genetic diversity are independent of either species or ecosystem diversity, genetic diversity can provide a unique measure by which to assess the value of regions for conservation. Regions can be valuable for conservation of their genetic resources regardless of their levels of species or ecosystem uniqueness or diversity. In general, the same methods and statistical programs that are used to answer questions about population genetics and phylogenetics are applicable to conservation genetics. Thus, numerous genetic techniques, laboratory methods, and statistical programs are available for assessing regional levels of genetic diversity for conservation considerations. Here, we provide the rationale, techniques available, field and laboratory protocols, and statistical programs that can be used to estimate the magnitude and type of genetic diversity in regions. We also provide information on how to obtain commonly utilized statistical programs and the type of analyses that they include. The guide that we present here can be used to conduct investigations of the genetic diversity of regions under consideration for conservation of their natural resources.

Florida queen conch stocks once supported a significant fishery, but overfishing prompted the state of Florida to institute a harvest moratorium in 1985. Despite the closure of the fishery, the queen conch population has been slow to recover. One method used in the efforts to restore the Florida conch population has been to release hatchery-reared juvenile conch into the wild; however, suboptimal predator avoidance responses and lighter shell weights relative to their wild counterparts have been implicated in the high mortality rates of released hatchery juveniles. We conducted a series of experiments in which hatchery-reared juvenile conch were exposed to a predator, the spiny lobster (Panulirus argus), to determine whether they could develop behavioral and morphological characteristics that would improve survival. Experiments were conducted in tanks with a calcareous sand substrate to simulate a natural environment. Conditioned conch were exposed to caged lobsters while conch in the control tanks were exposed to empty cages. Conditioned conch moved significantly less and buried themselves more frequently than the naive control conch. Morphometric data indicated that the conditioned conch grew at a significantly slower rate than the naive conch, but the shell weights of the two groups were not significantly different. This implies that the conditioned conch had thicker or denser shells than the control group. As a result, the conditioned conch had significantly higher survival than naive conch in a subsequent predation experiment in which a lobster was allowed to roam free in each tank for 24 hours. In the future, the conditioning protocols documented in this study will be used to increase the survival of hatchery-reared conch in the wild.

The reproductive organs of Florida manatees (Trichechus manatus latirostris) are surrounded by thermogenic locomotory muscles and insulating fat. Manatees are reported to maintain core body temperatures of 35.6 degrees -36.4 degrees C, temperatures known to interfere with production and maturation of viable sperm in terrestrial mammals. We describe two novel venous plexuses associated with the manatee epididymis. Each epididymis is located in a hypogastric fossa at the caudolateral extremity of the abdominal cavity. Each hypogastric fossa is lined by an inguinal venous plexus that receives cooled blood from a superficial thoracocaudal plexus. We conclude that male manatees may prevent hyperthermic insult to their reproductive tissues by feeding cooled superficial blood to venous plexuses deep within their bodies. Female manatees also possess hypogastric fossae and venous structures similar to those found in male manatees. The ovaries, uterine tubes, and distal tips of the uterine horns are located in the hypogastric fossae. We suggest that the thermovascular structures we describe also prevent hypothermic insult to female manatee reproductive tissues. The venous structures in manatees are functionally similar to structures associated with reproductive thermoregulation in cetaceans and phocid seals. Thus, these thermovascular structures appear to be convergent morphological adaptations that occur in three clades of diving mammals with independent evolutionary histories.

Dinoflagellates can be classified both botanically and zoologically; however, they are typically put in the botanical division Pyrrhophyta. As a group they appear most related to the protistan ciliates and apicomplexans at the ultrastructure level. Within the Pyrrhophyta are both unarmored and armored forms of the dominant, motile flagellated stage. Unarmored dinoflagellates do not have thecal or wall plates arranged in specific series, whereas armored species have plates that vary in thickness but are specific in number and arrangement. In armored dinoflagellates, the plate pattern and tabulation is a diagnostic character at the family, subfamily, and even genus levels. In most cases, the molecular characterization of dinoflagellates confirms the taxonomy on the basis of external morphology; this has been demonstrated for several groups. Together, both genetic and morphological criteria are becoming increasingly important for the characterization, separation, and identification of dinoflagellates species. Pfiesteria and Pfiesteria-like species are thinly armored forms with motile dinospore stages characterized by their distinct plate formulae. Pfiesteria piscicida is the best-known member of the genus; however, there is at least one other species. Other genetically and morphologically related genera, now grouped under the common names of "Lucy," "Shepherd's crook," and cryptoperidiniopsoid, are being studied and described in separate works. All these other heterotrophic dinoflagellate groups, many of which are thought to be benign, co-occur in estuarine waters where Pfiesteria has been found.

Relative to many other mammals, little is known about the functional morphology of the four extant species of the order Sirenia. In this study, 166 Florida manatee (Trichechus manatus latirostris) carcasses fresh enough to collect detailed anatomical information were examined to describe the gross anatomy of the diaphragm. Our results show that the Florida manatee's diaphragm differs from those of other mammals in that it: lies in a dorsal plane, rather than in the more typical transverse plane; is located dorsal to the heart and does not attach to the sternum; and attaches medially at the "I"-shaped central tendon to bony projections extending ventrally from the vertebral bodies, forming two distinct hemidiaphragms. The manatee's transverse septum is a separate structure that lies at a right angle to the diaphragm and separates the heart from the liver and other viscera. The extreme muscularity of the diaphragm and the ability of manatees to adjust their position in the water column with minimal external movement suggest that diaphragmatic contractions may change the volume of each pleural cavity to affect buoyancy, roll, and pitch. We also hypothesize that such contractions, in concert with contractions of powerful abdominal muscles, may compress gas in the massive large intestine, and thereby also contribute to buoyancy control., (Copyright 2000 Wiley-Liss, Inc.)

The ovarian germinal epithelium in the common snook, Centropomus undecimalis, is described. It consists of epithelial and prefollicle cells that surround germ cells, either oogonia or oocytes, respectively. The germinal epithelium borders a body cavity, the ovarian lumen, and is supported by a basement membrane that also separates the epithelial compartment of the ovarian lamellae from the stromal compartment. During folliculogenesis, the epithelial cells, whose cytoplasmic processes encompass meiotic oocytes, transform into prefollicle cells, which become follicle cells at the completion of folliculogenesis. The follicle is a derivative of the germinal epithelium and is composed of the oocyte and surrounding follicle cells. It is separated from the encompassing theca by a basement membrane. The cells that form the theca interna are derived from prethecal cells within the extravascular space of the ovarian stroma. The theca externa differentiates from undifferentiated cells within the stromal compartment of the ovary, from within the extravascular space. The theca interna and the theca externa are not considered to be part of the follicle and are derived from a different ovarian compartment than the follicle. Meiosis commences while oocytes are still within the germinal epithelium and proceeds as far as arrested diplotene of the first meiotic prophase. The primary growth phase of oocyte development also begins while oocytes are still within the germinal epithelium or attached to it in a cell nest. The definitions used herein are consistent between sexes and with the mammalian literature., (Copyright 2000 Wiley-Liss, Inc.)

Phylogenetic relationships among the 12 recognized fish species in the New World genus Centropomus (Pisces, Centropomidae) were analyzed using allozyme electrophoresis and 618 bp of the mitochondrial DNA 16S ribosomal RNA (rRNA) gene. Molecular phylogenetic trees were generally consistent with previously published partial hypotheses based on morphological evidence. However, previously undefined sister group relationships between major species groups were resolved using molecular data, and phylogenetic hypotheses for Centropomus based on 16S rRNA sequences were better supported than were allozyme-based hypotheses. The high level of congruence among the trees inferred from the nuclear and mitochondrial characters provided a firm phylogenetic basis for analysis of ecological diversification and molecular evolution in the genus. Compared to basal Centropomus species, members of the most nested species group were significantly larger in body size and occupied a marine niche only peripherally utilized by their congeners. We also observed substitution rate heterogeneity among 16S rRNA lineages; in contrast to expectations based on "metabolic rate" and "generation interval" models, relative substitution rates were faster than expected for the group of large-bodied snooks. Using the Pliocene rise of the Central American isthmian marine barrier to calibrate rates of 16S ribosomal gene evolution in Centropomus, we found that the rates for the genus were similar to those reported for higher vertebrates. Analysis of the three sets of transisthmian geminate taxa in Centropomus indicated that two of the pairs were probably formed during the Pliocene rise of the isthmus while the third pair diverged several million years earlier., (Copyright 1999 Academic Press.)

Electron microscopy has been used successfully to study and identify single-celled marine dinoflagellates including parasitic ones and others, such as those that cause red tide. Delicate cells can be preserved for scanning electron microscopy with a combined glutaraldehydeosmium tetroxide mixture that is adjusted for the osmolality of the medium. The protocol allows resolution of fine morphological features. Preservation for transmission electron microscopy can be accomplished with a standard glutaraldehyde fixation and osmium-tetroxide post-fixation in a suitable buffer, but again, the osmolality of the mixture must be adjusted. The protocol allows ultrastructural resolution of vesiculated cells and has been modified for small sample sizes.

Many investigators categorize individuals from hybrid zones to facilitate comparisons among genotypic classes (e.g., parental, F 1 , backcross) for comparative studies in which components of fitness or geographic variation are being analyzed. Frequently, multiple character sets representing genetically independent traits are used to classify these individuals and various methodologies are employed to combine the classifications obtained from the different character sets. We adapted the principles of total evidence and taxonomic congruence (two formalized approaches used by systematists in formulating phylogenetic hypotheses) to address the problem of discriminating hybridizing species and classifying individuals from hybrid zones. As our model, we used two morphological (coloration and morphometric) and two molecular (allozyme and mitochondrial DNA restriction-fragment-length polymorphism) character sets that differentiate two stone crab species (Menippe adina and M. mercenaria). Using principal-components analysis, we determined that combining character sets and eliminating characters or character sets that did not have large eigenvector coefficients for the principal component that best separated the two species yielded the highest level of discrimination between species and allowed us to classify a broad range of morpho-genotypes as hybrids. For the stone crabs, three diagnostic allozyme loci and five diagnostic coloration characters best separated the species. The two character sets were not completely congruent, but they agreed in their classification of 50% of the individuals from the hybrid zone and rarely strongly disagreed in their classifications. Classification discrepancies between the two character sets probably represent variation between traits in interspecific gene flow rather than intraspecific, ecologically mediated variation. Our results support the assertions of previous investigators who espoused the benefits associated with using multiple character sets to classify individuals from hybrid zones and demonstrate that, if character sets are reasonably congruent and numerically balanced, combining diagnostic characters from multiple character sets (a total-evidence approach) can enhance discriminatory power between species and facilitate the assignment of hybrid-zone individuals to genotypic classes. On the contrary, classifying hybrid-zone individuals using character sets separately (a taxonomic-congruence approach) provides the opportunity to compare levels of introgression between species and to assess reasons for discordance among the data sets., (© 1996 The Society for the Study of Evolution.)

An experimental investigation of drift macroalgal accumulation in seagrass beds was conducted to determine if the relationship between passively dispersed plant structure and the spatial arrangement of rooted macrophytes differed when examined across two spatial scales. Experiments were performed from December 1992 to April 1993 at four different sites in Tampa Bay, Florida, utilizing artificial seagrass units (ASUs) of uniform shoot length and density but with different areal dimensions [1 m 2 (S) versus 4 m 2 (L)]. Drift macroalgae were also collected from 1 m×1 m plots of natural seagrass at each of the experimental sites from November 1990 to May 1992 to determine the relationship between macroalgal abundance and structural characteristics of natural seagrass. Disproportionately higher amounts of macroalgae were captured in L compared to S plots suggesting that macroalgal accumulation does not scale up directly with the areal dimensions of ASU patches. Higher amounts of algae recovered in L plots is in accordance with patterns expected if algae accumulate in zones of attenuated water flow. Neither seagrass shoot density nor blade length could adequately describe the patterns of algal accumulation. These combined results suggest that explanations for trapping/retention of passively dispersed particles should extend beyond traditional measures of vegetation complexity.

In this report, the gonads of 32 glandulocaudine species, representing 18 genera, are compared with 11 outgroup characiform species. Through the presence of spermatozoa within the ovarian cavity, internal fertilization of the female is confirmed for the 16 genera for which mature ovaries were available. No outgroup ovary studied contains spermatozoa. All mature glandulocaudine testes have a large portion of the posterior testis, which is devoid of developing germ cells and spermatocysts (aspermatogenic), devoted to sperm storage, with the degree of partitioning in that region varying greatly within the group. All outgroup species examined have spermatozoa with spherical nuclei. With the exception of the species of the genus Planaltina, which also have spherical nuclei, all glandulocaudines have elongated nuclei, which vary among the species from 3.6 μm to 31.6 μm in length. Distinct sperm packets (spermatozeugmata) are formed in five genera by two different methods. In the genera Xenurobrycon, Tyttocharax, and Scopaeocharax, all of the tribe Xenurobryconini, the spermatozeugmata are formed within the spermatocysts and released fully formed. In all genera of the tribe Glandulocaudini, which includes Glandulocauda and Mimagoniates, loose spermatozoa are released which cluster into spermatozeugmata within the posterior storage areas. These morphological specializations are discussed within a phylogenetic framework as adaptations for internal fertilization and are hypothesized to be independently derived. © 1995 Wiley-Liss, Inc., (Copyright © 1995 Wiley-Liss, Inc.)

Two models developed to discern the mode of selection in hybrid zones differ in some predictions. The tension-zone model predicts that selection acts against hybrids and independently of the environment (endogenous selection) and that selection is invariant throughout the hybrid zone. The ecological selection-gradient, or ecotone, model maintains that fitness of different genotypes varies in response to environmental variation (exogenous selection) and thus, that in a region of the zone, fitness of hybrids is at least equal to that of the parental species. Therefore, to assess the predominant mode of selection operating in a hybrid zone, it is fundamental to evaluate whether selection is acting specifically against hybrid individuals, that is, whether hybridity alone is the basis for deficiencies of hybrids, and to evaluate whether the relative fitness of hybrids versus that of pure species varies across the zone. In a hardclam (genus Mercenaria) hybrid zone located in a polyhaline lagoon in east-central Florida, we used age-specific and location-specific analyses to determine that a hybrid deficit occurrs, that the deficit seems to be due to selection against hybrids, and that selection varies across the zone. Various measures of deviation from Hardy-Weinberg equilibrium, linkage disequilibrium analyses, and shifts in allele frequencies at semidiagnostic loci support the idea that selection is strongest in the northern region of the lagoon, the zone of sympatry and hybridization. Southward, into the range of M. mercenaria (the numerically predominant species), the percentage of hybrids remains relatively high and selection against hybrids decreases. For some genetic linkage groups, selection for M. mercenaria alleles seems to be occurring, but selection seems to be acting principally against alleles characteristic of M. mercenaria and, to a lesser degree, for alleles characteristic of M. campechiensis (the rarer species). These findings and others from previous analyses we have done on this hybrid zone demonstrate that selection in the zone is complex, and that characteristics of both the tension-zone and ecotone models are present. Supporting the tension-zone model, selection against hybrids per se clearly occurs, but specific genotypes seem to be at a selective disadvantage, whereas others have a selective advantage, and selection operates differentially on the two parental species within the zone. Supporting the ecotone model, the strength of overall selection varies throughout the zone, and environmentally mediated selection in which each species and hybrids have an advantage in specific habitats occurs, but some selection against hybrids is invariant throughout the zone. Thus, the structure and genetic architecture of this hybrid zone appear to be products of a complicated interaction between both types of selective forces cited in the two competing models., (© 1995 The Society for the Study of Evolution.)

Restriction-site analyses of mitochondrial DNA (mtDNA) from the loggerhead sea turtle (Caretta caretta) reveal substantial phylogeographic structure among major nesting populations in the Atlantic, Indian, and Pacific oceans and the Mediterranean sea. Based on 176 samples from eight nesting populations, most breeding colonies were distinguished from other assayed nesting locations by diagnostic and often fixed restriction-site differences, indicating a strong propensity for natal homing by nesting females. Phylogenetic analyses revealed two distinctive matrilines in the loggerhead turtle that differ by a mean estimated sequence divergence p = 0.009, a value similar in magnitude to the deepest intraspecific mtDNA node (p = 0.007) reported in a global survey of the green sea turtle Chelonia mydas. In contrast to the green turtle, where a fundamental phylogenetic split distinguished turtles in the Atlantic Ocean and the Mediterranean Sea from those in the Indian and Pacific oceans, genotypes representing the two primary loggerhead mtDNA lineages were observed in both Atlantic-Mediterranean and Indian-Pacific samples. We attribute this aspect of phylogeographic structure in Caretta caretta to recent interoceanic gene flow, probably mediated by the ability of this temperate-adapted species to utilize habitats around southern Africa. These results demonstrate how differences in the ecology and geographic ranges of marine turtle species can influence their comparative global population structures., (© 1994 The Society for the Study of Evolution.)

To address aspects of the evolution and natural history of green turtles, we assayed mitochondrial (mt) DNA genotypes from 226 specimens representing 15 major rookeries around the world. Phylogenetic analyses of these data revealed (1) a comparatively low level of mtDNA variability and a slow mtDNA evolutionary rate (relative to estimates for many other vertebrates); (2) a fundamental phylogenetic split distinguishing all green turtles in the Atlantic-Mediterranean from those in the Indian-Pacific Oceans; (3) no evidence for matrilineal distinctiveness of a commonly recognized taxonomic form in the East Pacific (the black turtle C.m. agassizi or C. agassizi); (4) in opposition to published hypotheses, a recent origin for the Ascension Island rookery, and its close genetic relationship to a geographically proximate rookery in Brazil; and (5) a geographic population substructure within each ocean basin (typically involving fixed or nearly fixed genotypic differences between nesting populations) that suggests a strong propensity for natal homing by females. Overall, the global matriarchal phylogeny of Chelonia mydas appears to have been shaped by both geography (ocean basin separations) and behavior (natal homing on regional or rookery-specific scales). The shallow evolutionary population structure within ocean basins likely results from demographic turnover (extinction and colonization) of rookeries over time frames that are short by evolutionary standards but long by ecological standards., (© 1992 The Society for the Study of Evolution.)

Histological detail in sections from tissues embedded in glycol methacrylate was improved by counterstaining PAS/iron-hematoxylin stained sections with a dilute solution of metanil yellow. The addition of the counterstain increases contrast in tissue sections and highlights PAS-positive entities. The staining protocol provides sharp definition of tissue morphology, differentiates cell types and other tissue components and does not produce background staining.

Female green turtles exhibit strong nest-site fidelity as adults, but whether the nesting beach is the natal site is not known. Under the natal homing hypothesis, females return to their natal beach to nest, whereas under the social facilitation model, virgin females follow experienced breeders to nesting beaches and after a "favorable" nesting experience, fix on that site for future nestings. Differences shown in mitochondrial DNA genotype frequency among green turtle colonies in the Caribbean Sea and Atlantic Ocean are consistent with natal homing expectations and indicate that social facilitation to nonnatal sites is rare.