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Onychophorans (velvet worms) are closely related to the arthropods, but their limb morphology represents a stage before arthropodization (i.e., the segmentation of the limbs). We investigated the expression of onychophoran homologs of genes that are involved in dorso-ventral (DV) and proximo-distal (PD) limb patterning in arthropods. We find that the two onychophoran optomotor-blind (omb) genes, omb-1 and omb-2, are both expressed in conserved patterns in the dorsal ectoderm of the limbs, including the onychophoran antennae (the frontal appendages). Surprisingly, the expression of decapentaplegic (dpp), which acts upstream of omb in Drosophila, is partially reversed in onychophoran limbs compared to its expression in arthropods. A conserved feature of dpp expression in arthropods and onychophorans, however, is the prominent expression of dpp in the tips of developing limbs, which, therefore, may represent the ancestral pattern. The expression patterns of wingless (wg) and H15 are very diverged in onychophorans. The wg gene is only expressed in the limb tips and the single H15 gene is expressed in a few dorsal limb cells, but not on the ventral side. The expression of wg and dpp at the limb tips is one of the three possible alternatives predicted by the topology model of arthropod limb patterning and is, thus, compatible with a conserved function of wg and dpp in the patterning of the PD axis. On the other hand, DV limb gene expression is less conserved, and the specification of ventral fate appears to involve neither wg nor H15 expression.

SUMMARY All trunk segments in the pill millipede Glomeris marginata (Myriapoda: Diplopoda) are initially patterned genetically, (as visualized by the embryonic expression pattern of the even-skipped gene) and formed morphologically, (as visualized by 4-6-diamidin-2-phenylindol stained embryos) in a single segmental period. In addition, formation of every nascent trunk segment concerns ventral as well as dorsal segmental units. Only after the formation of the nascent posterior trunk segments, the dorsal segmental units of two adjacent segments fuse to form a single dorsal segmental unit that subsequently covers two ventral leg-bearing segmental units. The formation of a diplosegmental unit, or in short a diplosegment, is thus the result of dorsal fusion of embryonic tissue and not the result of any splitting-process or fusion of dorsal tergites. The new data also argue against heterochrony as a primary causative factor for the formation of the diplosegments during the formation of dorsal versus ventral segmental units. Furthermore, no evidence was found supporting the hypothesis that anterior trunk segments in diplopods represent degenerate diplosegments. Two possible scenarios arise from the ontogenetic data presented here, whether this represents an ancestral feature of the diplopods, or alternatively if they represent an isolated case only found in Glomeris (and close relatives). If the former is the case, my work may provide an impressive example of Haeckel's recapitulation theory.

In arthropods, such as Drosophila melanogaster, the leg gap genes homothorax (hth), extradenticle (exd), dachshund (dac), and Distal-less (Dll) regionalize the legs in order to facilitate the subsequent segmentation of the legs. We have isolated homologs of all four leg gap genes from the onychophoran Euperipatoides kanangrensis and have studied their expression. We show that leg regionalization takes place in the legs of onychophorans even though they represent simple and nonsegmented appendages. This implies that leg regionalization evolved for a different function and was only later co-opted for a role in leg segmentation. We also show that the leg gap gene patterns in onychophorans (especially of hth and exd) are similar to the patterns in crustaceans and insects, suggesting that this is the plesiomorphic state in arthropods. The reversed hth and exd patterns in chelicerates and myriapods are therefore an apomorphy for this group, the Myriochelata, lending support to the Myriochelata and Tetraconata clades in arthropod phylogeny.

The phylum Priapulida is a small group of marine worms that is allied with the nematodes, kinorhynchs, loriciferans and nematomorphs in a clade called the Cycloneuralia or Introverta. Together with the arthropods they are generally considered to comprise the Ecdysozoa, a clade of moulting animals. A number of recent priapulid species possess features that resemble the predicted Ecdysozoan ancestor. In addition, recent molecular studies have also shown that they are basal within the Ecdysozoa/Cycloneuralia (Garey 2001, Webster et al. 2006). Their putative basal position thus makes priapulids highly interesting research objects for understanding the evolution of Ecdysozoa. Earlier investigations of the early embryology of the priapulid Priapulus caudatus are critically revised with the aid of modern techniques and equipment, confirming earlier studies that the early cleavages are highly symmetrical, total, subequal, radial and stereotypical. New results show that up to the sixth cleavage, the spindles are oriented along the animal/vegetal axis at both poles. This unique cleavage pattern has only limited similarities to other animals. During the sixth cleavage two cells move inwards and gastrulation commences. If the mesoderm is derived from both cells, its origin differs from that of many other protostomes.Two previously undescribed larval stages of P. caudatus; the light bulb shaped hatchling and the first lorica larva are described. The second lorica larva superficially resembles the previously described type 2 lorica larva (Higgins et al 1993). Differences between the second lorica larva and the type 2 lorica larva, with respect to possible ecophenotypical variation and sub-specialization, are described. Preliminary data are presented on musculature development of P. caudatus. Preliminary data have also been obtained on the early development of a second priapulid, Halicryptus spinulosus. Comparison of Halicryptus and Priapulus may help to resolve developmental ground pattern of the priapulids.

Heart development exhibits some striking similarities between vertebrates and arthropods, for example in both cases the heart develops as a linear tube from mesodermal cells. Furthermore, the underlying molecular pathways exhibit a significant number of similarities between vertebrates and the fruit fly Drosophila, suggesting a common origin of heart development in the last common ancestor of flies and vertebrates. However, there is hardly any molecular data from other animals. Here we show that many of the key genes are also active in heart development in the spider Cupiennius salei. Spiders belong to the chelicerates and are distantly related to insects with respect to the other arthropods. The tinman/Nkx2.5 ortholog is the first gene to be specifically expressed in the presumptive spider heart, like in flies and vertebrates. We also show that tinman is expressed in a similar way in the beetle Tribolium castaneum. Taken together this demonstrates that tinman has a conserved role in the specification of the arthropod heart. In addition, we analyzed the expression of other heart genes (decapentaplegic, Wnt5, H15, even-skipped, and Mef2 ) in Cupiennius. The expression of these genes suggests that the genetic pathway of heart development may be largely conserved among arthropods. However, a major difference is seen in the earlier expression of the even-skipped gene in the developing spider heart compared with Drosophila, implying that the role of even-skipped in heart formation might have changed during arthropod evolution. The most striking finding, however, is that in addition to the dorsal tissue of the fourth walking leg segment and the opisthosomal segments, we discovered tinman-expressing cells that arise from a position dorsal to the cephalic lobe and that contribute to the anterior dorsal vessel. In contrast to the posterior heart tissue, these cells do not express the other heart genes. The spider heart thus is composed of two distinct populations of cells.