The engrailed gene is well known from its role in segmentation and central nervous system development in a variety of species. In molluscs, however, engrailed is involved in shell formation. So far, it seemed that engrailed had been co-opted uniquely for this particular process in molluscs. Here, we show that, in the gastropod mollusc Patella vulgata, an engrailed ortholog is expressed in the edge of the embryonic shell and in the anlage of the apical sensory organ. Surprisingly, a dpp-BMP2/4 ortholog is expressed in cells of the ectoderm surrounding, but not overlapping, the engrailed-expressing shell-forming cells. It is also expressed in the anlage of the eyes. Earlier it was shown that a compartment boundary exists between the cells of the embryonic shell and the adjacent ectoderm. We conclude that engrailed and dpp are most likely involved in setting up a compartment boundary between these cells, very similar to the situation in, for example, the developing wing imaginal disc in Drosophila. We suggest that engrailed became involved in shell formation because of its ancestral role, which is to set up compartment boundaries between embryonic domains.

The orthodenticle/ otx and orthopedia/ otp classes of homeobox gene families have been identified in all three major classes of bilaterians: deuterostomes, lophotrochozoans, and ecdysozoans. Otx genes have been studied extensively and play a role in the development of anterior neural structures. Otp genes have been found to be involved in nervous system development in mouse and Drosophila. To date, no members of these genes are known in molluscs. We cloned orthologs of orthodenticle/ otx and orthopedia/ otpfrom the gastropod Patella vulgata, and designated them Pv-otx and Pv-otprespectively. Our analysis of the spatio-temporal expression pattern of otx and otp orthologs during P. vulgata embryogenesis leads to the following conclusions. First, Pv-otx is expressed in and around the stomodaeum and our analysis thus supports the previously suggested conservation of the protostome and deuterostome larval mouth regions. Second, we find that Pv-otp is involved in the development of the larval apical sensory organ, suggesting a conserved role for this gene family in nervous system development. A similar conserved role in nervous system development has been proposed for orthodenticle/otx genes and we suggest that part of the cells expressing Pv-otx are involved in the development of the anterior nervous system. Last, we postulate that otx genes were ancestrally involved in the development of ciliary bands in bilaterians.

The twist gene is known to be involved in mesoderm formation in two of the three clades of bilaterally symmetrical animals: viz. deuterostomes (such as vertebrates) and ecdysozoans (such as arthropods and nematodes). There are currently no data on the spatiotemporal expression of this gene in the third clade, the lophotrochozoans (such as mollusks and annelids). To approach the question of mesoderm homology across bilaterians, we decided to analyze orthologs of this gene in the gastropod mollusk Patella vulgata that belongs to the lophotrochozoans. We present here the cloning, characterization, and phylogenetic analysis of a Patella twist ortholog, Pv-twi, and determine the early spatiotemporal expression pattern of this gene. Pv-twi expression was found in the trochophore larva in a subset of the ectomesoderm, one of the two sources of mesoderm in Patella. These data support the idea that twist genes were ancestrally involved in mesoderm differentiation. The absence of Pv-twi in the second mesodermal source, the endomesoderm, suggests that also other genes must be involved in lophotrochozoan mesoderm differentiation. It therefore remains a question if the mesoderm of all bilaterians is homologous.

Snail genes have been found to play a role in mesoderm formation in two of the three clades of bilaterians, deuterostomes (comprising the chordates) and ecdysozoans (comprising the arthropods). No clear data are available on the role these genes play in development of the mesoderm in the third clade, that of lophotrochozoans (comprising annelids and molluscs). We identified two new members of the snail gene family in the gastropod mollusc Patella vulgata. Phylogenetic analysis showed that the two genes clearly belong to the snail sub-family. Their expression patterns do not indicate a role during early mesoderm formation. In fact, contrary to expectations, the snail genes of Patella were mostly expressed in the ectoderm. In view of the location of their expression sites, we suggest that these genes could be involved in regulating epithelial-mesenchymal transitions (EMT) and cell motility, as has recently been demonstrated for snail genes in vertebrates. This may well correspond to the ancestral function of these genes. The results are discussed in the light of the evolutionary origin of the mesoderm. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00427-002-0228-1.

No abstract