Analysis of snail genes in the crustacean Parhyale hawaiensis : insight into snail gene family evolution

The transcriptional repressor snail was first discovered in Drosophila melanogaster , where it initially plays a role in gastrulation and mesoderm formation, and later plays a role in neurogenesis. Among arthropods, this role of snail appears to be conserved in the insects Tribolium and Anopheles gambiae , but not in the chelicerates Cupiennius salei and Achaearanea tepidariorum , the myriapod Glomeris marginata , or the Branchiopod crustacean Daphnia magna . These data imply that within arthropoda, snail acquired its role in gastrulation and mesoderm formation in the insect lineage. However, crustaceans are a diverse group with several major taxa, making analysis of more crustaceans necessary to potentially understand the ancestral role of snail in Pancrustacea (crustaceans + insects) and thus in the ancestor of insects as well. To address these questions, we examined the snail family in the Malacostracan crustacean Parhyale hawaiensis . We found three snail homologs, Ph-snail1 , Ph-snail2 and Ph-snail3 , and one scratch homolog, Ph-scratch . Parhyale snail genes are expressed after gastrulation, during germband formation and elongation. Ph-snail1 , Ph-snail2 , and Ph-snail3 are expressed in distinct patterns in the neuroectoderm. Ph-snail1 is the only Parhyale snail gene expressed in the mesoderm, where its expression cycles in the mesodermal stem cells, called mesoteloblasts. The mesoteloblasts go through a series of cycles, where each cycle is composed of a migration phase and a division phase. Ph-snail1 is expressed during the migration phase, but not during the division phase. We found that as each mesoteloblast division produces one segment’s worth of mesoderm, Ph-snail1 expression is linked to both the cell cycle and the segmental production of mesoderm.