Comparative analysis of neurogenesis in the branchiopod Daphnia magna and the malacostracan Orchestia cavimana.
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Neuroblasts – stem cell-like neural precursor cells - have long been known in insects and higher crustaceans. They have been discussed in favour of the Tetraconata hypothesis, which unites insects and crustaceans in a monophyletic group. The lack of unambiguous evidence for neuroblasts in non-malacostracan crustaceans and underrepresented molecular data regarding crustacean neurogenesis as compared to other arthropod groups led to this first comprehensive molecular and morphological study of neurogenesis in a crustacean, the branchiopod Daphnia magna. It is unambiguously shown that stem cell-like neuroblasts are present in D. magna, based on division patterns and molecular markers (ASH, asense, snail-related, prospero). As in insects and malacostracans, D. magna neuroblasts divide asymmetrically to generate smaller ganglion mother cells, which divide again to generate two small cells of equal size. In contrast to insects, but similar to malacostracans, branchiopod neuroblasts do not delaminate, rather, they remain in the neuroectoderm dividing parallel to the surface so that the ganglion mother cells come to lie interiorly. Snail, asense and prospero - so called panneural genes - are expressed in all insect neuroblasts; likewise Dam snail-related, Dam asense and Dam prospero are expressed in the neuroblasts of D. magna. However, the expression pattern of Dam ASH (achaete-scute homologue) does not show proneural clusters in D. magna, in contrast to all other euarthropod groups. Rather it is expressed in the whole neuroectoderm and up-regulated in the neuroblasts. Furthermore Dam ASH expression starts after Dam Snail-related, which leads to the suggestion that evolutionary changes have taken place, ASH having lost its proneural function, presumably reflecting the difference in the final position of neuroblasts in insects and crustaceans. Stem cell-like neuroblasts in a branchiopod crustacean provide further support for the Tetraconata-hypothesis. Unfortunately, the data cannot contribute to the question of monophyly or paraphyly of crustaceans.
