How far does cell lineage influence cell fate specification in crustacean embryos?
In crustaceans, invariant cell lineages have been shown to occur (i) in early cleavages of several taxa and (ii) in the course of formation and differentiation of the post-naupliar germ bands in malacostracans. Work on early cleavages is still in its infancy. In contrast, the generation and proliferation of mesoteloblasts and ectoteloblasts and the subsequent proliferation and differentiation of bandlet cells have been studied in members of several subgroups of Malacostraca. Similarities and differences have been determined in order to interpret the interdependencies of the steps in the differentiation process. Some of these steps are highly conserved, as in the case of the generation of four pairs of mesoteloblasts, others are prone to phylogenetic change, as in the case of the primary ring of 19 ectoteloblasts which has been altered at least twice in evolution. A stereotyped cleavage pattern in the germ band has been shown to be independent of the origin of the precursor cells. The question whether neuroblasts in crustaceans and insects are homologous or are the result of convergent evolution is still open. However, the homology of early differentiating neurons in crustaceans and insects seems to be well established. In addition, similarities in the expression patterns of the engrailed gene are likely to be homologous and point to a close relationship between these two groups.
Freie Universität Berlin, Institut für Zoologie, Germany.
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Dohle
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Scholtz