| When and how cells commit to different fates is critical to an embryo. The mechanisms underlying cell fate specification fall into two categories: cell-autonomous and cell-nonautonomous specification. Historically, early specification events leading to the three germ layers were thought to be primarily cell autonomous in embryos with invariant cell lineages. However, it is now clear that both mechanisms are employed during the specification of all types of embryos.;Parhyale hawaiensis, an amphipod crustacean, displays a stereotypical cleavage pattern with an invariant cell lineage during early embryogenesis. Fate mapping of the eight-cell stage embryo demonstrates that each blastomere is fated to give rise to only one of the three germ layers or in the case of one of the blastomeres (g), the germline. However, it is not known if these cells are committed at this stage; therefore I set out to investigate how tightly early cell lineage and cell fates are coupled in this species. Using cell ablations, a classic approach for determining the developmental potential of early blastomeres, I show that much regulation occurs within the three germ layers following ablation indicating that the fate of early blastomeres may not yet be committed at the eight-cell stage. Furthermore, these findings demonstrate the presence of mesoderm and ectoderm equivalence groups in Parhyale.;In addition, using molecular approaches, I cloned the Parhyale orthologs of genes with known roles in cell fate specification or germline development. Characterization of vasa suggests a conserved role for this gene in germ cell development, while the novel localization of beta-catenin in what may be analogous to the germ plasm, which becomes restricted specifically to the germline, indicates a possible new role for beta-catenin. In Parhyale, the germ cells are specified early in development by preformation (segregation of germline determinants). Ablation of the g blastomere, which exclusively gives rise to the germ cells, or its progeny, later in development, results in a loss of embryonic germ cells. However, these g-ablated animals are fertile and produce normal offspring demonstrating that replacement germ cells must be induced in post-embryonic stages. This is the first report of an animal that preferentially specifies its germline early in development, yet if forced, can also undergo germ cell induction suggesting that both cell-autonomous and nonautonomous germline specification mechanisms exist in Parhyale . |
