Animal embryos specify four early cell types. Determining the underlying mechanism is one central question of developmental biology. At present, it is studied how the crustacean
Parhyale hawaiensis manages such developmental program at the eight-cell stage. Specification of germ cells, e.g., depends on the genes
vasa and
nanos, however, in a different way than in
Drosophila melanogaster , demonstrating change of gene function during evolution.
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