Divergence of germ cell-less roles in germ line development across insect species

During development, sexually reproducing animals must specify and maintain the germ line, the lineage of cells that gives rise to the next generation of animals. In the fruit fly Drosophila melanogaster, germ cell-less (gcl) is required for the formation of primordial germ cells in the form of cells that cellularize at the posterior pole of the embryo, called pole cells. Forming pole cells is a mechanism of germ cell formation unique to a subset of insects. Even though most animals do not form pole cells as primordial germ cells, gcl is conserved across Metazoa, raising the question of how this conserved gene acquired its central role in the evolutionarily derived process of pole cell formation. Here, we examine the functions of gcl in two other insects with different modes of germ cell specification: the milkweed bug Oncopeltus fasciatus and the cricket Gryllus bimaculatus. We found that gcl is involved in germ cell development, but not strictly required for germ cell specification, in O. fasciatus, although it appears to function through a mechanism different from that in D. melanogaster. In contrast, we could not detect any impact on the embryonic germ line upon gcl knockdown in G. bimaculatus. This work serves as a case study into how the roles of genes in the process of germ line development can change over evolutionary time across animals.