Comparative functional and evolutionary analysis of essential germline stem cell genes across the genus Drosophila and two outgroup species.

In () encodes a protein essential for germline stem cell daughter (GSC) differentiation in early gametogenesis. Despite its essential role in , direct functional evaluation of in other closely related species reveal this essential function is not necessarily conserved. In , for example, is not essential for GSC daughter differentiation. Here, we generated null alleles using CRISPR-Cas9 in a species more distantly related to , to interrogate whether 's essential GSC differentiation function is novel to the species group or a function more basal to the genus. To further characterize the extent of the functional flexibility of other GSC regulating genes, we generated a gene ortholog dataset for 366 GSC regulating genes essential in across 15 additional and two outgroup species. We find that 's essential GSC function is conserved between and and therefore originated prior to the formation of the species group. Additionally, we find that ~8% of the 366 GSC genes essential in are absent in at least one of the 17 species in our ortholog dataset. These results indicate that developmental systems drift (DSD), in which the specific genes regulating a function may change, but the final phenotype is retained, occurs in stem cell regulation and the production of gametes across species.