Comparative functional and evolutionary analysis of essential germline stem cell genes across the genus <i>Drosophila</i> and two outgroup species

In Drosophila melanogaster, bag of marbles (bam) encodes a protein essential for germline stem cell (GSC) daughter differentiation in early gametogenesis. Despite its essential role in D. melanogaster, direct functional evaluation of bam in other closely related Drosophila species reveal this essential function is not necessarily conserved. In Drosophila teissieri, for example, bam is not essential for GSC daughter differentiation. Here, we generated bam null alleles using CRISPR-Cas9 in a species more distantly related to D. melanogaster, Drosophila americana, to interrogate whether bam's essential GSC differentiation function is novel to the melanogaster species group or a function more ancestral to the Drosophila 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 D. melanogaster across 15 additional Drosophila and two outgroup species. We find that bam's essential GSC function is conserved between D. melanogaster and D. americana and therefore originated prior to the formation of the melanogaster species group. Additionally, we find that ∼8% of the 366 GSC genes essential in D. melanogaster are absent in at least one of the 17 species in our ortholog dataset. These results indicate that developmental systems drift, 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 Drosophila species.