In D. melanogaster females, bag of marbles (bam) acts as the master switch for germline stem cell differentiation during oogenesis and in males, plays a key role in regulating spermatogenesis. Interestingly, bam is rapidly evolving across the Drosophila genus with a strong burst of nonsynonymous changes in both the D. melanogaster and D. simulans lineages. We have previously shown that the major functional change associated with interspecific differences in the Bam protein is in oogenesis, and not spermatogenesis. We have also discovered a genetic interaction between Wolbachia and a bam hypomorphic mutant in D. melanogaster, where Wolbachia infection rescues the bam fertility defect. However, the nature of this rescue remains unclear. Our preliminary data shows that Wolbachia does not rescue the tumorous ovary phenotype characteristic of the bam hypomorph, implying the rescue is indirect. These results have led us to hypothesize that Wolbachia may be a driver of positive selection at bam in D. melanogaster and D. simulans. However, since bam shows high levels of amino acid diversification across the Drosophila genus, it is also possible that bam’s function as the master switch in oogenesis has arisen in the lineage leading to D. melanogaster and D. simulans and the refinement of this function has thus been under positive selection in these species. We are currently characterizing the nature of the interaction between Wolbachia and bam as well as defining bam function in D. simulans using CRISPR-Cas9 editing to distinguish between these hypotheses.