Functional divergence of the bag of marbles gene in the Drosophila melanogaster species group

The combination of recent advances in both genomic and gene editing technologies have opened up new possibilities for assessing the functional consequences and drivers of positive selection. In Drosophila melanogaster, a key germline stem cell differentiation factor, bag of marbles (bam) shows rapid bursts of amino acid fixations between its D. melanogaster and its sibling species D. simulans, but not in the outgroup species D. ananassae. We previously hypothesized that a genetic conflict with the maternally inherited, intracellular bacteria W. pipientis could be driving the adaptive evolution of bam as W. pipientis increases the fertility of a bam partial loss of function mutant. However, we have not been able to further test this hypothesis by assessing bam variation in other Drosophila lineages and their interactions with W. pipientis because bam function has not been examined in non-melanogaster Drosophila species. Since bam is rapidly evolving at the protein level, its function may not be conserved between species, and therefore different evolutionary pressures may be shaping bam in individual lineages. Here, we ask if bam is necessary for GSC daughter differentiation in five Drosophila species in the melanogaster species group that span approximately 15 million years of divergence and show different patterns of nucleotide sequence evolution at bam. We find that bam function is not fully conserved across these species, and that bam function may change on a relatively short time scale. Ultimately, we conclude that a simple gain in function as the germline stem cell differentiation factor alone does not explain our population genetic and functional genetic results we have observed. Our findings provide a foundation on which to explore the evolution of bam as a GSC differentiation factor and its interactions with W. pipientis in specific lineages.