Comparative Analysis ofDrosophilaBam and Bgcn Sequences and Predicted Protein Structural Evolution

The protein encoded by theDrosophila melanogastergenebag of marbles(bam) plays an essential role in early gametogenesis by complexing with the gene product ofbenign gonial cell neoplasm(bgcn) to promote germline stem cell daughter differentiation in males and females. Here, we compared the AlphaFold2 and AlphaFold Multimer predicted structures of Bam protein and the Bam:Bgcn protein complex betweenD. melanogaster, D. simulans, and D. yakuba, wherebamis necessary in gametogenesis to that inD. teissieri, where it is not. Despite significant sequence divergence, we find very little evidence of significant structural differences in high confidence regions of the structures across the four species. This suggests that Bam structure is unlikely to be a direct cause of its functional differences between species and that Bam may simply not be integrated in an essential manner for GSC differentiation inD. teissieri. Patterns of positive selection and significant amino acid diversification across species is consistent with the Selection, Pleiotropy, and Compensation (SPC) model, where detected selection atbamis consistent with adaptive change in one major trait followed by positively selected compensatory changes for pleiotropic effects (in this case perhaps preserving structure). In the case ofbam, we suggest that the major trait could be genetic interaction with the endosymbiotic bacteriaWolbachia pipientis. Following up on detected signals of positive selection and comparative structural analysis could provide insight into the distribution of a primary adaptive change versus compensatory changes following a primary change.