Abstract
Selective pressures on DNA sequences often result in signatures of departures from neutral evolution that can be captured by the McDonald-Kreitman (MK) test. However, the nature of such selective forces mostly remains unknown to the experimentalists. Here we use the bag of marbles (bam) gene in Drosophila to investigate different types of driving forces behind positive selection. We examine two evolutionary models for bam. The Conflict model originates from a conflict of fitness between Drosophila and Wolbachia that causes reciprocal adaptations in each, resulting in diversifying selection on the bam protein. In the alternative Buffering model, Wolbachia protects bam from deleterious mutations during an infection and thereby allows such mutations to accumulate and even fix in the population. If Wolbachia is subsequently lost from the species, mutations that revert the gene back towards its original biological function become advantageous. We use simulations to show that both models produce signals of positive selection, though the levels of positive selection under the Conflict model are more easily detected by the MK test. By fitting the two models to the empirical divergence of D. melanogaster from an inferred ancestral sequence, we found that the Conflict model reproduced strong signals of positive selection like those observed empirically, while the Buffering model better recapitulated the physicochemical signatures of the amino acid sequence evolution at bam. Our demonstration that the Buffering model can lead to positive selection suggests a novel mechanism that needs to be considered behind observed signals of positive selection on protein coding genes.
Competing Interest Statement
The authors have declared no competing interest.
