Influence of the male germline on B chromosome transmission in D. melanogaster

Selfish genetic elements that bias their inheritance to the detriment to their host are in an evolutionary arms race with host-derived suppressors that neutralize the spread of these elements. The supernumerary B chromosomes recently found in a mutant Drosophila melanogaster laboratory stock are subject to drive (>50% transmission) when passed through the female germline, but through the male germline they experience the opposite of drive, a phenomenon referred to as drag (<50% transmission). Here we show that males from this mutant stock do not have fertility defects, excluding the possibility that sperm carrying more B chromosomes are inviable. Though we revealed irregularities in the germline stem cell (GSC) niche, we show that other aspects of GSCs activity is normal: there is no delay in their mitoses and the B chromosomes appear to be symmetrically segregated during the GSC divisions, suggesting that the asymmetry of these divisions is not likely the source of B chromosome drag. Together, this work lays the foundation for future studies into the mechanism of paternal B chromosome drag in D. melanogaster, which is poised to significantly advance on our understanding of how various genetic and developmental contexts promote disparate chromosome behavior.