Mechanisms of Transmission Ratio Distortion at Hybrid Sterility Loci Within and Between Species.

Hybrid incompatibilities are a common correlate of genomic divergence and a potentially important contributor to reproductive isolation. However, we do not yet have a detailed understanding of how hybrid incompatibility loci function and evolve within their native species, or why they are dysfunctional in hybrids. Here, we explore these issues for a well-studied, two-locus hybrid incompatibility between () and () in the closely related yellow monkeyflower species and By performing reciprocal backcrosses with introgression lines (ILs), we find evidence for gametic expression of the incompatibility. Surprisingly, however, hybrid transmission ratios at do not reflect this incompatibility, suggesting that additional mechanisms counteract the effects of gametic sterility. Indeed, our backcross experiment shows hybrid transmission bias toward through both pollen and ovules, an effect that is particularly strong when is homozygous for alleles. In contrast, we find little evidence for transmission bias in crosses within , providing no indication of selfish evolution at this locus. Although we do not yet have sufficient genetic resolution to determine if hybrid sterility and transmission ratio distortion (TRD) map to the same loci, our preliminary fine-mapping uncovers a genetically independent hybrid lethality system involving at least two loci linked to This fine-scale dissection of TRD at and provides insight into genomic differentiation between closely related species and reveals multiple mechanisms of hybrid dysfunction.