Genetic architecture of isolation between two species of Silene with sex chromosomes and Haldane's rule

Examination of the genetic architecture of hybrid breakdown can provide insight into the genetic mechanisms of commonly observed isolating phenomena such as Haldane’s rule. We used line-cross analysis to dissect the genetic architecture of divergence between two plant species that exhibit Haldane’s rule for male sterility and rarity, Silene latifolia and Silene diclinis. We made 15 types of crosses, including reciprocal F1, F2, backcrosses, and later-generation crosses, grew the seeds to flowering, and measured the number of viable ovules, proportion of viable pollen, and sex ratio. Typically, Haldane’s rule for male rarity in XY animal hybrids is explained by interactions involving recessive X-linked alleles that are deleterious when hemizygous (dominance theory), while sterility is explained by rapid evolution of spermatogenesis genes (faster-male evolution). In contrast, we found that the genetic mechanisms underlying Haldane’s rule between the two Silene species did not follow these...