Genetic Conflict and the Evolution of Sporophyte-Specific Gene Expression in Ceratodon purpureus

Population genetic theory is built on a foundation of Mendelian segregation. The success of this theory tells us that, in general, each allele at a heterozygous locus has an equal chance of being transmitted from one generation to the next. Striking exceptions to this generality are well known, in which nearly all of the offspring inherit a distorter allele, but these cases are seldom expected to be polymorphic. In contrast to this expectation, a few recent studies report subtle but persistent deviations from 1:1 meiotic segregation ratios in natural populations, suggesting that our knowledge of the distribution of segregation distortion may be biased toward strong drivers with major fitness effects. Here we generated meiotic sex ratio estimates for two populations of the dioecious moss Ceratodon purpureus. Using a general hierarchical likelihood model we found an overall male-biased sex ratio and significant genetic variation for offspring sex ratio in both populations. Furthermore, we found a positive association between a male-biased sex ratio and total spore production, pointing to a previously unexplored genetic correlation between meiotic sex ratio and resource allocation between maternal gametophytes and their nutritionally dependent offspring sporophytes. We are now using RADseq and comparative transcriptomics to evaluate the importance of weak distorters in the maintenance of polymorphism for fitness, the genomic architecture of recombination, and the evolution of reproductive isolation.