Hybrid zones and the genetic architecture of a barrier to gene flow between two sunflower species

Genetic analyses of reproductive barriers represent one of the few methods by which theories of speciation can be tested. However, genetic study is of ten restricted to model organisms that have short generation times and are easily propagated in the laboratory. Replicate hybrid zones with a diversit y of recombinant genotypes of varying age offer increased resolution for ge netic mapping experiments and expand the pool of organisms amenable to gene tic study. Using 88 markers distributed across 17 chromosomes, we analyze t he introgression of chromosomal segments of Helianthus petiolaris into H. a nnuus in three natural hybrid zones. Introgression was significantly reduce d relative to neutral expectations for 26 chromosomal segments, suggesting that each segment contains one or more factors that contribute to isolation . Pollen sterility is significantly associated with 16 of these 26 segments , providing a straightforward explanation of why this subset of blocks is d isadvantageous in hybrids. In addition, comparison of rates of introgressio n across colinear vs. rearranged chromosomes indicates that close to 50% of the barrier to introgression is due to chromosomal rearrangements. These r esults demonstrate the utility of hybrid zones for identifying factors cont ributing to isolation and verify the prediction of increased resolution rel ative to controlled crosses.