Low intraspecific variation for genomic isolation between hybridizing sunflower species

Barriers to gene flow between species result from selection against foreign linkage blocks in hybrids. When the geographic ranges of taxa meet at mult iple locations, the opportunity exists for variation in the genetic archite cture of isolating barriers. Hybrid zones between two sunflower species (He lianthus annuus and H. petiolaris) in Nebraska and California exhibited rem arkably similar patterns of introgression of mapped molecular markers. Cong ruence among hybrid zones may result from limited intraspecific variation a t loci contributing to isolation and from similar selective effects of alle les in the heterospecific genetic background. The observed consistency of i ntrogression patterns across distantly separated hybrid zones suggests that intrinsic forces predominate in determining hybrid zone dynamics and bound aries between these sunflower species.