Rapid hybrid speciation in wild sunflowers.

Hybrid or "recombinational" speciation refers to the origin of a new homoploid species via hybridization between chromosomally or genetically divergent parental species. Theory predicts that this mode of speciation is punctuated, but there has been little empirical evidence to support this claim. Here, we test the hypothesis of rapid hybrid speciation by estimating the sizes of parental species chromosomal blocks in Helianthus anomalus, a wild sunflower species derived via hybridization between H. annuus and H. petiolaris. Analysis of the frequency spectrum of parental species chromosomal blocks with respect to predictions based on R. A. Fisher's [Fisher, R. A. (1953) Heredity 8, 187-197] junctions approach, suggests that H. anomalus arose rapidly, probably in fewer than 60 generations. This result is corroborated by independent lines of evidence demonstrating (i) a significant concordance between the genomes of H. anomalus and early generation H. annuus x H. petiolaris synthetic hybrids, and (ii) a rapid recovery of pollen fertility in these synthetic hybrid lineages. These results are not only consistent with theory but also provide a new and general method for estimating the tempo of hybrid speciation and dating the origin of hybrid zones.