RAPID HYBRID SPECIATION IN WILD SUNFLOWERS

Hybrid or ''recombinational'' speciation refers to the origin of a new homoploid species via hybridization between chromosomally or genetica lly divergent parental species, Theory predicts that this mode of spec iation is punctuated, but there has been little empirical evidence to support this claim. Here, we test the hypothesis of rapid hybrid speci ation by estimating the sizes of parental species chromosomal blocks i n Helianthus anomalus, a wild sunflower species derived via hybridizat ion between H, annuus and H, petiolaris, Analysis of the frequency spe ctrum of parental species chromosomal blocks with respect to predictio ns 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 independe nt lines of evidence demonstrating (i) a significant concordance betwe en the genomes of H. anomalus and early generation H. annuus x H. peti olaris synthetic hybrids, and (ii) a rapid recovery of pollen fertilit y in these synthetic hybrid lineages. These results are not only consi stent with theory but also provide a new and general method for estima ting the tempo of hybrid speciation and dating the origin of hybrid zo nes.