RAPID HYBRID SPECIATION IN WILD SUNFLOWERS: EVIDENCE FROM GENETIC MAPPING

Department of Biology, Indiana University, Bloomington, IN 47405 USA

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. Using genetic mapping technology, we tested 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. The accuracy of these block size estimates was enhanced by the mapping of an additional 193 RAPD and 151 AFLP markers onto a pre-existing genetic linkage map of H. anomalus (701 markers in total). The acquisition of AFLP data was greatly facilitated through utilization of fluorescence technology. Analysis of the frequency spectrum of parental species chromosomal blocks with respect to predictions based on R. A. Fisher's junctions approach, suggests that H. anomalus arose rapidly, probably in fewer than 60 generations. This result is corroborated by independent lines of evidence demonstrating 1) a significant concordance between the genomes of H. anomalus and early generation H. annuus X H. petiolaris synthetic hybrids, and 2) 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.