Homoploid hybrid speciation has traditionally been considered a rare event, dependent on the establishment of both a novel, balanced genotype and reproductive isolating barriers between the new species and its progenitors. However, more recent studies have shown that synthetic hybrids converge to match the chromosomal structure of natural hybrids after only a few generations, suggesting that this phenomenon may be more frequent than previously assumed. Here the possibility that the diploid hybrid species Helianthus deserticola has arisen from more than one hybrid speciation event was investigated using chloroplast DNA PCR-RFLPs. The haplotypes present in eight different populations of H. deserticola from locations spanning its known geographical range were assayed based on four PCR amplified fragments of chloroplast DNA, digested with a total of seven restriction enzymes. The cpDNA haplotypes for H. deserticola were compared to the haplotypes found in seven populations of the parental species H. annuus and six populations of the parental species H. petiolaris with the same geographic distribution. A total of eleven different haplotypes were detected in the three different species; seven of the H. deserticola populations had haplotypes characteristic of H. petiolaris, while one had a haplotype characteristic of H. annuus. This variation in cpDNA haplotype may be explained by a single origin for H. deserticola followed by cytoplasmic introgression or by multiple diploid hybrid speciation events. Microsatellite markers were used to further investigate these two possibilities.

Key words: cpDNA PCR-RFLPs, Helianthus deserticola, homoploid hybrid speciation, hybridization, multiple origins