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Origin(s) of the diploid hybrid species Helianthus deserticola (Asteraceae)¹

Department of Biology, Indiana University, Jordan Hall 142, 1001 East Third Street, Bloomington, Indiana 47405 USA

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 toward 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 arose from more than one hybrid speciation event was investigated using patterns of variation from cpDNA, 18 nuclear microsatellite loci, and population interfertility. Helianthus deserticola contains cpDNA haplotypes characteristic of both parental species, is polyphyletic with one parental species based on nine microsatellite loci, and has a high degree of interfertility among populations. The data are consistent with either a single origin followed by introgression with the parental species or multiple origins. Analysis of microsatellite variation places the origin of H. deserticola between 170 000 and 63 000 years before present, making it unlikely that anthropogenic disturbances influenced its origin. Finally, the hybrid species generally has lower levels of genetic diversity but higher levels of differentiation among populations than either parental species.

Key Words: Asteraceae • Helianthus • hybrid speciation • hybridization • parallel speciation • phylogeography • sunflowers

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