Distribution of parental DNA markers in Encelia virginensis (Asteraceae: Heliantheae), a diploid species of putative hybrid origin

Morphological, geographical and ecological evidence suggests that Encelia virginensis is a true-breeding diploid species derived from hybrids of E. actoni and E. frutescens. To test this hypothesis, we examined the chloroplast and nuclear DNA of several Encelia species. PCR amplification targeted three separate regions of chloroplast DNA: trnK-2621/trnK-11, rbcL/ORF106, and psbA3/TrnI-51, which amplify 2600 bp, 3300 bp and 3200 bp fragments respectively. Restriction fragment analysis of chloroplast DNA revealed no variation that could be used to discriminate between the parent species. A RAPD analysis using 109 dekamer primers was used to analyze the nuclear genome. Encelia actoni and E. frutescens were distinguished by several high-frequency RAPD markers. In populations of E. virginensis, these markers were detected in varying proportions, and no unique markers were found. Evidence from the nuclear genome supports the hypothesis that E. virginensis is of hybrid origin. That E. virginensis may have arisen by normal divergent speciation followed by later introgression remains a possibility, however, and is not formally ruled out here. Diploid hybrid speciation in Encelia differs from other documented cases in that there are no discernible chromosome differences between the species, and all interspecific hybrids are fully fertile. In addition, apparent ecological selection against backcross progeny provides an external barrier to reproduction between F₁ progeny and the parental species. These characteristics suggest that hybrid speciation in Encelia may represent an alternative model for homoploid hybrid speciation involving external reproductive barriers. In particular, this may be the case for other proposed diploid hybrid taxa that also exhibit little chromosomal differentiation and have fertile F₁s.