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

Morphological, geographical and ecological evidence suggests thatEncelia virginensis is a true-breeding diploid species derived from hybrids ofE. actoni andE. frutescens. To test this hypothesis, we examined the chloroplast and nuclear DNA of severalEncelia species. PCR amplification targeted three separate regions of chloroplast DNA:trnK-2621/trnK-11,rbcL/ORF106, andpsbA3/TrnI-51, which amplify 2600bp, 3300bp and 3200bp 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 andE. frutescens were distinguished by several high-frequency RAPD markers. In populations ofE. virginensis, these markers were detected in varying proportions, and no unique markers were found. Evidence from the nuclear genome supports the hypothesis thatE. virginensis is of hybrid origin. ThatE. 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 inEncelia 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 inEncelia 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.