DISTRIBUTION OF PARENTAL DNA MARKERS IN ENCELIA-VIRGINENSIS (ASTERACEAE, HELIANTHEAE), A DIPLOID SPECIES OF PUTATIVE HYBRID ORIGIN

Morphological, geographical and ecological evidence suggests that Ence lia virginensis is a true-breeding diploid species derived from hybrid s of E. actoni and E. frutescens. To test this hypothesis, we examined the chloroplast and nuclear DNA of several Encelia species. PCR ampli fication 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 o f chloroplast DNA revealed no variation that could be used to discrimi nate between the parent species. A RAPD analysis using 109 dekamer pri mers was used to analyze the nuclear genome. Encelia actoni and E. fru tescens were distinguished by several high-frequency RAPD markers. In populations off. virginensis, these markers were detected in varying p roportions, and no unique markers were found. Evidence from the nuclea r genome supports the hypothesis that E. virginensis is of hybrid orig in. That E. virginensis may have arisen by normal divergent speciation followed by later introgression remains a possibility, however, and i s not formally ruled out here. Diploid hybrid speciation in Encelia di ffers from other documented cases in that there are no discernible chr omosome 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 betwee n F-1 progeny and the parental species. These characteristics suggest that hybrid speciation in Encelia may represent an alternative model f or homoploid hybrid speciation involving external reproductive barrier s. In particular, this may be the case for other proposed diploid hybr id taxa that also exhibit little chromosomal differentiation and have fertile F(1)s.