The three related genera Encelia, Enceliopsis, and Geraea comprise the alliance. The first consists primarily of shrubs and the latter two of herbaceous perennials and an annual. With the exception of two Encelia species of arid South America, all inhabit southwestern North America. Enceliopsis and Geraea are sister groups, and together form the sister group to Encelia, which includes two major clades. Especially in Encelia, there are diverse morphologies and a variety of ecological strategies marked by differences in habitat, vestiture, water balance, and photosynthetic parameters. The North American species of all three genera are obligate outcrossers, all with n = 18 chromosomes. Although intergeneric hybrids are largely sterile, interspecific hybrids in Encelia are fertile in the wild and in cultivation. Hybrids in the wild are largely restricted to F ,s, except in areas of human disturbance. Two true-breeding species are of homoploid hybrid origin, and are evidently isolated from the parent species through external ecological barriers involving selection against backcross progeny. Studies of the chloroplast genome and the intercistronic transcribed spacer (ITS) of nrDNA show clear differentiation of the genera, but much less variation within Encelia, even between phenotypically disparate species, suggesting recent divergence. Because the species are interfertile, it will be possible to study the genetics of the traits that distinguish the species and contribute to their differences.
Morphological, geographical and ecological evidence suggests that EnceIia virginensis is a true-breeding diploid species derived from hybrids ofE. 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/ORFl06, and psbA3/TrnI-51, which amplify 2600bp, 3300bp 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 F 1 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 Fls.
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