Abstract
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 F1s, 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.
Highlights
(1995) provided preliminary phylogenetic data from the internal transcribed spacer of nuclear ribosomal DNA (ITS). These phylogenetic studies have seemed a "work in progress," since full character support for a specific tree has never been published, they agree in a number of major features. They strongly support the monophyly of Encelia, Enceliopsis sensu stricto, and a sister-group relationship between Enceliopsis and Geraea
Species of hybrid origin may share features of the parent species that otherwise would be autapomorphies of those species, producing a pattern of shared characters that could not be explained by divergent evolution. These criteria can never prove that a species is of hybrid origin, but they can provide a weight of evidence
The parent species differ by 9 bases; at each site, every sampled E. virginensis has either the E. actoni base, the E. jrutescens base, or a polymorphism consisting of both bases
Summary
These phylogenetic studies have seemed a "work in progress," since full character support for a specific tree has never been published, they agree in a number of major features They strongly support the monophyly of Encelia (diagnosed by a constricted apical notch of the achene and the general absence of achene awns; the clade has occurred in every tree in every analysis, with 100% bootstrap and jackknife in combined morphology and ITS [Clark 1995]), Enceliopsis sensu stricto (scapose habit and large capitula [Sanders and Clark 1987]; again occurring in all trees), and a sister-group relationship between Enceliopsis and Geraea (thickened unpigmented crown of the achene [Sanders and Clark 1987]; occurring in all trees). Is it sterile? How do the VOLUME 17, NUMBER 2
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