Phylogeny and Adaptation in the Encelia Alliance (Asteraceae: Helliantheae)

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 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.

Natural selection maintains species despite widespread hybridization in the desert shrubEncelia

“…Still, no evidence of intrinsic barriers to reproduction has been found in experimental crosses between different taxon pairs in the genus (16,17) . Additionally, hybrid zones are found between eight unique species pairs (18) , yet these zones are almost always narrowly limited to ecotones or zones of disturbance, a pattern known to be indicative of extrinsic or ecological control over hybrid zone structure (1,19) . Moreover, the same pollinators are frequently observed moving between sympatric and parapatric species (16,17) .…”

Diversification, disparification, and hybridization in the desert shrubsEncelia

“…Most species of Encelia are distributed in the arid lands of southwestern North America, the dry lands of Chile, Peru, and Argentina, and the (arid) Galapagos Islands (Clark, 1998) . These plants inhabit various types of deserts, including inland deserts, coastal dunes, as well as high and low deserts; E. actoni even passes the frost line in the Sierra Nevada mountain range of California.…”

Potential Host Plants ofTrirhabda geminata(Coleoptera: Chrysomelidae): Impacts on Survival, Development, and Feeding

“…In the area of Riverside, CA, at least three Encelia species co-occur, including E. actoni, E. californica, and E. farinosa, with E. farinosa being the most abundant. The evolutionary trajectories of the species are thought to have diverged relatively recently (Fehlberg and Ranker 2007), and hybrids of all combinations are known to be fertile in an artiÞcial setting (Clark 1998), although the status of each as a distinct species is not in question (Clark 1998, Fehlberg andRanker 2007). Additionally, all three species are known to have similar secondary chemistries, with defenses including chromenes and sesquiterpene lactones (Geissman and Mukherjee 1967;Geissman 1969, 1971;Wisdom and Rodriguez 1982;Proksch and Rodriguez 1984;Wisdom 1985;Srivastava et al 1990;Kunze et al 1995).…”

“…Both larval and adult beetles feed on plant foliage, and there is likely only one generation per year (Paine et al 1993). Although T. geminata (Coleoptera, Chrysomelidae) is frequently referred to as a specialist on Encelia farinosa (Hogue 1970;OÕBrien 1980;Wisdom 1982Wisdom , 1985Redak et al 1995), there have also been reports of T. geminata feeding on E. californica (OÕBrien 1980) and on E. virginensis (Hogue 1970), a species likely originated from hybridization of E. actoni ϫ E. frutescens (Allen et al 1997, Clark 1998.…”

“…The species T. geminata Horn occurs in the southwestern United States from Arizona west into southern California and south into Baja California, Mexico, as well as northeastern mainland Mexico. The range of the beetles overlaps with, but is somewhat smaller than, the range of the beetleÕs reported host plant, Encelia farinosa (Compositae, Asteraceae) (Wisdom 1982), and includes the known ranges of a number of other Encelia species (Clark 1998).…”