The allopolyploid origin and population genetics of the rare Solidago houghtonii (Asteraceae).

摘要

Solidago houghtonii is a federally threatened hexaploid plant species of likely hybrid origin, limited geographic distribution, and high habitat specificity. An understanding of the evolutionary history and population genetic structure of rare species is critical to ensuring their long-term survival. To study the hybrid origin of S. houghtonii I sequenced four noncoding chloroplast (cp) DNA loci, and the nuclear ribosomal DNA ITS and 3' ETS regions from four individuals of S. houghtonii, which span its geographic range, and 25 other species of Solidago including all sympatric species. Polymorphisms within the nrDNA sequences indicated the presence of multiple homeologue types which were separated by molecular cloning Phylogenetic analyses of cloned sequence data indicated that S. riddellii, S. ptarmicoides, and S. ohioensis are parents of S. houghtonii, however cpDNA sequence data indicated a fourth species, S. gigantea, as the maternal genome donor. These data reveal both a single origin and a complex pattern of reticulation that is consistent not only with the hypothesized allohexaploid nature of this species, but also with chloroplast capture of cpDNA from an unexpected source through introgression.;The amount of genetic diversity and the degree of population subdivision was analyzed in 23 S. houghtonii populations spanning the geographical range and habitat specificities of the species. 452 individual plants were characterized for cpDNA haplotype by screening amplicons of three noncoding cpDNA regions for length polymorphism. A total of 14 haplotypes were detected. Gene diversity (HE) across populations ranged from 0.00 -- 0.59 and a high degree of population structure was found (FST = 0.76). An AMOVA partitioned most of the variation among populations (76%), and the remainder among individuals within populations (24%). Similar structure was found when the populations were grouped according to either geographic region or substrate type. A Mantel test found significant isolation-bydistance between all populations except the most disjunct population (P = 0.003). A statistical parsimony network analysis of the 14 S. houghtonii haplotypes indicated a northern Michigan origin for the species. The genetic structure of S. houghtonii populations is consistent with postglacial range expansion and contraction during the hypsithermal period ∼6000YBP.