Ecological distributions, phenological isolation, and genetic structure in sympatric and parapatric populations of the Larrea tridentata polyploid complex

Abstract

PREMISE OF THE STUDY: Polyploidy is widely recognized as a mechanism of diversifi cation. Contributions of polyploidy to specific pre- and postzygotic barriers—and classifi cations of polyploid speciation as “ecological” vs. “non-ecological”—are more contentious. Evaluation of these issues requires comprehensive studies that test ecological characteristics of cytotypes as well as the coincidence of genetic structure with cytotype distributions. M ETHODS: We investigated a classical example of autopolyploid speciation, L arrea tridentata, at multiple areas of cytotype co-occurrence. Habitat and phenological diff erences were compared between diploid, tetraploid, and hexaploid populations on the basis of edaphic, community composition, and fl owering time surveys. Frequency of hybridization between diploids and tetraploids was investigated using a diploid-specifi c chloroplast DNA (cpDNA) marker; genetic structure for all cytotypes was assessed using amplifi ed fragment length polymorphisms (AFLPs). K EY RESULTS: Across contact zones, we found cytotypes in habitats distinguished by soil and vegetation. We observed modest diff erences in timing and production of flowers, indicating a degree of assortative mating that was asymmetric between cytotypes. Nonetheless, cpDNA analyses in diploid–tetraploid contact zones suggested that ~5% of tetraploid plants had hybrid origins involving unilateral sexual polyploidization. Genetic structure of AFLPs largely coincided with cytotype distributions in diploid–tetraploid contact zones. In contrast, there was little structure in areas of contact between tetraploids and hexaploids, suggesting intercytotype gene fl ow or recurrent hexaploid formation. CONCLUSIONS: Diploid, tetraploid, and hexaploid cytotypes of L. tridentata are segregated by environmental distributions and fl owering phenology in contact zones, with diploid and tetraploid populations having corresponding differences in genetic structure.