Cytotype segregation on regional and microgeographic scales in snow buttercups (Ranunculus adoneus: Ranunculaceae)

Polyploid speciation is an important source of angiosperm diversity. Insights into the origin and establishment of new polyploid species may be gained by studying the distributions of ancestral and derivative cytotypes at multiple spatial scales. Diploid (2n = 16) and tetraploid (2n = 32) snow buttercups (Ranunculus adoneus: Ranunculaceae) occur in the alpine of the central and southern Rocky Mountains. Root-tip squashes and flow cytometry were used to determine the ploidy of 1618 individuals from 35 populations. Samples from 31 of the 35 sites were entirely of one cytotype, either diploid or tetraploid. Diploid and tetraploid snow buttercups have nonoverlapping regional distributions. Where both cytotypes occur on the same site, the two are spatially segregated despite no apparent change in habitat. Triploid snow buttercups were only found at a diploid/tetraploid contact zone, while two hexaploid plants were found in tetraploid populations. Tetraploid establishment once or twice in the history of the species complex could account for the regional distribution of the two cytotypes. Habitat differentiation between cytotypes or reproductive exclusion of minority cytotypes may explain the observed segregation at both microgeographic and regional scales.