Spatial scales of genetic structure and gene flow in Calochortus albus (Liliaceae)

Abstract

Calochortus (Liliaceae) displays high species richness, restriction of many individual taxa to narrow ranges, geographic coherence of individual clades, and parallel adaptive radiations in different regions. Here we test the first part of a hypothesis that all of these patterns may reflect gene flow at small geographic scales. We use amplified fragment length polymorphism variation to quantify the geographic scales of spatial genetic structure and apparent gene flow in Calochortus albus, a widespread member of the genus, at Henry Coe State Park in the Coast Ranges south of San Francisco Bay. Analyses of 254 mapped individuals spaced 0.001-14.4 km apart show a highly significant decline in genetic identity with ln distance, implying a root-mean-square distance of gene flow σ of 5-43 m. STRUCTURE analysis implies the existence of 2-4 clusters over the study area, with frequent reversals among clusters over short distances (<200 m) and a relatively high frequency of admixture within individuals at most sampling sites. While the intensity of spatial genetic structure in C. albus is weak, as measured by the Sp statistic, that appears to reflect low genetic identity of adjacent plants, which might reflect repeated colonizations at small spatial scales or density-dependent mortality of individual genotypes by natural enemies. Small spatial scales of gene flow and spatial genetic structure should permit, under a variety of conditions, genetic differentiation within species at such scales, setting the stage ultimately for speciation and adaptive radiation as such scales as well.

Keywords: Endemism; parallel adaptive radiations; spatial genetic structure; species richness.

Figure 1

Calochortus albus in bloom at Henry Coe State Park (photo: Jillian Henss).

Figure 2

Map of the distribution of sample sites at Henry Coe State Park. There is an extensive, two-dimensional area of oak woodlands on north-facing slopes inhabited by Calochortus albus surrounding the essentially one-dimensional array of 16 sample sites in the northern portion of the park.

Figure 3

Mean ± SD kinship coefficient plotted against the logarithm of geographic distance for each distance class. (A) All 20 sites; (B) the northern 16 sites only).

Figure 4

Plot of Pr (X|K) against K, based on data for (A) for all 20 sites, and (C) the 16 northern sites. Error bars represent standard deviations after the burn-in period. The plateau of these curves suggest a K between 2 and 5. ΔK versus K, implying a K = 2 for (B) all 20 sites and (D) the 16 northern sites.

Figure 5

(A) STRUCTURE plots of individual membership in two, three, four, or five color-coded clusters for all 20 sites. Black vertical lines separate sets of individuals at different sites, which are ordered from left to right in their geographic sequence from west to east at Henry Coe State Park. Admixture of indivuals is indicated by the presence of two or more cluster colors within individual vertical bands. (B) Plots of individual members in two, three, four, or five clusters for the northern 16 sites.