Partial intersterility and evidence of allopatric speciation in Exidiopsis plumbescens (Exidiaceae)

Two partially intersterile populations of Exidiopsis plumbescens are documented by pairing tests and morphological analyses. One population, E. plumbescens var. plumbescens, extends from Vancouver, British Columbia, to northern California. The second population, E. plumbescens var. fabalispora, is known from Brackendale, Whistler, and Manning Park, British Columbia and Priest Lake, Idaho. Significant differences between the two populations were noted in basidial length/width ratio and basidiospore length; however, the most significant and consistent difference was basidiospore length/width ratio. On the basis of mean basidiospore length/width ratios, two specimens from Ontario are included in var. fabalispora. The type specimen of E. plumbescens, from Bingen, Washington, has a mean basidiospore length/width ratio within the range of var. plumbescens. No obvious differences in substrata between the populations were noted. Lower winter temperatures within the known range of var. fabalispora and variations in the amount of rainfall may be the basis for the partial reproductive isolation. The varying levels of interfertility between the two populations suggest that several polygenic loci control interfertility with each positive locus exerting an additive effect, i.e., slightly enhancing interfertility. Although some overlapping of the two populations was noted, based on mean basidiospore length/width ratios and interfertility of specimens collected near the boundary between var. plumbescens and var. fabalispora, variations in interfertility and basidiospore length/width ratios do not seem to be consistently correlated with distance from the common boundary. Our data suggest that speciation in E. plumbescens is allopatric and is a gradual process responding to differences in climatic factors.