Population genetic structure in Cheilanthes gracillima

Population genetic structure was examined in five populations of the xerically adapted homosporous fern Cheilanthes gracillima. Fstatistics using allozymic data indicated substantial genetic structure in all populations. To determine the factors responsible for genetic structure, we calculated levels of intragametophytic selfing and the fixation index for each subpopulation of each population and estimated levels of intrapopulational gene flow in each population. These analyses indicated that each subpopulation was a panmictic unit; thus, population genetic structure is not due to family structure, arising via matings between relatives. Intrapopulational gene flow was surprisingly low, given the typically high dispersibility of fern spores. However, it seems unlikely that spore dispersal in C. gracillima is significantly reduced relative to other homosporous ferns. Instead, we propose that the low rates of intrapopulational gene flow reflect limited availability of safesites for spore germination and gametophyte establishment. This ecological factor may play a primary role in generating and/or maintaining population genetic structure in C. gracillima. PLANTS ARE NOT DISTRIBUTED randomly in natural populations. Rather, populations are often structured both physically and genetically. Physical patchiness in plant populations may result from the spatial distribution of suitable habitat. Several factors may contribute to genetic structure of populations (see review by Loveless and Hamrick, 1984). The selection of different genotypes in a heterogeneous environment may produce a genetically structured population. Low levels of intrapopulational gene flow may also contribute to population genetic structure, producing patches of related, and thus genetically similar, individuals (i.e., family structure). All of these factors have been implicated in studies of plant population genetic structure. Furthermore, a correlation between population genetic structure and the mating system has also been observed. Highly inbreeding plant species typically exhibit genetically structured populations whereas populations of outcrossing species often lack genetic structure. Homosporous ferns represent a stimulating group of plants for analysis of population genetic structure. They are often dominant I Received for publication 14 July 1988; revision accepted 3 February 1989. This research was supported in part by NSF Grant BSR8620444 and a Grant-in-Aid from the Washington State University Office of Grant and Research Development. We thank Rick Noyes for technical assistance and Tom Ranker, Chris Haufler, and an anonymous reviewer for helpful comments on the manuscript. understory elements in mesic forest communities and may be characterized by large populations. The production of thousands of wind-dispersed spores by each reproductively mature sporophyte provides the potential for extensive intrapopulational gene flow and limited population genetic structure. Studies of genetic structure in two such forest-dwelling fern species, Polystichum munitum and Blechnum spicant, substantiated this prediction and indicated genetic homogeneity throughout two populations of P. munitum (Soltis and Soltis, 1987) and four of five populations of B. spicant (Soltis and Soltis, 1988). Many ferns are xerically adapted and occupy crevices in rocky cliffs and slopes, where suitable habitat may be limited. A physically patchy habitat may result in genetic structure of a population through family structure, selection in different microhabitats, or chance colonization of adjacent microsites by different genotypes. Thus, populations of xeric fern species may be more likely than mesic ferns to exhibit genetic structure. However, no genetic analyses ofpopulation structure have been conducted for xeric ferns. In this paper we analyze population genetic structure in Cheilanthes gracillima, a xeric, saxicolous fern from western North America. To explore the factors responsible for genetic structure or lack thereof in C. gracillima, we estimated rates of intragametophytic selfing, the fixation index, and levels of intrapopulational gene flow. Cheilanthes gracillima occurs on rock out-