Amer. J. Bot. 76(8): 1114-1118. 1989. POPULATION GENETIC STRUCTURE IN CHEILANTHES GRACILLIMA1 PAMELA S. SOLTIs, DOUGLAS E. SOLTIs, AND BRYAN D. NESS Department of Botany, Washington State University, Pullman,Washington 99164 ABSTRACT Population geneticstructure was examined in five populations of the xerically adapted homo- sporous fernCheilanthes gracillima. Fstatisticsusing allozymic data indicated substantial genetic structure in all populations. To determine the factors responsible for genetic structure, we calculated levels of intragametophytic selfingand the fixationindex for each subpopulation of each populationand estimatedlevels 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 betweenrelatives. Intrapopulational gene flow was surprisingly low, given the typicallyhigh dispersibility of fern spores.However, it seems unlikelythat spore dispersal in C. gracillimais significantly reducedrelativeto other homosporous ferns.Instead, we proposethat the low ratesof intrapopulational gene flow reflect limited availability of safesites for spore germinationand gametophyteestablishment.This ecologicalfactormay play a primary role in generating and/or maintaining population genetic structure in C. gracillima. PLANTS ARE NOT DISTRIBUTED randomly in nat- ural populations.Rather, populations areoften structured both physically and genetically. Physical patchiness in plant populations may result from the spatial distributionof suitable habitat. Several factors may contributeto ge- netic structureof populations (see review by Loveless and Hamrick, 1984). The selection of different genotypes in a heterogeneous envi- ronment may producea geneticallystructured population. Low levels of intrapopulational gene flow may also contribute to population genetic structure, producing patchesof related, and thus genetically similar, individuals (i.e., family structure). All of these factors have been implicated in studies of plant population ge- netic structure. Furthermore, a correlation be- tween population genetic structure and the mating system has also been observed. Highly inbreeding plant species typically exhibit ge- netically structured populations whereas pop- ulations of outcrossing species often lack ge- netic structure. Homosporous ferns represent a stimulating group of plants for analysis of population ge- netic structure. They are often dominant I Received for publication 14 July 1988; revision ac- cepted 3 February 1989. This research was supported in part by NSF Grant BSR- 8620444 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. understoryelements in mesic forest commu- nities and may be characterized by largepop- ulations. The production of thousands of wind-dispersedspores by each reproductively mature sporophyteprovides the potential for extensive intrapopulational gene flow and lim- ited population genetic structure. Studies of genetic structurein two such forest-dwelling fern species,Polystichum munitumand Blech- num spicant,substantiated this predictionand indicatedgenetichomogeneitythroughout two populations of P. munitum (Soltis and Soltis, 1987) and four of five populationsof B. spicant (Soltis and Soltis, 1988). Manyfernsarexerically adapted and occupy crevices in rocky cliffs and slopes, where suit- ablehabitat maybe limited.A physically patchy habitatmay resultin geneticstructure of a pop- ulation through family structure,selection in different microhabitats, or chancecolonization of adjacentmicrosites by differentgenotypes. Thus, populationsof xeric fern species may be more likely than mesic ferns to exhibit genetic structure. However,no geneticanalyses ofpop- ulation structure have been conducted for xeric ferns. In this paperwe analyze population ge- netic structure in Cheilanthes gracillima,a xe- ric, saxicolousfern from westernNorth Amer- ica. To explore the factors responsible for genetic structure or lack thereof in C. gracil- lima, we estimated rates of intragametophytic selfing,the fixation index, and levels of intra- populationalgene flow. Cheilanthes gracillima occurs on rock out- 1114 This content downloaded from 128.32.146.106 on Thu, 17 Oct 2013 14:47:48 PM All use subject to JSTOR Terms and Conditions