GENETIC VARIATION AND POPULATION STRUCTURE IN THE FERN BLECHNUM SPICANT (BLECHNACEAE) FROM WESTERN NORTH AMERICA

Population genetic structure in the homosporous fern Blechnum spicant was analyzed in six populations from western North America. Each population was divided into approximately 10 m by 10 m subpopulations, and genetic variation within and among subpopulations was compared using enzyme electrophoresis and F statistics. These analyses indicated that there was no evidence of genetic structure in four of the six populations examined. However, significant genetic heterogeneity among subpopulations was observed for the other two populations. The genetic structure of these populations may be attributable, in part, to family structure resulting from high rates of intragametophytic selfing and/or spatial patchiness in the distribution of individuals due to limited habitat availability in these areas. Outcrossing populations of B. spicant generally lack genetic structure, whereas the most highly inbreeding population maintains significant genetic structure. The information obtained in this investigation of population genetic structure in Blechnum spicant is consistent with data for angiosperms and gymnosperms. It appears that the outcrossing mating system and effective mechanism of spore dispersal in B. spicant may account for the general lack of genetic structure within populations of this species. STUDIES OF POPULATION STRUCTURE in angiosperms and gymnosperms indicate that in some species genetic variation is randomly distributed within populations, whereas in others there is a discrete pattern of intrapopulational genetic variation (Loveless and Hamrick, 1984). Recent investigations of genetic variation in homosporous ferns have documented substantial levels of genetic variability within populations of several fern species, including Bommeria hispida (Haufler and Soltis, 1984; Haufler, 1985), Pellaea andromedifolia (Gastony and Gottlieb, 1985), Polystichum munitum (P. Soltis and D. Soltis, 1987), and P. imbricans (Soltis and Soltis, unpublished data). However, the manner in which the observed genetic variation is spatially distributed within populations of homosporous ferns is, in most cases, unknown. An analysis of population genetic structure in Polystichum munitum indicated that there was no spatial genetic heterogeneity within populations of this species (P. Soltis and D. Soltis, 1987). Additional studies are required to elucidate the patterns of intrapopulational genetic variation in other species of homosporous ferns. In this study we examined the population genetic structure of Blechnum spicant (Blechnaceae). Blechnum spicant occupies moist to wet habitats, generally in heavy shade, from I Received for publication 1 December 1986; revision accepted 10 April 1987. near sea level to midmontane elevations. This species is interruptedly circumboreal and in western North America occurs from Alaska southward to California, primarily west of the Cascades; a single disjunct population of B. spicant is known from northern Idaho. Blechnum spicant is a long-lived perennial. Reproduction is primarily sexual; the fronds are strongly dimorphic, the fertile leaves producing thousands of wind-dispersed spores which are released in midsummer. Vegetative reproduction in B. spicant is limited; the rhizomes are short-creeping and do not form extensive clones (Hitchcock and Cronquist, 1973). MATERIALS AND METHODS-Plant materialPopulation genetic structure in Blechnum spicant was analyzed by assessing levels and patterns of electrophoretically detectable genetic variation within and among artificially delimited population subdivisions. Six populations of B. spicant were investigated in this manner; collection data and sample sizes for each population are given in Table 1. The Russian Gulch population occupies a steep, north-facing slope and comprises several hundred individuals. The Mill Creek population inhabits a roadside ditch and consists of several hundred individuals. The habitat and population size of the South Fork Smith population are similar to those of Mill Creek. The Twanoh population occurs along a stream at the base of a steep ravine and comprises several hundred indi-