Abstract
Levels and distribution of genetic variation were analyzed in 15 populations of the homosporous fern Gymnocarpium dryopteris ssp. disjunctum to evaluate the mating system and the genetic structure of this taxon. Estimated rates of intragametophytic selling were 0.000 in all populations, signifying that all sporophytes examined arose via intergametophytic matings. Furthermore, values of F, the fixation index, ranged from –0.171 to 0.038, indicating that all populations approach Hardy‐Weinberg equilibrium. Therefore, not only is intragametophytic selling rare in G. dryopteris ssp. disjunctum, but intergametophytic selling is also uncommon. Factors promoting outcrossing in G. dryopteris ssp. disjunctum include an antheridiogen system, an ontogenetic sequence that favors intergametophytic matings, and high levels of genetic load. Analysis of F‐statistics indicated statistically significant genetic differentiation among populations despite a mean genetic identity of 0.973 and high levels of interpopulational gene flow (Nm = 3.41 to 4.09). These patterns may reflect levels of gene flow that prevent significant interpopulational divergence while permitting slight genetic differentiation among populations.
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