Genetic structure in the nonrewarding, bumblebee-pollinated orchid Calypso bulbosa

Among- and within-population variation at neutral loci is governed by an interaction between stochastic processes and gene flow. A previous study of pollen dispersal in which the transfer of individually marked pollinia was monitored, indicated not only that populations of the nonrewarding, bumblebee-pollinated orchid Calypso bulbosa are connected by substantial levels of gene flow, but also that seed production may be the result of self-fertilization to a considerable extent. We examined the genetic structure of 21 C. bulbosa populations in northern Sweden by F-statistics analyses of variation at three polymorphic allozyme loci. Four populations each consisted of two or three distinct patches, which also allowed us to evaluate among-patch differentiation. The inbreeding coefficient over all loci within populations was high (F-IS=0.283). F-statistics indicated that the level of genetic differentiation among patches within populations varied among populations. F-ST among patches within populations ranged between -0.021 and 0.119 and was significantly different from zero in two of the populations. There was low to moderate genetic differentiation among populations (F-ST=0.072). A Mantel test indicated a positive correlation between geographical and genetic distances among populations, but this correlation was dependent on the difference in allele frequencies between the southernmost population sampled and all other populations. Self-fertilization and substructuring within sampling units (within patches and populations) may have contributed to the high inbreeding coefficients observed in many C. bulbosa populations. Long-distance seed and pollen dispersal may account for the low to moderate genetic differentiation among populations.