Allopolyploid origin and population genetics of the rare orchid Spiranthes diluvialis

The process of becoming and the attributes of being polyploid play a major role in the development and maintenance of genetic variation in allopolyploid species. A genetic survey employing protein electrophoresis on 12 populations of S. diluvialis, as well as on populations of eight congeneric species, was conducted to assess the putative allopolyploid origin of S. diluvialis and to determine the genetic variability within and among populations. Genetic identity values indicated S. diluvialis was more similar to S. magnicamporum (0.619) and S. romanzoffiana (0.727) than to any of the other congeneric species assayed. Similar to most allopolyploids, S. diluvialis showed high levels of fixed, or nearly fixed, heterozygosity and a high percentage of polymorphic loci (57.1-71.4%). The mean number of alleles per polymorphic locus in populations of S. diluvialis (2.6-3.3), however, was similar to mean values for both animal-pollinated, outcrossing, diploid species, and geographically restricted, diploid species (2.6 and 2.5, respectively). Genetic divergence among populations (mean F-st = 0.083) was low, leading to relatively high estimates of interpopulational gene flow (mean Nm = 5.41). Thus, each population harbors most of the genetic variability found within the species. The genetic variation observed within S. diluvialis supports the occurrence of at least two separate hybridization events giving rise to S. diluvialis.