Effects of population size, mating system, and evolutionary origin on genetic diversity in Spiranthes sinensis and S. hongkongensis

Hong Kong once supported more than 109 species of wild orchids, of which approximately 30% were endemic. Most of the local wild orchids have now become rare or endangered. I conducted a comparative study of genetic diversity in two closely related terrestrial orchids, an allotetraploid, Spiranthes hongkongensis, and its diploid progenitor, S. sinensis, to assess the effects of the population bottleneck associated with the origin of the polyploid and to investigate the relationships between number of breeding individuals, mating system, and level of isozyme variation in their populations. Nearly complete genetic uniformity was observed both within and among populations of S. honkongensis. In contrast, S. sinensis had high levels of genetic variation for all of the genetic parameters examined. Regression analysis of population size and several components of genetic diversity in S. sinensis revealed that, among various measures of within-population variation, the proportion of polymorphic loci (P) and average number of alleles per locus (A) or per polymorphic locus (Ap) were the most sensitive to population size (R2 = 0.942, p = 0.001; R2 = 0.932, p = 0.002; and R2 = 0.932, p = 0.002 respectively). The highly negative correlation $(r = -0.999, p < 0.01)$ between population size and the mean frequency of private alleles in pairwise population comparisons, p̄(1), indicated that population size may also be used to predict the extent of population differentiation caused by random genetic drift. Conservation of genetic diversity in S. sinensis could be maximized by protecting several of both large and small populations, whereas fewer populations may be needed to achieve this goal for S. hongkongensis.