Fixation of alleles and depleted levels of genetic variation within populations of the endangered lithophytic orchid Amitostigma gracile (Orchidaceae) in South Korea: implications for conservation

摘要

To date, empirical studies on levels of genetic diversity in highly isolated populations of lithophytic orchids are few. We investigated levels of allozyme diversity and spatial distribution of genotypes in small, isolated populations of the endangered, self-compatible lithophyte Amitostigma gracile from southern Korea. At the regional level, genetic diversity was extremely low within nine populations (mean expected heterozygosity, H e = 0.005); among 20 putative loci screened, only one locus (Dia-1) was polymorphic. At this locus Dia-1 a was fixed in two populations, whereas Dia-1 b was in four populations, which as well as the scarcity of genetic variation might be indicative of random genetic drift. Similarly, at the landscape level, Dia-1 a was fixed in a population, whereas Dia-1 b with a high frequency (0.917) was detected in another population (two populations were separated by only 150 m). Analysis of fine-scale genetic structure within populations revealed significant declines in kinship with distance, suggesting that most seeds fall around maternal plants. Furthermore, a significantly high degree of population differentiation and a considerable deficit of heterozygosity were found among and within populations (F ST = 0.857; F IS = 0.940). These results suggest that genetic drift as a consequence of a small effective population size (genetic bottleneck), coupled with a limited gene flow would be major factors in harboring extremely low levels of genetic variation and in the shaping of population genetic structure of A. gracile in South Korea.