Effects Of Habitat Fragmentation On Spatial Genetic Structure Within Populations Of Castanopsis Sclerophylla In Qiandao Lake

Habitat fragmentation is widely recognized as one of the most dangerous human-made processes to the maintenance of biodiversity. However, no consistent conclusion has been made on whether habitat fragmentation will affect the genetic variation within populations, and our former study also failed to find out significant effect of fragmentation on the genetic diversity within populations of Castanopsis sclerophylla in medium and large island, a dominant plant species in evergreen broad-leaved forests in Qiandao Lake, southeastern China. Recently, many researches argued that fragmentation will initially impact the spatial genetic structure (SGS) within a population instead of affecting the genetic diversity directly. Based on the limited seed dispersal ability of C. sclerophylla and some results from our former study, we proposed a question that whether habitat fragmentation could influence the pattern of SGS within island population of C. sclerophylla, when the population genetic diversity was not significantly changed by this process.In this study, three sites with different extent of fragmentation were selected, among them, Heyang island (medium size) and Laoshan island (large size) was deeply and slightly affected by fragmentation respectively, and Xianshan peninsula was chosen as control site. Using 4 pairs of polymorphic SSR primers and a total of 728 C. sclerophylla sampled individuals, we analyzed the genetic diversity and SGS within pre- and post-fragmented subpopulations. The mean number of alleles, allelic richness, and observed and expected heterozygosities didn't show significant difference between pre- and post-fragmented subpopulations, indicating no obvious effect of fragmentation on genetic diversity. The most possible reason for the phenomenon is that C. sclerophylla is a wind-pollinated species with great pollen dispersal ability. This character helped to maintain the pattern of gene flow pre-and post-fragmentation.Spatial autocorrelation analysis pointed out that the strength of SGS within post-fragmented subpopulation of Heyang island (Sp=0.0082) is greater than that within pre-fragmented subpopulation (Sp=-0.0043). Moreover, the heterogeneity test also showed significant difference of SGS between these two subpopulations in the first distance class (0-22m) (p<0.05). These two evidences indicated that the negative effects of fragmentation had influenced on SGS within Heyang island population. However, no obvious differentiation could be found in SGS between pre- and post-fragmented subpopulations in other study sites. In Heyang island, the effective gene dispersal range in pre-fragmented subpopulation was farther than that in post-fragmented subpopulation by 36%, indicating that the limited gene flow was the major cause of SGS change. Simultaneously, the comparatively small population size might also contribute to the strengthened SGS.In a word, as predicted, high extent of fragmentation will rapidly impact on SGS within fragmented plant population, and this conclusion may be valuable for biological conservation and restoration.