| Habitat fragmentation has threatened the survival of species and caused worldwide loss of biodiversity. The reduction and isolation of habitats severely impact the composition and function of ecosystem, erode the genetic variation and evolution potential of species, thus jeopardize the long-term survival of species and increase their risk of extinction. Unlike the easy movement of animals, the immobility of plants makes them more easily affected by habitat fragmentation. However, plant species with different habitat requirements and life characteristics may respond differently to the effects of habitat fragmentation. Understanding the genetic effects of habitat fragmentation can provide efficient strategies for species conservation, having important theoretical and empirical meaning for the biodiversity conservation. The "sky islands" of warm-temperate forests in East Asia formed through historical habitat fragmentation and the land-bridge islands which caused by historical climate changes and human activities in southeast China provide ideal candidates for examining the genetic effects of habitat fragmentation on plant species. In this context, we choose three plant species with different life characteristics in southeast of China and southwest of Japan, to explore the genetic effects of habitat fragmentation at different landscape scales, detangle the relative effects of historical climate changes and recent human activities. The results of our study are as follow:1. Nine polymorphic microsatellite makers were used to assess the levels of genetic diversity, effective population size, contemporary and historical gene flow of six of the seven known Kirengeshoma palmate populations. We detected 164 alleles in nine loci in total. The observed heterozygosity ranged from 0.37-0.51, with an average of 0.44. The expected heterozygosity ranged from 0.54 to 0.69, the mean value is 0.60. Both contemporary and historical migration rates among populations were low, and a test of alternate models of population history strongly favored a model of long-term migration-drift equilibrium. We inferred declines in population size ca.10 000-100 000 yr ago in five of the six populations, but failed to detect recent declines. Bayesian clustering divided K. palmata populations into three genetic clusters (HMR, TMR, Japan), which was consistent with a glacial refugium hypothesis from former studies based on cpDNA and ISSR data.2. Using dual-suppression-PCR based approach, we developed 12 polymorphic microsatellite makers for Hedyotis chrysotricha. Seven of them were used to investigate the genetic diversity and spatial genetic structure of 18 H. chrysotricha populations (384 individuals) from 10 islands and 8 adjacent mainland sites in the TIL region. Results showed that there are 61 alleles across seven loci in total. The observed heterozygosity ranged from 0.286-0.621, with an average of 0.444. The expected heterozygosity ranged from 0.275 to 0.523, with an average of 0.461. Island populations (HE = 0.381) had significantly lower mean genetic diversity than those from the mainland populations (HE = 0.461) and also displayed higher mean subdivision [FST = 0.12 (island populations); FST = 0.042/0.051 (western/eastern mainland islands)]. The levels of migration rates among most populations were moderate (mc≤0.15); Bottleneck test only detected one island population (IP05) has experienced reduction of effective population size. In consequence,2MOD strongly favored a migration-drift model over a pure drift model in all the populations, but island populations have significantly higher probabilities of allelic coancestry [F = 0.184 (island populations; F = 0.085 (mainland populations)], which indicated a relatively greater influence of drift.3. Eight locus-specific nSSR markers were developed for Loropetalum chinense using dual-suppression-PCR based approach, and then we used them to compare the patterns of nuclear microsatellite variation between insular populations of L. chinense from the human activity created Thousand-Island Lake (TIL) and the Holocene-dated Zhoushan Archipelago of Southeast China. The results demonstrated that 126 alleles were detected in eight loci. The observed heterozygosity ranged from 0.53-0.72, with an average of 0.60. The expected heterozygosity ranged from 0.57 to 0.70, with an average of 0.615. Populations from the TIL region (AR = 5.163; HO = 0.563; HE = 0.597) harboured higher levels of genetic diversity than those from the Zhoushan Archipelago (mean:AR = 5.398; HO = 0.629; HE = 0.652), but these differences were not significant (mc = 0.006-0.187). There was no correlation between genetic diversity and most island features, except for a negative effect of mainland-island distance on allelic richness and expected heterozygosity in the Zhoushan Archipelago. In general, levels of gene flow among island populations were moderate to high (mc = 0.006-0.187), and tests of alternative models of population history strongly favoured a migration-drift model over a pure drift model in each region. In sum, our results showed that neither recent (artificial) nor past (natural) island formation has negtive genetic effects on Loropetalum chinense. Rather, our study highlights the importance of gene flow (most likely via seed) in maintaining genetic variation and preventing inter-population differentiation in the face of habitat’insularization’ at different temporal and spatial scales.Summarizing the genetic analysis of three plant species with different life characteristics, our results indicated the genetic effects of habitat fragmentation are far more complicated than we expected. The reduction of effective population size and genetic differentiation of Kirengeshoma palmate populations were caused by the historical climate changes rather than recent human activities. In land-bridge island systems, recent habitat fragmentation has impacted the genetic diversity of island populations, while population structures were not affected by habitat fragmentation. Our results highlight the importance of life characteristics of species, especially the gene dispersal ability, in counteracting the negative effects of habitat fragmentation on plant species. |
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