Studies on gene flow and inbreeding depression in Magnolia stellata for its conservation

本研究的目的是阐明通过花粉和近交抑制的基因流动，以利用分子生态遗传学中的微卫星标记方法，为保护受威胁物种星玉兰建立基线。研究结果如下：1.体型较大且位于光照较好部位的基因组往往开花较多，开花时间较早，开花时间较长。大量的花朵增加了基因对传粉者的吸引力，进而增加了雌性生殖的成功，这超过了地理配子法的负面影响。基因显然是通过克隆生长(萌发和分层)产生新的分株，通常由几个大的分株和许多小的分株组成。小基因组表现出种群内的遗传结构，表明种子的传播在空间上是有限的。小基因组中纯合子的过剩可能是由于遗传亚结构和/或近交，而纯合子频率从小到大的下降可能是由于遗传结构的丧失和/或近交衰退。在核DNA的集合种群中，种群间的遗传分化水平不到叶绿体DNA的一半，这表明花粉流动和种子传播显著降低了种群间的遗传分化。亲子关系分析表明，亲本间距离对当代花粉流动有很大的制约作用。虽然大多数花粉流发生在研究种群内部，但来自种群外的花粉流频率很低，表明种群之间存在基因流联系。在研究的小种群中，种子生产受到花粉短缺、自交以及遗传漂移和近亲交配导致的遗传退化的严格限制，这会加速灭绝的风险。

The objective of this research project was to elucidate gene flow via pollen and inbreeding depression to establish a baseline for conservation of a threatened species, Magnolia stellata, using the methods with microsatellite markers in molecular ecological genetics. The findings are as follows :1. Genets that are large and located in well-lit sites tend to have many flowers, and blossom both earlier and longer. Large numbers of flowers increase the attractiveness of genets for pollinators and then female reproductive success, and this outweighs the negative effects of geitonogamy.2. Genets, which apparently produced new ramets through clonal growth (sprouting and layering), were generally composed of several large ramets and many small ramets. Small genets showed within-population genetic structure, suggesting spatially limited seed dispersal. The excess homozygotes in the small genets may be due to genetic substructuring and/or inbreeding, and the reduction in homozygote frequency from the small to large genets may be due to loss of genetic structure and/or inbreeding depression.3. The level of genetic differentiation among populations within a metapopulation for nuclear DNA was less than half that for chloroplast DNA, suggesting that pollen flow, as well as seed dispersal, has significantly reduced genetic differentiation among populations. Paternity analysis of seeds showed that contemporary pollen flow was greatly restricted by the distance between parents. Although most pollen flow occurred within the study population, pollen flow from outside the population occurred with a low frequency, indicating that the populations were linked by gene flow.4. Seed production in the study small population is strictly limited by elevated pollen shortage, selfing, and genetic deterioration through genetic drift and inbreeding, which accelerate the risk of extinction.