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

摘要本研究旨在利用分子生态遗传学中的微卫星标记技术，研究白玉兰（Magnolia stellata）在花粉和近交抑制中的基因流动，为白玉兰的保护奠定基础。研究结果如下：1。位于光照充足的地方的大基因往往有很多花，开花时间也更早更长。大量的花增加了基因对传粉者的吸引力，从而提高了雌性繁殖的成功率，这超过了地理配的负面影响。通过无性系生长（萌发和分层）产生新分株的基因通常由几个大分株和许多小分株组成。小基因表现为群体内遗传结构，表明种子传播空间有限。小基因中过多的纯合子可能是由于遗传亚结构和/或近交，而从小基因到大基因的纯合子频率减少可能是由于遗传结构的丧失和/或近交抑制。核DNA在元居群内居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居群间的遗传分化。种子父系分析表明，当代花粉流受到亲本距离的极大限制。虽然花粉流主要发生在种群内，但来自种群外的花粉流发生的频率较低，表明种群间存在基因流联系。本研究小种群的种子生产受到花粉短缺、自交以及遗传漂变和近亲繁殖导致的遗传恶化的严格限制，加速了灭绝的风险。

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.