Data from: Polyploidy and microsatellite variation in the relict tree Prunus lusitanica L.: how effective are refugia in preserving genotypic diversity of clonal taxa?

Refugia are expected to preserve genetic variation of relict taxa, especially in polyploids, because high gene dosages could prevent genetic erosion in small isolated populations. However, other attributes linked to polyploidy, such as asexual reproduction, may strongly limit the levels of genetic variability in relict populations. Here, ploidy levels and patterns of genetic variation at nuclear microsatellite loci were analysed in Prunus lusitanica, a polyploid species with clonal reproduction that is considered a paradigmatic example of a Tertiary relict. Sampling in this study considered a total of 20 populations of three subspecies: mainland lusitanica (Iberian Peninsula and Morocco), and island azorica (Azores) and hixa (Canary Islands and Madeira). Flow cytometry results supported an octoploid genome for lusitanica and hixa, whereas a 16-ploid level was inferred for azorica. Fixed heterozygosity of a few allele variants at most microsatellite loci resulted in levels of allelic diversity much lower than those expected for a high-order polyploid. Islands as a whole did not contain higher levels of genetic variation (allelic or genotypic) than mainland refuges, but island populations displayed more private alleles and higher genotypic diversity in old volcanic areas. Patterns of microsatellite variation were compatible with the occurrence of clonal individuals in all but two island populations, and the incidence of clonality within populations negatively correlated with the estimated timing of colonization. Our results also suggest that gene flow has been very rare among populations, and thus population growth following founder events was apparently mediated by clonality rather than seed recruitment, especially in mainland areas. This study extends to clonal taxa the idea of oceanic islands as important refugia for biodiversity, since the conditions for generation and maintenance of clonal diversity (i.e. occasional events of sexual reproduction, mutation and/or seed immigration) appear to have been more frequent in these enclaves than in mainland areas.避难所（Refugia）被认为能够保存孑遗类群的遗传变异，在多倍体（polyploids）类群中尤为如此，因为高基因剂量可阻止小型隔离种群发生遗传侵蚀（genetic erosion）。然而，与多倍体相关的其他特征，如无性繁殖（clonal reproduction），可能会显著限制孑遗种群的遗传变异水平。本研究针对葡萄牙李（Prunus lusitanica）开展分析，该物种为兼具克隆繁殖的多倍体类群，被视为第三纪孑遗（Tertiary relict）的典型代表，研究者对其倍性水平及核微卫星位点（nuclear microsatellite loci）的遗传变异模式进行了检测。本次采样共涵盖3个亚种的20个种群：大陆分布的lusitanica亚种（伊比利亚半岛与摩洛哥）、azorica亚种（亚速尔群岛）以及hixa亚种（加那利群岛与马德拉群岛）。流式细胞术（Flow cytometry）结果显示，lusitanica亚种与hixa亚种的基因组为八倍体（octoploid），而azorica亚种则被推断为十六倍体（16-ploid）。多数微卫星位点上少量等位变异呈现固定杂合性（fixed heterozygosity），导致其等位基因多样性（allelic diversity）远低于高等多倍体的预期水平。整体而言，岛屿种群的遗传变异（包括等位基因与基因型变异）水平并未高于大陆避难所，但古老火山区域的岛屿种群拥有更多私有等位基因（private alleles），且基因型多样性（genotypic diversity）更高。微卫星变异模式显示，除两个岛屿种群外，其余所有种群均存在无性系个体（clonal individuals），且种群内无性繁殖的发生率与估测的定殖时间呈负相关。研究结果还表明，种群间的基因流（gene flow）极为罕见，因此奠基者事件（founder events）后的种群增长显然主要通过无性繁殖而非种子更新（seed recruitment）实现，这一现象在大陆种群中尤为显著。本研究将“海洋岛屿作为生物多样性重要避难所”的观点拓展至无性繁殖类群，因为相较于大陆区域，这些隔离生境中克隆多样性的产生与维持条件（即偶尔发生的有性繁殖、突变及/或种子迁入）似乎更为频繁。