Disentangling the influence of mutation and migration in clonal seagrasses using the Genetic Distance Spectrum for microsatellites

The recurrent lack of isolation by distance reported at regional scale in seagrass species was recently suggested to stem from stochastic events of large-scale dispersal. We explored the usefulness of phylogenetic information contained in microsatellite loci to test this hypothesis by using the Genetic Diversity Spectrum (GDS) on databases containing, respectively, 7 and 9 microsatellites genotypes for 1541 sampling units of Posidonia oceanica and 1647 of Cymodocea nodosa. The simultaneous increase of microsatellite and geographic distances that emerges reveals a coherent pattern of isolation by distance in contrast to the chaotic pattern previously described using allele frequencies, in particular, for the long-lived P. oceanica. These results suggest that the lack of isolation by distance, rather than the resulting from rare events of large-scale dispersal, reflects at least for some species a stronger influence of mutation over migration at the scale of the distribution range. The global distribution of genetic polymorphism may, therefore, result predominantly from ancient events of step-by-step (re)colonization followed by local recruitment and clonal growth, rather than contemporary gene flow. The analysis of GDS appears useful to unravel the evolutionary forces influencing the dynamics and evolution at distinct temporal and spatial scales by accounting for phylogenetic information borne by microsatellites, under an appropriate mutation model. This finding adds nuance to the generalization of the influence of large-scale dispersal on the dynamics of seagrasses.

此前有研究指出，区域尺度上海草物种反复出现的距离隔离缺失现象，可能源于大规模扩散的随机事件。本研究借助包含微卫星位点系统发育信息的遗传多样性谱系（Genetic Diversity Spectrum, GDS）对上述假说展开验证；所用数据库分别涵盖1541份波西多尼亚海草（Posidonia oceanica）样本与1647份卵叶喜盐草（Cymodocea nodosa）样本的7个和9个微卫星基因型数据。研究结果显示，微卫星遗传距离与地理距离同步增加，这表明海草存在一致的距离隔离模式，与此前基于等位基因频率分析得到的混乱模式形成鲜明对比，该现象在长寿的波西多尼亚海草中尤为显著。上述结果表明，部分海草物种的距离隔离缺失现象，并非由罕见的大规模扩散事件导致，而是在其分布范围内，突变对遗传结构的影响强于迁移。因此，遗传多态性的全球分布格局，可能主要源于古老的逐步（再）定殖事件，后续伴随本地种群补充与克隆生长，而非当代的基因交流。在合适的突变模型下，借助微卫星携带的系统发育信息，遗传多样性谱系（GDS）分析能够阐明不同时空尺度下影响种群动态与演化的进化驱动力，该方法具备较高应用价值。本研究结果为“大规模扩散对海草种群动态具有普遍性影响”这一论断增添了更细致的认知。