Data from: Parallel speciation or long-distance dispersal? Lessons from seaweeds (Fucus) in the Baltic Sea

Parallel evolution has been invoked as a forceful mechanism of ecotype and species formation in many animal taxa. However, parallelism may be difficult to separate from recently monophyletically diverged species that are likely to show complex genetic relationships as a result of considerable shared ancestral variation and secondary hybridization in local areas. Thus, species’ degrees of reproductive isolation, barriers to dispersal and in particular, limited capacities for long-distance dispersal, will affect demographic structures underlying mechanisms of divergent evolution. Here, we used nine microsatellite DNA markers to study intra- and interspecific genetic diversity of two recently diverged species of brown macroalgae, Fucus radicans (L. Bergström & L. Kautsky) and F. vesiculosus (Linnaeus), in the Baltic Sea. We further performed biophysical modelling to identify likely connectivity patterns influencing the species’ genetic structures. For each species we found intraspecific contrasting patterns of clonality incidence and population structure. In addition, strong genetic differentiation between the two species within each locality supported the existence of two distinct evolutionary lineages (FST =0.15-0.41). However, overall genetic clustering analyses across both species’ populations revealed that all populations from one region (Estonia) were more genetically similar to each other than to their own taxon from the other two regions (Sweden and Finland). Our data supports a hypothesis of parallel speciation. Alternatively, Estonia may be the ancestral source of both species, but is presently isolated by oceanographic barriers to dispersal. Thus, a limited gene flow in combination with genetic drift could have shaped the seemingly parallel structure.平行演化被认为是诸多动物类群中生态型形成与物种形成的关键机制。然而，要将平行演化与近期单系分化的物种区分开来往往颇具难度：这类物种因共享大量祖先遗传变异，且在局部区域发生过次生杂交，往往呈现出复杂的遗传关系。因此，物种的生殖隔离程度、扩散障碍，尤其是有限的长距离扩散能力，都会影响驱动趋异演化的种群结构基础。 本研究利用9个微卫星DNA标记（microsatellite DNA markers），对波罗的海海域两种近期分化的褐藻——辐射墨角藻（Fucus radicans, L. Bergström & L. Kautsky）与墨角藻（Fucus vesiculosus, Linnaeus）的种内及种间遗传多样性展开分析。此外，本研究通过生物物理模型（biophysical modelling），解析了影响这两个物种种群遗传结构的潜在连通格局。 针对两个物种各自的种内分析均发现，其克隆发生率与种群结构呈现出显著的分化格局。此外，各采样点内两个物种间均呈现出强烈的遗传分化，这支持了两个独立演化支系的存在（遗传分化系数FST=0.15~0.41）。然而，针对两个物种种群的整体遗传聚类分析结果显示：来自爱沙尼亚区域的所有种群之间的遗传相似度，均高于其与瑞典、芬兰另外两个区域内同类群的遗传相似度。 本研究数据支持平行物种形成假说。另一种可能是，爱沙尼亚是这两个物种的祖先起源地，但目前因海洋学扩散屏障而与其他区域隔绝。因此，有限的基因流与遗传漂变共同作用，或许塑造了本次研究中观察到的看似平行的遗传结构。