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（Parallel evolution）已被证实是诸多动物类群中生态型形成与物种形成的强力驱动机制。然而，平行演化信号往往难以与近期单系分化的物种相区分——这类物种由于存在大量共享祖先遗传变异，且局部区域发生过次级杂交，往往呈现复杂的遗传关系。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，均会影响驱动趋异演化的种群统计结构。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——辐射岩藻（Fucus radicans，L. Bergström & L. Kautsky）和泡叶藻（Fucus vesiculosus，Linnaeus）的种内及种间遗传多样性展开分析。本研究还通过生物物理建模（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，我们均发现了种内克隆性发生率与种群结构的显著分化模式。此外，各采样点内两个物种间均存在强烈的遗传分化，证实存在两个独立的进化谱系（FST=0.15~0.41）。但针对两个物种种群的整体遗传聚类分析结果显示，来自单一区域（爱沙尼亚）的所有种群彼此间的遗传相似度，均高于其与另外两个区域（瑞典与芬兰）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，爱沙尼亚种群是这两个物种的祖先来源，但当前已被海洋学扩散阻隔所隔离。因此，有限的基因流与遗传漂变共同作用，或塑造了观测到的类平行演化结构。