Data from: Spatial genetic structure in Beta vulgaris subsp. maritima and Beta macrocarpa reveals the effect of contrasting mating system, influence of marine currents and footprints of postglacial recolonisation routes

Understanding the factors that contribute to population genetic divergence across a species' range is a long-standing goal in evolutionary biology and ecological genetics. We examined the relative importance of historical and ecological features in shaping the present-day spatial patterns of genetic structure in two related plant species, Beta vulgaris subsp. maritima and Beta macrocarpa. Using nuclear and mitochondrial markers, we surveyed 93 populations from Brittany (France) to Morocco – the southern limit of their species' range distribution. Whereas B. macrocarpa showed a genotypic structure and a high level of genetic differentiation indicative of selfing, the population genetic structure of B. vulgaris subsp. maritima was consistent with an outcrossing mating system. We further showed (1) a strong geographic clustering in coastal B. vulgaris subsp. maritima populations that highlighted the influence of marine currents in shaping different lineages and (2) a peculiar genetic structure of inland B. vulgaris subsp. maritima populations that could indicate the admixture of distinct evolutionary lineages and recent expansions associated with anthropogenic disturbances. Spatial patterns of nuclear diversity and differentiation also supported a stepwise recolonization of Europe from Atlantic-Mediterranean refugia after the last glacial period, with leading-edge expansions. However, cytoplasmic diversity was not impacted by postglacial recolonization: stochastic long-distance seed dispersal mediated by major oceanic currents may mitigate the common patterns of reduced cytoplasmic diversity observed for edge populations. Overall, the patterns we documented here challenge the general view of reduced genetic diversity at the edge of a species' range distribution and provide clues for understanding how life-history and major geographic features interact to shape the distribution of genetic diversity.阐明物种分布范围内驱动种群遗传分化（population genetic divergence）的各类因素，是进化生物学与生态遗传学领域长期以来的核心研究目标。本研究以两种近缘植物物种——甜菜滨海亚种（Beta vulgaris subsp. maritima）与长角甜菜（Beta macrocarpa）——为研究对象，解析历史与生态特征对其现今种群遗传结构空间格局的相对影响强度。我们利用核基因标记（nuclear markers）与线粒体标记（mitochondrial markers），对从法国布列塔尼（Brittany）至摩洛哥——即该类群分布范围南界——的93个种群开展了遗传普查。研究结果显示，长角甜菜的基因型结构与高水平遗传分化特征符合自交繁育（selfing）系统的特点；而甜菜滨海亚种的种群遗传结构则与异交交配系统（outcrossing mating system）相符。我们进一步发现：（1）滨海甜菜海岸种群呈现显著的地理聚类特征，表明海流对其不同演化支系的形成具有重要塑造作用；（2）内陆滨海甜菜种群存在特殊的遗传结构，这可能源于不同演化支系的遗传混合（admixture）以及与人为干扰相关的近期种群扩张事件。核基因多样性与分化的空间格局同样支持，末次冰期（last glacial period）后，该类群从大西洋-地中海避难所（Atlantic-Mediterranean refugia）逐步重新定殖欧洲，并伴随前沿种群扩张（leading-edge expansions）过程。然而，细胞质多样性（cytoplasmic diversity）并未受到冰期后重新定殖过程的影响：由主要洋流介导的随机性长距离种子扩散（long-distance seed dispersal），或可缓解边缘种群普遍存在的细胞质多样性降低现象。综上，本研究揭示的种群遗传模式，挑战了“物种分布边缘区域遗传多样性降低”的通用认知，并为解析生活史特征与主要地理要素如何相互作用进而塑造遗传多样性分布格局提供了新的线索。