Background and Aims

The distribution and genetic structure of most plant species in Britain and Ireland bear the imprint of the last ice age. These patterns were largely shaped by random processes during recolonization but, in angiosperms, whole genome duplication may also have been important. We investigate the distribution of cytotypes of Campanula rotundifolia, considering DNA variation, post-glacial colonization, environmental partitioning, and reproductive barriers.

Methods

Cytotypes and genome size variation from across the species’ range were determined by flow cytometry and genetic variation was assessed using cpDNA markers. A common garden study examined growth and flowering phenology of tetraploid, pentaploid and hexaploid cytotypes and simulated a contact zone for investigation of reproductive barriers.

Key Results

Irish populations were entirely hexaploid. In Britain, hexaploids occurred mostly in western coastal populations which were allopatric with tetraploids, and in occasional sympatric inland populations. Chloroplast markers resolved distinct genetic groups, related to cytotype and geographically segregated; allopatric hexaploids were distinct from tetraploids, whereas sympatric hexaploids were not. Genome downsizing occurred between cytotypes, Progeny of open-pollinated clones from a contact zone showed that maternal tetraploids rarely produced progeny of other cytotypes, whereas the progeny of maternal hexaploids varied, with frequent pentaploids and aneuploids.

Conclusions

The presence of distinctive hexaploid chloroplast types in Ireland, Scottish islands and western mainland Britain indicate that its establishment preceded separation of these land masses by sea-level rise c 16000 years BP. This group did not originate from British tetraploids and probably diverged before post-glacial invasion from mainland Europe. The combination of cytotype, molecular, contact zone and common garden data shows an overall pattern reflecting post-glacial colonization events, now maintained by geographic separation, together with more recent occasional local in situ polyploidisation. Reproductive barriersfavour the persistence of the tetraploid to the detriment of the hexaploid.

中文翻译：

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入侵、隔离和进化塑造了圆叶风铃的种群遗传结构

 背景和目标

英国和爱尔兰大多数植物物种的分布和遗传结构都带有最后一个冰河时代的印记。这些模式很大程度上是由重新定殖过程中的随机过程决定的，但在被子植物中，全基因组复制也可能很重要。我们研究了圆叶风铃的细胞型分布，考虑了 DNA 变异、冰川后定植、环境分配和生殖障碍。

 方法

通过流式细胞术确定不同物种范围内的细胞型和基因组大小变异，并使用 cpDNA 标记评估遗传变异。一项常见的花园研究检查了四倍体、五倍体和六倍体细胞型的生长和开花物候学，并模拟了接触区以研究生殖障碍。

 主要结果

爱尔兰人口完全是六倍体。在英国，六倍体主要出现在与四倍体异域的西部沿海种群中，偶尔也出现在同域内陆种群中。叶绿体标记解析了不同的遗传群体，与细胞类型相关并在地理上隔离；异域六倍体与四倍体不同，而同域六倍体则不然。基因组缩小发生在细胞型之间，来自接触区的开放授粉克隆的后代表明，母本四倍体很少产生其他细胞型的后代，而母本六倍体的后代则多种多样，常见的是五倍体和非整倍体。

 结论

爱尔兰、苏格兰群岛和英国西部大陆存在独特的六倍体叶绿体类型，这表明它的建立早于距今 16000 年海平面上升导致的这些陆地的分离。这个群体并非起源于英国四倍体，可能在冰期后欧洲大陆入侵之前就分化了。细胞型、分子、接触区和常见花园数据的结合显示了反映冰期后殖民事件的总体模式，现在通过地理分离以及最近偶尔的局部原位多倍化来维持。生殖障碍有利于四倍体的持续存在，而不利于六倍体。