Abstract
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, postglacial colonization, environmental partitioning and reproductive barriers. 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. 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 the contact zone showed that maternal tetraploids rarely produced progeny of other cytotypes, whereas the progeny of maternal hexaploids varied, with frequent pentaploids and aneuploids. The presence of distinctive hexaploid chloroplast types in Ireland, Scottish islands and western mainland Britain indicates that its establishment preceded separation of these land masses by sea-level rise c. 16 000 years BP. This group did not originate from British tetraploids and probably diverged before postglacial invasion from mainland Europe. The combination of cytotype, molecular, contact zone and common garden data shows an overall pattern reflecting postglacial colonization events, now maintained by geographic separation, together with more recent occasional local in situ polyploidisation. Reproductive barriers favour the persistence of the tetraploid to the detriment of the hexaploid.
Highlights
Genetic structure in British and Irish plant species has been enduringly shaped by events during and since the last glacial period (Cottrell et al 2002; Rendell and Ennos 2002; Provan and Bennett 2008; Wachowiak et al 2011)
We investigate the distribution of cytotypes of Campanula rotundifolia, considering DNA variation, postglacial colonization, environmental partitioning and reproductive barriers
While early flowering of plants in Ireland was noted by Jeffree (1960), we found that Irish hexaploids were the last to flower in the common garden (Table 2, common garden was related to climate (CCI) 5, hexaploids), which may indicate strong genotype × environment interactions for these eight clones
Summary
Genetic structure in British and Irish plant species has been enduringly shaped by events during and since the last glacial period (Cottrell et al 2002; Rendell and Ennos 2002; Provan and Bennett 2008; Wachowiak et al 2011). 27 000 years BP (during the British Devensian and Irish Midlandian stages), only parts of southern and eastern England remained unglaciated, and grounded ice reached to the edge of the continental shelf along its whole boundary from SW Ireland to the Shetland Isles (Clark et al 2012). With sea levels around 130 m lower than at present, land bridges connected Britain and Ireland to each other and to continental Europe, and more southerly, non-glaciated parts of the continental shelf were dry land. A retreat of the ice sheet and rise in sea level followed, so that most of the land mass was deglaciated by 15 000 years BP, and the land bridges and continental shelf were progressively inundated. The process of recolonization of deglaciated land resulted in distinctive genetic patterns, still evident today (Hewitt 1999), and offered opportunities for species with adaptive strategies, such as genome doubling, to take advantage of the new space
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