Polyploid Harebells – a commentary on Julia Wilson’s recent paper

Very many plant species have polyploid forms, in which more than one paired set of chromosomes are present in all the cells. The recent paper by Julia Wilson and colleagues describes the situation in the Harebell, Campanula rotundifolia. In Europe, this species may have 34, 68, 85 or 102 pairs of chromosomes, forming a diploid – tetraploid – pentaploid – hexaploid sequence of cytotypes which has arisen naturally by doubling, crossing and back-crossing between cytotypes.

Campanula rotundifolia pictured at St Abbs Head; photo by Chris Jeffree

Julia and colleagues determined the ploidy level of 1,700 samples from 700 locations in the British Isles. Counting chromosomes under the microscope is possible but very tedious, and these days a faster technique called flow cytometry is used, where a laser beam scans a stream of suspended nuclei that have been extracted from a fragment of plant tissue. No diploids were found in Britain or Ireland. However, they found that tetraploids (2n=68) are common throughout much of Britain, but in Ireland and parts of the west of Britain, only hexaploids (2n=102) are found. Most plant populations contained a single cytotype, but intriguingly, a few hexaploids turn up in tetraploid areas in Scotland and England, sometimes with pentaploids (2n=85), at sites including Wanlockhead and Teesdale.

Like humans, the ancient migration routes of plants are reflected in their genetic make-up today, and we can use clues in the chloroplast DNA of plants to inform us of their pathways. During the last glaciation, Ireland and Britain were buried under ice or were tundra. Plants retreated to refugia in central Europe. With sea-levels about 130m lower than they are now, land bridges connected Britain and Ireland to each other and to continental Europe. As the climate warmed, plants moved northwards, and the land bridges disappeared. Ireland became an island about 15000 years ago, while Britain remained connected to Europe for another 7000 years. Chloroplast DNA studies showed that the Irish and western British hexaploids had formed on mainland Europe and moved north more than 15000 years ago. Tetraploids didn’t make it to Ireland in time, but had another 7000 years to move to Britain before that land bridge closed. The hexaploid populations at Wanlockhead and Teesdale are genetically similar to tetraploids and it is suspected that they have arisen more recently due to local mutations.

One might expect high ploidy plants to be larger and lusher. She grew tetraploids, pentaploids and hexaploids together in a common garden in Midlothian. They all grew very well, better than in the wild, but there were no real differences except that the pentaploid was highly floriferous. Four years after planting, there was good survival of all the ploidies, and some Irish hexaploids transplanted to a garden in dry East Lothian are thriving 10 years later. We have no idea whether the tetraploids would survive in the oceanic cool summers of the far west. This raises the question of ‘why aren’t British hexaploids more widespread?’ – studies of the seedlings of common garden plants showed that while tetraploid mothers produced many tetraploid offspring, hexaploid mother plants often produced offspring with odd numbers of chromosomes which are probably infertile. Thus hexaploids are likely to die out in mixed populations.

Campanula rotundifolia in the common garden experiment

Today’s western hexaploid populations are isolated on islands or by unfavourable terrain and are vulnerable to introductions from elsewhere. Ebay sellers from China, Canada, England and Hungary are advertising harebell seed today, what are its origins and cytotype? – we don’t know.

But we do know that ploidy levels can influence fitness. One quirky example of that: a recent Chinese study on the Black Goji Berry Lycium ruthenicum showed how ploidy levels may influence drought tolerance (Rao et al 2020). Those authors induced polyploidy with colchicine, trying to make better berries.

You can download Julia’s paper, free of charge, by clicking this:

https://doi.org/10.1093/aobpla/plaa011

Footnote: I reached for my copy of Stace’s New Flora of the British Isles (4th edition). He recognises two subspecies: Ssp. rotundifolia (narrow stem leaves with 2n= 68) and Ssp. montana (broader stem leaves with 2n= 102 chromosomes). The true relationship between taxonomic status, ploidy and ‘fitness’ remains a moot point.

John Grace 16th May 2020

References

Rao, Shupei et al. (2020) Chromosome doubling mediates superior drought tolerance in Lycium ruthenicum via abscisic acid signalling. Horticultural Research 7, 40.

Wilson, Julia et al.  (2020) Invasion, isolation and evolution shape population genetic structure in Campanula rotundifolia. AoB PLANTS 12: plaa011; doi: 10.1093/aobpla/plaa011

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