Northumberland Diary Discussion: 20 November 2011
Started by: /diaries/Northumberland/
Sternbergias and cyclamen from southern Greece. Entry 197.Go to latest contribution by Cliff Booker, 28 November 2011, 17:07. Go to bottom of this page.
John, I have grown Cyclamen hederifolium for very many years in the garden but don't have experience of seeing it in the wild. From seed the leaves can vary enormously, though this must to a strong degree be due to the selection that gardeners have made over the years. However, I have had one plant with much larger and circular leaves, and after reading your description wonder whether this may be a tetraploid that has arisen in the garden? I wonder how much is known about the 'rate' at which such mutations occur?
Since polyploids are still effectively genetically the same (ie: the 'genes' are not mutated, it seems hard to class them as different species, even if morphologically very distinct. Presumably if geographically isolated there will come a time when divergence is sufficient to class them as different species. What of C. africanum which Kit Grey-Wilson also describes as existing in diploid and tetraploid forms?
I hadn't realised until recently how ubiquitous and complex polyploidy is amongst plants (after reading 'The Secret Life of Trees' by Colin Tudge) and see how it can lead to some debate!
Yes, it is generally reckoned that at least half of all plant species are polyploid, that is they are related to species with half, or a third, or a quarter of the number of chromosomes that they have, and from ancestors of which they are presumed to have derived (polyploid evolution is a one-way process; a species cannot usually halve its chromosome number, although it may double it quite readily). Occasionally you find 'autopolyploids' which are essentially genetically identical to their diploid progenitors, but this is unusual. More often, polyploids are hybrids, containing the genomes of two distinct species, and so quite different from both, and reproductively isolated from them too. Because of this reproductive isolation (which can be just as significant as geographical isolation) they may evolve further in isolation from their parents, so it is rare to find that polyploids are genetically closely related to their parents. This is one of many reasons why polyploids are often classified at specific rank, but as I said, they DO have to be different!!
John - many thanks for that. I had assumed that polyploidy occurred, like colchicine treatment, just within a single species. It makes things even more complicated than I had thought! Presumably if there is 'hybridisation' between diploid and tetraploid cyclamen (say of hederifolium) then the resultant triploids are sterile and never perpetuated. Just as you begin to get some handle on the diversity of plants it surprises you all over again!!
Yes thats right. Any hybrids between a diploid and a tetraploid are triploid and sterile, although odd-number hybrids (e.g. pentaploids or heptaploids) between higher level polyploids may be partially fertile. Polyploid cells arise quite frequently, and may total 1% of all cells in diploid plant tissue, but the likelihood of polyploid cells commandeering reproductive tissue and so giving rise to a new polyploid generation (vegetatively or through sex) is much less. Newly arising autopolyploids are usually reproductively disadvantaged and lack the vigour of hybrid (allopolyploid) polyploids, which may be why nearly all polyploids are alloploids.
Gee John and Tim,
My tiny brain doth ache enough already ... time for a lie down with a nice picture book and a single malt. LOL.
Well you know Cliff, some of us are incorrigible and somehow the mind finds it hard to shut down! I do like poetry too!