Visions from nature

Purple saxifrage and morphological and genetic diversity

By Anders L Kolstad

Purple saxifrage (Saxifraga oppositifolia) is a really though plant. High up in the mountains and far to the north, that’s where you’ll find it. Flowering occurs as soon as the snow melts and is therefore a precious sight for those awaiting the flower season. If you walk around in the mountains and keep an observant eye, you might start to notice that this plant looks a bit different from place to place. On the wind swept ridges it grows as dense cushions, resembling the growth of the moss campion (Silene aucaulis). But perhaps close by, down in a sheltered bump in the landscape where there’s less wind and more water available, there it may take on a creeping habit, forming long strings along the ground searching for a place to set root. For plants like this we often think they have high plasticity; that they can adapt to the environment they live in. Another way to look at it is to say they have a large ecological amplitude, meaning they can grow under very diverse conditions. No scientists have revealed there’s more than just a response to the environment and that there is a genetic component to the variation in purple saxifrage.

 

Øverst: Rødsildre (Saxifraga oppositifolia) kan vokse som tette tuer og blomster gjerne veldig rikt tidlig om våren. Denne formen av arten er alltid diploid, dvs. med to sett kromosomer. Nedersts: Den krypende formen av rødsildre vokser på litt mer beskyttede steder ned mot snøleiene og karakteriseres ved rasker vekst, mer vegetativ spredning og mindre blomstring. Denne formed av arten kan være enten diploid eller tetraploid (med fire sett kromosomer). Foto: Anders L. Kolstad NTNU Vitenskapsmuseet CC BY-SA 4.0.
Top: Purple saxifrage (Saxifraga oppositifolia) can grow as dense cushions and then they can become completely covered in pink flowers. This variety of the species is always diploid, meaning it has two sets of chromosomes. Bottom: The creeping form in purple saxifrage is found in more sheltered places down towards the snowbeds. It is characterised by growing quickly and having more vegetative reproduction and less flowering (sexual reproduction). The creeping form can be either diploid or tetraploid (with four sets of chromosomes). Photo: Anders L. Kolstad NTNU University museum, CC BY-SA 4.0.

Humans have two sets of chromosomes, one from our mother and one from our father. Other animals (usually) also have it this way. We say we are diploid (di = two). Plants on the other hand, they do things a bit more complicated. They can have just about any number of chromosomes, for example due to a mistake during a critical stage in the cell division. Such species are said to be polyploid (poly = many). Individuals can usually only reproduce with individuals with the same chromosome number (ploidilevel). Purple saxifrage exists as diploid (most common), triploid (uncommon), and tetraploid (quite common).

By comparing the habitat (growing location), growth form and chromosome number in 192 individuals of purple saxifrage on Svalbard, scientists could conclude that tetraploid individuals never grow as cushions, but stuck to the creeping habit and was found more commonly in snowbeds and other sheltered sites. They also required a relatively high soil pH in order to grow. The diploid individuals on the other hand, they were very flexible, with a high ecological amplitude both in terms of pH and exposure. From before we also know that diplod individuals produce much more flowers than the tetraploid individuals, whereas the latter usually spread more through fragments (vegetative spread).

Purple saxifrage is a great example of hidden diversity in nature. The flowers and the leaves look identical between the two ploidy types, yes they are ecologically very different. In time we may imagine two species developing from one. Purple saxifrage show us how ecology (habitat, reproduction) and evolution (genetic change) is connected phenomenon.

 

This post is part of a large series of short plant stories.

 

Source:

Eidesen, P. B., E. Müller, C. Lettner, I. G. Alsos, M. Bender, M. Kristiansen, B. Peeters, F. Postma, and K. F. Verweij. 2013. Tetraploids do not form cushions: association of ploidy level, growth form and ecology in the High Arctic Saxifraga oppositifolia L. s. lat. (Saxifragaceae) in Svalbard. Polar Research 32:20071.

 



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