Can a cold snap put a stop to sex?

The alpine plant Ranunculus kuepferi can face spring frosts while growing reproductive organs. Does the cold explain why sometimes the plant doesn't bother with sex?

In an Open Access (so it’s FREE) study, Simeone Klatt and colleagues take a look at the sex life of Ranunculus kuepferi and why, sometimes, it doesn’t happen.

The common name for R. kuepferi is Kuepfer’s buttercup. Its leaves could be mistaken for stalks of grass. It’s flowers are white with a golden centre and sit between 5cm (a couple of inches) to over 30cm (over a foot) above the ground. You’d find it in wet grasslands in the Alps, and that’s the root of a big problem for it.

Ranunculus kuepferi
Ranunculus kuepferi. Photo: jdevos / Tele Botanica

R. kuepferi is used to sitting under a blanket of snow. As the seasons turn the ice thaws, but the weather in the Alps can be changeable. There’s always the possibility of an overnight frost. This is a particular danger to the reproductive parts of a plant, which do not welcome being exposed to a freezing chill.

Prior work (in AoB PLANTS, so also free access) by the team found there was a correlation between elevation and polyploidy in R. kuepferi. The higher up a mountain a buttercup was, the more likely it was to be polyploid. The connection with sex is that the polyploid plants, with more than one pair of chromosomes, were much more likely to treat sex as optional or skip it altogether. Without needing sex, they could compress the time spent on growing offspring. That means a shorter time with delicate flowers out, and less chance of being intercepted by a serious frost.

It’s a good story, but is it accurate?

Klatt and colleagues decided to test how cold was connected to reproduction in the lab. They collected plants from over a hundred sites across the Alps. They then tested the diploid plants against the tetraploid plants over three conditions to see how they reacted. One test was against the normal outside temperatures between March and June. Some plants got a warm treatment, basking between 15°c in the day and 10°c at night. Unluckiest were the plants in the cold group. During the day they were at 7°c, but for three nights a week, they suffered temperatures of -1°c. For the remaining four days they only at temperatures of 2°c a night. This schedule continued till seed harvest.

They found the diploid plants started to flower first, but overall their performance varied on a few things. One was the temperature they were feeling. The other was their origin. Klatt et al. say: “A highly significant positive correlation of time until flowering and altitude of origin was observed for the outdoor-grown diploid plants… as well as a significant positive correlation for the other diploid study groups… For the tetraploid cytotype, no significant correlations between time until flowering and altitude of origin were found.”

That seems to be a common theme for the tetraploid plants. They had a plan, and they were sticking to it regardless of temperature. But then, they weren’t planning for sex to be an issue.

The results for the seeds showed why diploids bothered with sex. They consistently set more seeds than the tetraploids, but for both groups, cold temperatures hurt seed development. What did differ was the difference. For tetraploids, the negative impact wasn’t as severe as it was for the diploids.

They also found temperature affected the parentage. When the temperature turned cold, the plants were more likely to be apomictic – meaning they set seed without sex. In warm temperatures, things could also turn odd, literally in the case of the diploid plants. They could produce offspring with three chromosomes (a triploid bridge) instead of the usual two.

An interesting finding was that the ability for diploid plants to turn apomictic was plastic – meaning it could change. A plant that tried sex one year could give up the next – or vice versa. Klatt and colleagues say this could be due to the “influence of epigenetic control mechanisms for expression of reproductive phenotypes.” and suggest that experiments in timing frost shocks could find how the epigenetic switches work.

Further reading

Klatt, S., Schinkel, C. C. F., Kirchheimer, B., Dullinger, S., & Hörandl, E. (2018). Effects of cold treatments on fitness and mode of reproduction in the diploid and polyploid alpine plant Ranunculus kuepferi (Ranunculaceae). Annals of Botany, 121(7), 1287–1298.

Ladinig, U., Hacker, J., Neuner, G., & Wagner, J. (2013). How endangered is sexual reproduction of high-mountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment. Oecologia, 171(3), 743–760.

Schinkel, C. C. F., Kirchheimer, B., Dellinger, A. S., Klatt, S., Winkler, M., Dullinger, S., & Hörandl, E. (2016). Correlations of polyploidy and apomixis with elevation and associated environmental gradients in an alpine plant. AoB Plants, 8, plw064.

Schinkel, C. C. F., Kirchheimer, B., Dullinger, S., Geelen, D., De Storme, N., & Hörandl, E. (2017). Pathways to polyploidy: indications of a female triploid bridge in the alpine species Ranunculus kuepferi (Ranunculaceae). Plant Systematics and Evolution, 303(8), 1093–1108.