The ecology of geographical parthenogenesis in alpine plants

Asexual organisms have larger distribution areas than their sexual relatives, tend to range to higher elevations and to previously glaciated areas. This phenomenon, called "Geographical Parthenogenesis", contradicts the predominance of sexual reproduction in nature and has long been enigmatic. Empirical studies on alpine plants are largely missing. The Frozen Niche Variation model predicts that niche specialization of different asexual genotypes would allow for a more efficient occupation of the available resource space. Asexuals might be therefore more successful in occupying their potential distribution range than sexual populations. Alternatively or additionally, the prevalence of asexuality in colder climates might also be influenced by effects of cold temperatures on gamete formation and the quantitative expression of apomixis. We will test these hypotheses for geographical parthenogenesis in the alpine plant species Ranunculus kuepferi. We will combine analyses of wild populations and modeling across the range of the species to study 1) ecological niche differentiation and mode of reproduction in wild populations, 2) seedling establishment under controlled conditions, and 3) direct influence of temperature on mode of reproduction in experimental approaches 4) potential migration rates by means of simulation studies. The data from all projects will be used to model potential and actual range filling of sexuals vs. apomicts. Results will enable us to develop a comprehensive and predictive ecological model for geographical parthenogenesis.

The fact that asexual plant and animal species have larger geographical ranges than their sexual relatives is called Geographical Parthenogenesis. This frequent phenomenon is at odds with the predominance of sexual reproduction in plants and animals, its drivers are contentious. Different hypotheses emphasize the role of possible niche broadening, the advantages of uniparental reproduction for range expansion, and the possibly higher competitive abilities of the asexual, usually polyploid organisms. A comprehensive evaluation of the relative roles of these factors is missing so far. Here, we tried such a comprehensive evaluation using the alpine herb species Ranunculus kuepferi as a model system. Geographical parthenogenesis is pronounced in R. kuepferi: diploid populations are geographically restricted to the most southwestern parts of the Alps which have also served as glacial refugia. By contrast, tetraploid populations, which have emerged from diploid ancestors after the Last Glacial Maximum, have subsequently colonized nearly the whole Alpine chain. Our investigations combine field work (for characterizing the ecological niches of the two cytotypes, measuring their competitive abilities as well as several functional and demographic traits) with statistical modelling and the computer-based simulation of the Holocene range dynamcis of both cytotypes. The results suggest that their competitive responses to other alpine plants are similar, but that their ecological niches actually differ. In contrast to our expectations, the niche of the tetraploids is not broader but has merely shifted towards cooler conditions. Our simulations suggest that this shift is, however, not sufficient to explain the more successful range expansion of the tetraploid cytotype. Instead, it obviously was just a prerequisite of this success: increased tolerance of cold temperatures facilitated migration towards the high mountain chains in the southwestern Alps; however, without a change towards uniparental reproduction the tetraploids would still not have been able to overcome this barrier. In summary, these results suggest that Geographical Parthenogenesis is a multi-causal phenomenon. In particular, the combination of ecological tolerance of extreme conditions and increased spatial mobility as a consequence of uniparental reproduction have likely contributed to the rapid expansion of asexual taxa into formerly glaciated, cool areas of the globe. In an add-on study, we have moreover analysed the role of the reproductive system in biological invasions. Plants that have been dispersed outside their native ranges by human agency often occupy altered realized niches in their adventive distribution areas. The reasons of this phenomenon are not clear yet, but rapid evolutionary change/adaptation is under debate. If this driver would actually be important, asexual plants may be disadvantaged because reduced genetic diversity and lack of recombination should decrease their evolutionary flexibility. In our add-on study we compared 13 congeneric pairs of sexual and apomictic plant species, that all have become naturalized outside their native ranges, with respect to such niche shifts. We found that niche shifts actually are frequent but that their incidence and magnitude do not differ between sexual and asexual species. We conclude that rapid evolution is likely to play a minor role in driving niche shifts during biological (plant) invasions.