Geographical Parthenogenesis in Ranuculus kuepferi

Geographical parthenogenesis (GP) refers to the phenomenon of apomictic taxa showing latitudinally and altitudinally larger distribution ranges than their sexual relatives and tending to colonize previously glaciated areas more frequently. Various models try to explain GP. Among the most commonly debated intrinsic factors are polyploidy, uniparental reproduction and genetic interactions between sexuals and apomicts. It is assumed that the polyploid apomicts show a higher developmental plasticity and therefore cope better with stressful habitats than the diploid parents. Apomicts are usually self-compatible and capable of uniparental reproduction. In contrast, their self-incompatible sexual relatives depend on pollinators and mating partners to set seeds. According to Baker`s law, apomicts are therefore seen as better colonizers because they theoretically can found a population with a single propagule, while sexual outcrossers cannot. Finally, genetic interactions between apomicts and sexuals can result in a reproductive barrier against the sexual parents which might facilitate the spread of apomicts into new habitats. There is little direct evidence of how relevant these factors actually are for GP. We therefore pursue an experimental ecological approach by testing the response of the sexual/apomictic model plant Ranunculus kuepferi to different climatic conditions along an altitudinal gradient of 1200 m in the Austrian Alps. Our approach is expected to yield important insights into the mechanisms that might have driven GP. Ideally, short-term processes that occur within 1-2 generations provide indications of mechanisms for long-term evolution. In experimental plots with apomictic and sexual individuals established in the subalpine, alpine and subnival zone we investigate growth performance and reproductive success, the colonization potential of apomicts and sexuals by imitating founder scenarios, the role of pollen limitation, and the phenotypic plasticity of reproductive modes. Additionally, some of the experiments are carried out in natural sexual populations in the Western Alps and apomictic populations in the Austrian Alps. Together with ongoing DFG and FWF projects, screening phenotypic expression of apomixis in wild populations and modelling the causality of niche partitioning and range filling from site parameters, the study makes an essential contribution to understanding environmentally driven evolution in mountain plants and the biogeographical success of apomixis.

Geographical parthenogenesis refers to the phenomenon of apomictic taxa that show latitudinally and altitudinally larger distribution ranges than their sexual relatives and tend to colonize previously glaciated areas more frequently. Various models try to explain this phenomenon. The most common ones are: polyploidy gives apomicts a fitness advantage over the diploid parents in stressful habitats; asexually reproducing apomicts are facilitated in the colonization of new habitats whereas self-incompatible sexuals rely on mating partners and adequate pollinators; low temperatures promote polyploidization as first step from the sexual to the apomictic mode; reproductive barriers between sexuals and apomicts prevent range expansion of sexuals. Recent distribution analyses on Ranunculus kuepferi, whose sexual populations are restricted to the Western Alps and whose apomictic populations colonize the entire Alpine Arc, suggest that a niche shift towards colder conditions and competitive advantages of asexuality might have been crucial for the immigration into previously glaciated mountain ranges. To obtain direct evidence, the present project pursued an experimental ecological approach by testing the performance of sexual diploids and apomictic tetraploids in different climatic conditions along a 1000m elevational gradient in the Austrian Alps. Growth performance, frost resistance and various reproductive parameters were recorded between 2015 and 2018. Additional examinations were carried out in natural populations and in transplanted single founder individuals at different elevations. Growth parameters did not indicate clear fitness advantages of polyploidy over diploidy in R. kuepferi. On the contrary, diploids tended to develop more ramets and leaves per individual and showed a higher specific leaf area than tetraploids. Only the rhizome biomass significantly increased with elevation in tetraploids but not in diploids. Frost resistance increased with elevation in both cytotypes but tended to be slightly higher in tetraploids than in diploids. Tetraploids developed slightly more flowers per ramet and about twice as much carpels per flower than diploids. Nevertheless, seed output was substantially higher in diploids at all sites - mainly due to the high number of dysfunctional ovules in the apomicts. Also seed germination rates were significantly higher in diploids than in tetraploids. Conversely, apomictic founder individuals had a clear reproductive advantage over sexual ones: apomicts produced regularly seeds which was an exception in sexuals. Overall, for R. kuepferi, the study did not unequivocally confirm the hypothesis that polyploid apomicts show a higher vegetative and reproductive plasticity and therefore perform better than diploid sexuals in cold climates. In the presence of mating partners and pollinators, sexuals produce substantially more offspring than apomicts even in the alpine-nival ecotone. At most, a higher frost tolerance could confer a survival benefit for apomicts. The capacity for uniparental reproduction, however, seems to have been a more essential factor for the successful spread of the apomicts throughout the Alps.