Background and AimsAsexual organisms are more widespread in previously glaciated areas than their sexual relatives (‘geographical parthenogenesis’). In plants, this pattern is probably dependent on reproductive isolation and stability of cytotypes within their respective distribution areas. Both partial apomixis and introgressive hybridization potentially destabilize the spatial separation of sexual and apomictic populations. The wide distribution of apomicts may be further enhanced by uniparental reproduction which is advantageous for colonization. These factors are studied in the alpine species Ranunculus kuepferi.MethodsGeographical distribution, diversity and mode of reproduction of cytotypes were assessed using flow cytometry and flow cytometric seed screening on samples from 59 natural populations of Ranunculus kuepferi. Seed set of cytotypes was compared in the wild.Key ResultsDiploid sexuals are confined to the south-western parts of the Alps, while tetraploid apomicts dominate in previously glaciated and in geographically isolated areas despite a significantly lower fertility. Other cytotypes (3x, 5x and 6x) occur mainly in the sympatric zone, but without establishing populations. The tetraploids are predominantly apomictic, but also show a partial apomixis via an uncoupling of apomeiosis and parthenogenesis in the seed material. Both pseudogamy and autonomous endosperm formation are observed which may enhance uniparental reproduction.ConclusionsDiploids occupy a glacial relic area and resist introgression of apomixis, probably because of a significantly higher seed set. Among the polyploids, only apomictic tetraploids form stable populations; the other cytotypes arising from partial apomixis fail to establish, probably because of minority cytotype disadvantages. Tetraploid apomicts colonize previously devastated and also distant areas via long-distance dispersal, confirming Baker's law of an advantage of uniparental reproduction. It is concluded that stability of cytotypes and of modes of reproduction are important factors for establishing a pattern of geographical parthenogenesis.

Asexual organisms usually have larger, and in the Northern Hemisphere, more northern distributions than their sexual relatives. This phenomenon, called geographical parthenogenesis, has been attributed to predispositions in certain taxa, advantages of polyploidy and/or hybrid origin, advantages of uniparental reproduction, introgression of apomixis into sexuals, niche differentiation of clones, and biotic interactions. Here we focus on the role of uniparental reproduction in colonisation, and the importance of different developmental pathways, i.e. autonomous apomixis which does not require pollination and fertilisation of endosperm nuclei for successful seed set, and pseudogamous apomixis which does. A literature survey suggests that geographical parthenogenesis occurs frequently in species with autonomous apomixis, while the correlation with pseudogamy is poorly documented. However, taxonomic patterns (e.g. predominance of Asteraceae) and also methodological bias may influence estimates of frequencies of geographical parthenogenesis. We demonstrate that a flow cytometric seed screen (FCSS) is a powerful method for assessing pseudogamous vs. autonomous apomixis. We show that population genetic studies provide insights into the genetic diversity of apomicts, but do not give strong support for uniparental reproduction being the only explanation of geographical parthenogenesis. Molecular studies help elucidate the evolutionary and biogeographical history of apomictic complexes, and we conclude that multidisciplinary studies are needed to understand fully the phenomenon of geographical parthenogenesis.

Geographical parthenogenesis describes the enigmatic phenomenon that asexual organisms have larger distribution areas than their sexual relatives, especially in previously glaciated areas. Classical models suggest temporary advantages to asexuality in colonization scenarios because of uniparental reproduction and clonality. We analyzed population genetic structure and selffertility of the plant species Ranunculus kuepferi on 59 populations from the whole distribution area (European Alps, Apennines and Corsica). Amplified fragment length polymorphisms (AFLPs) and five microsatellite loci revealed individual genotypes for all populations and mostly insignificant differences between diploid sexuals and tetraploid apomicts in all measures of genetic diversity. Low frequencies of private AFLP fragments/simple sequence repeat alleles, and character incompatibility analyses suggest that facultative recombination explains best the unexpectedly high genotypic diversity of apomicts. STRUCTURE analyses using AFLPs revealed a higher number of partitions and a stronger geographical subdivision for diploids than for tetraploids, which contradicts expectations of standard gene flow models, but indicates a reduction of genetic structure in asexuals. Apomictic populations exhibited high admixture near the sexual area, but appeared rather uniform in remote areas. Bagging experiments and analyses of pollen tube growth confirmed self-fertility for pollen-dependent apomicts, but self-sterility for diploid sexuals. Facultative apomixis combines advantages of both modes of reproduction: uniparental reproduction allows for rapid colonization of remote areas, whereas facultative sexuality and polyploidy maintains genetic diversity within apomictic populations. The density dependence of outcrossing limits range expansions of sexual populations.

The evolution of polyploids is strongly influenced by their mode of origin. Autopolyploidy is often hampered by disturbances of chromosome pairing and segregation at meiosis, while allopolyploids exhibit fewer meiotic problems and potentially have selective advantages because of hybridity. Apomixis, the asexual reproduction via seed in flowering plants, can overcome problems of meiotic reduction, favouring range expansions of polyploids. We elucidate the origin and genetic structure of apomictic cytotypes of the alpine species Ranunculus kuepferi by analyzing five microsatellite loci (SSRs) and Amplified Fragment Length Polymorphisms (AFLPs) on eight diploid and twelve polyploid populations from across the range of the species. Multiple allelism appears in three SSR loci in all polyploid cytotypes and is likely a result of facultative unbalanced meiotic events, as it is typical for autopolyploids. Low frequencies of private AFLP fragments (3%) and SSR alleles (4%) in tetraploids suggest that they evolved from diploid populations without the contribution of another divergent parental gene pool. A Neighbor‐joining tree of AFLPs revealed a low genetic divergence of cytotypes and at least three independent origins of tetraploid populations. Autopolyploidy is in R. kuepferi likely evolutionarily young and still has signatures of a facultative unbalanced meiosis. The shift to apomixis avoided consequences of meiotic problems and therefore enhanced rapid establishment of autotetraploids. The lack of hybridity in polyploids suggests that apomixis is successful even without genomic novelty. The distributional success of polyploid cytotypes is probably more due to the benefits of apomictic reproduction than to genetic consequences of polyploidization.

Background and AimsHigh alpine environments are characterized by short growing seasons, stochastic climatic conditions and fluctuating pollinator visits. These conditions are rather unfavourable for sexual reproduction of flowering plants. Apomixis, asexual reproduction via seed, provides reproductive assurance without the need of pollinators and potentially accelerates seed development. Therefore, apomixis is expected to provide selective advantages in high-alpine biota. Indeed, apomictic species occur frequently in the subalpine to alpine grassland zone of the European Alps, but the mode of reproduction of the subnival to nival flora was largely unknown.MethodsThe mode of reproduction in 14 species belonging to seven families was investigated via flow cytometric seed screen. The sampling comprised 12 species typical for nival to subnival plant communities of the European Alps without any previous information on apomixis (Achillea atrata, Androsace alpina, Arabis caerulea, Erigeron uniflorus, Gnaphalium hoppeanum, Leucanthemopsis alpina, Oxyria digyna, Potentilla frigida, Ranunculus alpestris, R. glacialis, R. pygmaeus and Saxifraga bryoides), and two high-alpine species with apomixis reported from other geographical areas (Leontopodium alpinum and Potentilla crantzii).Key ResultsFlow cytometric data were clearly interpretable for all 46 population samples, confirming the utility of the method for broad screenings on non-model organisms. Formation of endosperm in all species of Asteraceae was documented. Ratios of endosperm : embryo showed pseudogamous apomixis for Potentilla crantzii (ratio approx. 3), but sexual reproduction for all other species (ratios approx. 1·5).ConclusionsThe occurrence of apomixis is not correlated to high altitudes, and cannot be readily explained by selective forces due to environmental conditions. The investigated species have probably other adaptations to high altitudes to maintain reproductive assurance via sexuality. We hypothesize that shifts to apomixis are rather connected to frequencies of polyploidization than to ecological conditions.