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 factor 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 âadventiveâ 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 acutally 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.
Figure One: Niche change observed with coarse-grained environmental (a-d) and fine-grained environmental (e-h) variables comparing diploid and tetraploid Ranunculus kuepferi populations in their full (a, e), allopatric (b, f) and sympatric (c, g) range and comparing tetraploid Ranunculus kuepferi populations within the sympatric and outside the sympatric area (d, h). Area of niche unique to diploids, niche overlap and niche unique to tetraploids (a-c, e-g) are shown in green, blue and red respectively. Area of niche unique to tetraploids in the sympatric area, niche overlap and niche unique to tetraploids outside the sympatric area (d,h) are shown in orange, purple and brown respectively. The red arrow links the centroid of the diploids and tetraploids niche (a-c, e-g) and tetraploids niche in the sympatric and outside the sympatric area (d, h) respectively. The available environment in the study area(s) are defined by red lines when comparing populations with the same background area (a,c,e,g) and by green and red lines when comparing populations from the sympatric with populations outside the sympatric area (b,d,f,h). The correlation circle shows the loadings of individual environmental variables to the two PCA axes. bio5: maximum temperature of warmest month; bio7: annual temperature range; bio14: precipitation of driest month; Io: ombrothermic index; calcium: percentage of calcareous soils; slope: slope inclination; T: temperature, F: average soil moisture during the growing season; W: variability of soil moisture during the growing season; R: soil pH; N: soil nutrient content. T, F, W, R, N are mean Landolt indicator values for the communities occupying the sampling plots.
Figure Two: Violin plot and convex-hull areas of the cytotypes from the migration simulations. The violin plots (n=50) are grouped by niche, i.e. simulations run using the respective species specific niche (own N) or the merged niche of both species (mix N), and by the way of reproduction, i.e. using the respective species specific reproduction (own R) or both species using either asexual (asx R) or sexual (sex R) reproduction. The coloured areas of the violin plot show a smoothed approximation of the frequency distribution (a kernel density plot) while the white central bar indicates the interquartile range and the black line indicate the median values. The dashed line shows the current real convex hull area.
Polygons on the maps show the convex-hull of the occurrence sites of the two cytotypes of R. kuepferi in reality (a) and using the hybrid model with different settings i.e. simulations run using the respective species specific niche (b-d) or the merged niche of both species (e-g), and by the way of reproduction, i.e. using the respective species specific reproduction, diploids sexual and tetraploids asexual (b,e) or both species using either asexual (c,f) or sexual (d,g) reproduction.