Apomixis & the evolution of cytological complex populations

Apomictic plant species often show a conspicuous and ubiquitous cytological (i.e., karyological) polymorphism, which also may be regularly observed at the intrapopulational level. Although there are several studies demonstrating cytotype mixture in apomicts, i.e., mainly co-occurrence of individuals of differing ploidy, these investigations remained chiefly descriptive and did not focus on the evolutionary processes and forces giving rise to and maintaining cytological polymorphisms, or on the evolutionary significance of the differentiation of populations into various cytotypes. The proposed project will address these still underexplored evolutionary processes and associated consequences using a taxonomically clearly circumscribed, but cytologically highly complex species, Potentilla pusilla Host (Rosaceae). In particular, we will assess the (i) (AFLP-)phenotypic and cytological structure within and among populations of P. pusilla based on which, we will (ii) test possible explanations for the co-occurrence of cytotypes including recurrent formation of cytotypes, admixture, habitat segregation, and self-incompatibility of cytotypes. Furthermore, in order to estimate the effect of change in cytotype on fitness, we will (iii) experimentally analyze expected selection against newly formed cytotypes. Our goal is to particularly distinguish between effects which are due to variation in cytological constitution and effects attributable to genotypic differences. Central to these questions, the actually followed reproductive modes do not only depend on the genetic and cytological constitution of an individual, but also on abiotic habitat conditions and biotic factors, i.e., the pollen donor. We thus favor an in situ investigation based on a sample representative for the ecological as well as genotypic and cytotypic variation of the study species. For these purposes, we will combine molecular, karyological, sociological, and experimental ecological analyses, on the basis of data collected from wild populations in the field. The project thereby will be strongly interdisciplinary and will involve the competence of scientists from the fields of organismic evolution, vegetation science and biostatistics, population biology, and molecular and cytological apomixis research.

We studied evolutionary factors controlling the geographic and ecological distribution of individuals differing in the number of chromosome sets. In our study system, individuals are polyploid, i.e. carry more than two complete chromosome sets per cell nucleus. As a main result, we identified reproductive interrelationships as most important factor determining the observed distribution of polyploidy.A large number of higher plants, basically seed plants, exhibit variation in ploidy among individuals of a single species. Observation of more than one ploidy level (i.e. a particular number of chromosome sets) within a species seems paradox as difference in ploidy functions as a strong crossing barrier promoting speciation and because a demographic phenomenon called genetic drift eliminates variation including different ploidy levels from populations on the long run. However, several processes exist which can maintain ploidy variation within a species: A change in ploidy of the progeny compared to the parental ploidy is a genome mutation counteracting genetic drift and thus will maintain ploidy variation if occurring with sufficiently high rates. In addition, loss of ploidy variation is counteracted by three processes: first, divergence in the habitat preferences and, second, mutual reproductive dependence among cytotypes (i.e. here a term referring to individuals of the different ploidy levels); and, third, seed exchange (i.e. migration) among populations. In case of ecological divergence among cytotypes (in heterogenous environments), ploidy variation will be maintained because of fitness advantages of cytotypes in their respectively preferred habitats. Mutual reproductive dependence refers to dependency of certain cytotypes on pollination by less fit cytotypes. Although fitter, the so called self-incompatible cytotype cant replace because of the dependence its pollen donor and a frequency equilibrium between the cytotypes will evolve. Migration, finally, is another counter force of genetic drift. - We tested these four non-exclusive hypotheses for the rosaceous species Potentilla puberula which comprises five ploidy levels (four, five, six, seven, and eight chromosome sets). The study has been carried out on 2,000 individuals from fifty populations in East Tyrol, Austria. Two of the proposed hypotheses, recurrent formation and divergence in habitat preferences, were potentially supported, whereas our data did not evidence mutual reproductive dependence to explain cytotype mixture. Migration remained as an alternative cause, but its effect has not been analysed at the time point when this report had been written.