Evolution of apomixis in the Ranunculus cassubicus complex

Apomixis, i.e. asexual formation of seed (agamospermy), is a widespread mode of reproduction in Higher Plants. Despite of considerable information on genetic control of apomixis, understanding of the origin of apomixis in natural populations is still poor. Most authors regard hybridization and polyploidy as conditions for the origin of apomixis, other data indicate that apomixis might arise in diploid or autopolyploid populations as well. The project aims to study on a model system how apomictic lineages might have originated from sexual progenitors, and how this process is related to polyploidization. The study is based both on natural populations of Central European species of the Ranunculus cassubicus complex, where prior information on mode of reproduction, ploidy levels, and genetic control of apomixis (here most probably by one single gene), is available. In R. cassubicifolius, both diploid and auto-tetraploid populations are sexual, but apomixis might occur in single individuals. In R. carpaticola, both diploid sexuals and polyploid apomictic clones have been found. The polyploid apomicts might have been originated as allopolyploids from hybrids of R. carpaticola and R. cassubicifolius. The two species are useful as models to study whether apomixis occurs in autopolyploid, allopolyploid, or even diploid populations. Genetic diversity of natural populations will be analyzed with sensitive DNA markers (AFLPs and microsatellites) to assess genotypic diversity and sort out clones within populations. Molecular markers are also expected to show relationships of populations and taxa, and give evidence for the putative hybrid origin of apomicts. To address this question also material of other related microspecies will be studied. On individual level, a screening of the mode of reproduction will be performed by means of pollen studies, embryological analyses, and progeny tests. Studies on embryo sac development will show the presence of aposporous vs. meiotic embryo sacs; DNA analysis and ploidy level dtermination of progeny arisen from controlled crosses will show the degree of parthenogenesis and give final evidence for the functionality of apomixis in natural populations. The results are expected to give new insights in the evolution of apomictic lineages in natural populations. Further information on conditions, where the alteration from sexual to apomictic mode of reproduction occurs, will help to understand the role of hybridization and polyploidy for the origin of apomixis. This will be also essential for the understanding of genetic background of apomixis.

In flowering plants, the reproduction via asexually formed seed (apomixis) occurs in many widespread and abundant genera. The focus of the project was to study the mode of origin of asexual lineages, patterns and sources of genetic diversity on the population level in a biogeographical context, and the phylogenetic relationships of taxa. The results suggest that asexual (apomictic) reproduction does not evolve after mere genome doubling of sexual species, but requires additionally the combination of two different genomes via hybridization. The allelic and clonal diversity observed in asexual complexes is probably caused by both recent and historical factors, suggesting a considerable dynamics of speciation processes. Relationships among asexual taxa of the complex are highly reticulate. The study was based on natural populations of Central and Eastern European species of buttercups (Ranunculus cassubicus complex), where prior information on mode of reproduction, ploidy levels, and genetic control of asexual reproduction was already available. The complex includes two closely related species, one distributed in the Alps, being entirely sexual on diploid and tetraploid level, one distributed in the Carpathians with diploid sexual and polyploid asexual populations. Analysis of natural populations utilizing DNA fingerprinting markers, DNA content measurements, and progeny tests revealed that asexual lineages originated from hybridization events of the two sexual species, involving tetraploid populations from the alpine species and diploids from the Carpathian taxon. This was probably a single event within the last glacial periods, when climatic oszillations caused range fluctuations of species and offered opportunities for hybridization. The combination of genomes may have triggered asexual reproduction; polyploidization alone, as observed in the tetraploid sexuals of the Alps, increases genetic diversity, but is not connected with a shift to apomixis. Genetic diversity is harboured in two forms: whereas genotypic diversity within asexual populations is much lower than in sexual populations, allelic diversity within asexual individuals was much higher due to hybrid origin and accumulated mutations in the polyploid genome. A considerable clonal diversity among populations in the asexual lineages is probably caused both by mutations and by local facultative sexuality. This potential for a recurrent formation of new lineages implies a considerable dynamics in asexual complexes and may help to establish lineages in various habitats. Phylogenetic analyses of DNA sequences including other species of the genus confirmed a sister relationship of the two sexual taxa of the R. cassubicus complex, but manifested reticulate relationships within the whole complex. The results give insights into the circumstances and conditions for the evolution of apomictic lineages in natural populations and the biodiversity of apomictic complexes. The observed dynamics in the creation of genetic diversity imply a higher evolutionary potential than previously thought. Results will stimulate further research on ecological differentiation and biogeography of asexual complexes, but may be also of interest for a potential development and usage of asexually reproducing crops.