Format: ORAL

Elvira Hrandl1, Claudia Paetzold1,2, Birthe H. Barke1, Fuad Bahrul Ulum1,3, Ladislav Hodac1,4, Eleni Syngelaki1

1 Department of Systematics, Biodiversity and Evolution of Plants (with herbarium), University of Goettingen, Germany 2 Department of Botany and Molecular Evolution, Senckenberg Research Institute, Frankfurt am Main, Germany 3 Biology Department, Mathematic and Sciences Faculty, Jember University, Jember, Indonesia 4 Max Planck Institute for Biogeochemistry, Department Biogeochemical Integration, Jena, Germany

The predominance of sexual reproduction in eukaryotes despite high costs is still an evolutionary enigma. Major theories propose that sex could provide a two-fold DNA restoration tool, ie. repair of oxidative damage during meiosis, and selective elimination of harmful mutations via recombination. In angiosperms only about 2.3% of genera and 1% of species reproduce via apomixis (ie., reproduction via asexually formed seeds). Most natural apomicts are polyploids, but the reasons for this association remain still unclear. Previous hypotheses suggested that genome duplication would be the functional trigger for apomixis by altering gene expression and epigenetic control. However, several studies indicate that apomixis already starts in diploid populations at low frequencies and can be inherited by haploid gametes. Transcriptomics on diploid aposporous F2 hybrids suggest that a lucky combination of mutations in genes related to reproductive development could be a functional trigger of apomeiosis. However, polyploidy has indirect positive effects to establish apomictic lineages. First, polyploidy can lower abiotic stress, e.g., light stress, which will reduce the trigger of oxidative stress for initiation of meiosis, thereby favouring facultative asexual pathways. Second, polyploidy can buffer short-term effects of recessive deleterious mutations, and only a little bit of facultative recombination (6%) is efficient to for purifying selection even in big polyploid genomes. Third, polyploidy can facilitate niche shifts by adaptations to more extreme environments by epigenetic changes and phenotypic plasiticity, which will be demonstrated by DNA methylation and gene expression analysis. Fourth, polyploidy results in a breakdown of self-compatibility, which allows for self-fertilization and uniparental reproduction in pseudogamous apomicts, which is an advantage for colonization. Experimental and molecular work on Ranunculus supports these hypotheses. We conclude that hybridity rather than polyploidy is the primary functional trigger for apomixis, but polyploidy is essential for establishment of apomictic lineages in natural populations.