The evolutionary establishment of apomixis in hybrids of the Ranunculus auricomus complex
Apomixis, the asexual reproduction via seed, appears in natural plant populations mostly in polyploid hybrids. The actual effects of hybridization and polyploidy on development, however, and evolutionary steps towards functional apomixis are still unclear. Two main lines of hypotheses have been proposed to act as a functional trigger for spontaneous emergence of apomictic initial cells as the first step: (1) hybridization causes alteration of expression patterns of genes controlling normally the sexual development, thereby activating germ cell fate in otherwise somatic cells of the ovule; (2) hybridization causes disturbances or failure of meiosis and megasporogenesis, thereby suppressing the sexual pathway. Thus, selection will favour an alternative, otherwise ineffective asexual pathway ("escape from sterility"). Polyploidy is hypothesized to have gene dosage effects on quantitative expression of apomixis and to enhance parthenogenetic development of unreduced egg cells. In a precursor project on the model system (Ranunculus auricomus complex) first steps of apomixis, ie. aposporous initial cells, appeared spontaneously in diploid and polyploid synthetic F1 hybrids, but functional apomictic seeds developed only in polyploids. Here we propose an expanded experimental approach to continue this study on the next hybrid generation produced from sibling crosses. We will test the hypothesis that F2 sibling crosses will show increased frequencies of apomictic seed formation because of dosage effects of genetic control factors. We will study female development via microscopy and seed flow cytometry on polyploid hybrids, and on artificially polyploidized diploid hybrids. To test for a correlation of a meiotic disturbance with frequencies of apospory, male and female meiosis will be studied microscopically and cytogenetically with FISH techniques. We expect comprehensive insights into sexual and apomictic development and a deeper understanding of the evolution of apomixis in plant populations.