The relation of meiotic behaviour to hybridity, polyploidy and apomixis in the Ranunculus auricomus complex (Ranunculaceae)
Abstract Background Hybridization and polyploidization are powerful evolutionary factors that are associated with manifold developmental changes in plants such as irregular progression of meiosis and sporogenesis. The emergence of apomixis, which is asexual reproduction via seeds, is supposed to be connected to these factors and was often regarded as an escape from hybrid sterility. However, the functional trigger of apomixis is still unclear. Recently formed di- and polyploid Ranunculus hybrids as well as their parental species were analysed for their modes of mega- and microsporogenesis by microscopy. Chromosomal configurations during male meiosis were screened for abnormalities. Developmental abnormalities were documented qualitatively and collected quantitatively for statistical evaluations. Results Allopolyploids showed significantly higher frequencies of erroneous microsporogenesis than homoploid hybrid plants. Among diploids, F 2 hybrids had significantly more disturbed meiosis than F 1 hybrids and parental plants. Chromosomal aberrations included laggard chromosomes, chromatin bridges and disoriented spindle activities. Meiotic failure appeared to be much more frequent in female compared to male development. Conclusions Results suggest diverging selective pressures on female and male meiosis, with only minor effects of hybridity on male development, but fatal effects on the course of megasporogenesis. Hence, pollen development continues without major alterations, while selection will favour alternatives to the female meiotic pathway. Relation of investigated meiotic errors with the observed occurrence of apospory in Ranunculus hybrids identifies disturbed female meiosis as potential elicitor of apomixis in order to rescue these plants from hybrid sterility. Meiotic disturbance appears to be stronger in neopolyploids than in homoploid hybrids, which may contribute to the prevalence of apomixis in polyploid plants.