Environmental and biotic influences on the maintenance of apomixis

Abstract

Most plant species that reproduce by the production of clonal seed (apomicts) retain some capacity for sexual reproduction (facultative apomixis). The frequency of this residual sexual reproduction in apomicts varies, even within a species, and this variation has often been attributed to the environment. This study aims to investigate the role of the environment on the expression of apomixis in the facultative apomict, Hieracium pilosella. H. pilosella has been chosen for this study because it is a widespread, exotic, species that possesses a facultative reproductive system. Additionally, there is interest in its' reproductive pathway as a potential source of genetic material for apomictic cropping technology. Field experiments examining the reproductive mode of Hieracium pilosella found there was a positive relationship between the frequency of sexual reproduction, and both temperature and rainfall for the seven days prior to pollination (at capitulum anthesis). The frequency of sex in all experiments was determined by pollinating H pilosella with a marker pollen donor, either the morphologically distinct H aurantiacum, or transgenic accessions of Hieracium spp. with a kanamycin resistance insert, and using hybrid characteristics as a measure. Both field and glasshouse studies demonstrated a genetic component to the frequency of sex; while neither photoperiod, nutrient level, or the presence of a powdery mildew (Erysiphe spp.) had a significant effect on residual sexuality. Additionally, it was demonstrated that sexual events, although contributing only 0.2 6.0 % of total seed progeny in apomictic populations, are important in determining population genetic diversity. The existence a flexible reproductive strategy indicates that both reproductive pathways are beneficial, or at least the cost of reproducing sexually (as predicted by evolutionary models) is insufficient to lead to its' loss. The maintenance of this rate of sexual reproduction, and the potential for it to generate population variation, may increase the complexity of both the transfer of genetic material from Hieracium pilosella to crop species, and biological control.