Apomixis in flowering plants: Developmental and evolutionary considerations.

Apomixis refers to a set of reproductive mechanisms that invariably rely on avoiding meiotic reduction and fertilization of the egg cell to generate clonal seeds. After having long been considered a strictly asexual oddity leading to extinction, the integration of more than 100 years of embryological, genetic, molecular, and ecological research has revealed apomixis as a widely spread component of the dynamic processes that shape flowering plant evolution. Apomixis involves several flexible and versatile developmental pathways that can be combined within the ovule to produce offspring. Here we review the large body of classic and contemporaneous contributions that have addressed unreduced gamete formation, haploid induction, and parthenogenesis in flowering plants. We emphasize similarities and differences between sexual and apomictic reproduction, and highlight their implications for the evolutionary emergence of asexual reproduction through seeds. On the basis of these comparisons, we propose a model that associates the developmental origin of apomixis to a dynamic epigenetic landscape, in which environmental fluctuations reversibly influence female reproductive development through mechanisms of hybridization and polyploidization.