Apomixis in Systematics, Evolution and Phylogenetics of Angiosperms: Current Developments and Prospects

Apomixis, the reproduction via asexually formed seed, is taxonomically scattered in angiosperms. Here we review the major developmental pathways to apomixis and the occurrences on the level of orders and families. We provide surveys of large families for which occurrences and evolutionary pathways of apomixis are well documented (Poaceae, Ranunculaceae, Plumbaginaceae, Rosaceae, Brassicaceae, Rutaceae, and Asteraceae). Molecular phylogenetic, phylogenomic, and cytogenetic studies have confirmed that apomixis frequently arises in hybrids, with or without polyploidy. Multiple origins, different developmental pathways, and genetic/epigenetic control mechanisms confirm that apomixis is a derived trait. Pollen functions are in many taxa maintained for fertilization of polar nuclei and proper endosperm development, which allows also for maintenance of facultative sexuality and further intercrossing of lineages. This way, apomixis often results in huge and dynamic complexes of numerous hybrid genotypes and phenotypes with highly reticulate relationships. Such complexes are successful to establish in various habitats and geographical regions. Expression of apomixis appears to be in some genera influenced by environmental conditions. The diversity of evolutionary pathways is reflected in various, genus-wise taxonomic treatments. Future research is needed to understand the short-term evolutionary dynamics, the functional background for apomixis, and the long-term evolutionary fates of apomictic lineages.