Variability and coexistence of sexual and apomictic embryo sac development pathways in Fragaria x ananassa cv. ‘Camino real’

While most apomictic plants have a defined type of gametophytic apomixis, the cultivars of strawberry reported various mechanisms of unreduced embryo sac formation. In this study, the main events of megasporogenesis and megagametogenesis during the sexual and apomictic processes of embryo sac formation were analyzed in the cultivar "Camino real". Ovules at different developmental stages were sectioned and cleared and were observed using light microscopy and confocal laser scanning microscopy. Archesporial cells developed asynchronously and had diverse fates. These cells can alternatively be selected as mother cells, apomeiotic initials, persist in a resting stage, or degenerate. Other cells in the nucellus are able to acquire a functional megaspore identity. Apomixis did not involve callose deposition in their initial cells and derivatives. The initiation of apomixis was variable in time and eventually related to the interruption of the sexual process at pre-meiotic and post-meiotic stages of the ovule development. The occurrence of sexuality, apospory and diplospory of Antennaria and Taraxacum types is conserved across cultivars. The comparison with wild Fragaria species suggests that hybridization did not directly cause the observed phenotypic variation in apomixis mechanisms. An overview of the mode and time of unreduced sac formation can contribute to the development and management of clonal seed production in strawberry. The complex phenotypic variation additionally remains to be fully explained in light of emerging concepts regarding the regulatory mechanisms of plant reproductive development.