Unusual diversity of apomictic mechanisms in a species of Miconia, Melastomataceae

Apomixis, the asexual formation of seeds, seems to be a reproductive alternative for many angiosperms, involving various pathways with different genetic and ecological consequences. It is common in some megadiverse tropical groups such as Melastomataceae, of which approximately 70% of the species studied so far in the tribe Miconieae are autonomous apomictics. Hence, Miconia appears to be a good model for the study of the embryological pathways associated with apomixis. In the present study, we analyzed the polyploid and autonomous apomictic M. fallax and compared its embryology to that of the diploid and sexual M. pepericarpa, both treelets species common in the Cerrado, the Neotropical savanna areas of Central Brazil. Ovule structure and basic megasporogenesis and megagametogenesis events were similar in both species. However, M. fallax showed exclusive features associated with apomixis: aposporous embryo sac development, with the parthenogenetic development of unreduced egg cells, autonomous endosperm formation, nucellar embryony and polyembryony. Moreover, both gametophytic and sporophytic apomixis occurred in parallel to the development of a sexual embryo sac, a rarely described condition, which probably confers a great reproductive flexibility to the species.