Apomixis beyond trees in the Brazilian savanna: new insights from the orchid Zygopetalum mackayi

Abstract Knowledge on the biology of apomictic tropical plants is still fragmentary. Studies on trees from the Brazilian savanna show apomixis is mainly sporophytic, facultative and associated with polyploidy and polyembryony. Here we show, for the first time, how reproduction mode is associated with chromosome numbers and polyembryony in an herbaceous plant from the Brazilian savanna, the orchid Zygopetalum mackayi. We described megasporogenesis and megagametogenesis in all three cytotypes of this species and clarify apomixis is strictly sporophytic, facultative and restricted to triploids and tetrapoloids, while seed formation is strictly sexual in diploids. Polyembryony is mainly a result of apomixis, but also a consequence of the occurrence of multiple archesporium in all cytotypes. Fruit set is higher in tetraploids compared to other cytotypes and suggest the occurrence of a late-acting self-incompatibility system in diploids. Contrary to other Brazilian savanna species, apomixis in Z. mackayi does not allow for reproductive assurance. We hypothesize range expansion of tetraploids as a consequence of higher fruit set compared to diploids and polyembryony associated with putative physiological plasticity increased by polyploidy. Biological consequences of apomixis may be more diverse in tropical biomes than previously described for temperate species.