The embryology of Xanthoceras and its phylogenetic implications

Recent molecular analyses found the Chinese monotypic genus Xanthoceras to be either a sister genus to the rest of a broadly defined Sapindaceae sensu lato (s.l.) including Aceraceae and Hippocastanaceae or the only genus of the Xanthoceraceae. Here we investigate for the first time the embryology of Xanthoceras and compare the embryological data with related taxa to aid in understanding the phylogenetic relationships of this genus. The results indicate that Xanthoceras shares many embryological features with Sapindaceae sensu stricto (s.s.), Aceraceae, and Hippocastanaceae, including a glandular tapetum, simultaneous microsporogenesis, three-colporate pollen grains, a bitegmic and crassinucellate ovule, Polygonum type of embryo sac development, endosperm formation of the nuclear type, and exalbuminous mature seeds. The wide range of similarities in embryological features suggests that Xanthoceras is closely related to other members of Sapindaceae s.l. However, Xanthoceras differs from other Sapindaceae in several morphological characteristics, for instance, five to eight ovules in each locule in Xanthoceras versus one to two ovules in each locule in other Sapindaceae s.l., and five golden horn-like appendages present on both staminate and hermaphrodite flowers of Xanthoceras versus absent on the flowers of other Sapindaceae s.l. We propose that Xanthoceras merits recognition at the subfamily level as Xanthoceroideae, rather than at the family level as Xanthoceraceae. The present study also reveals that late-acting ovarian self-incompatibility (OSI) occurs in X. sorbifolium and that postzygotic OSI contributes to the self-sterility of the species.