Self-fertility and polyembryony in South American yellow trumpet trees (Handroanthus chrysotrichus and H. ochraceus, Bignoniaceae): a histological study of postpollination events

Abstract

Handroanthus chrysotrichus shows pollination-dependent self-fertility, polyploidy, and adventitious polyembryony, and it is closely related to H ochraceus, for which apparently conflicting reports of self-incompatibility and apomixis have been published The present study aims to investigate the polyembryony in these species by means of histological analysis of ovule/seed development in unpollinated, selfed, and crossed pistils/fruits (in H. chrysotrichus only) as well as seed germination experiments. Experimental pollinations were carried out to evaluate breeding systems in the studied populations, and the results indicated self-fertility in both species. Adventitious embryo precursor cells (AEPs) were formed in the ovules of unpollinated, selfed, and crossed pistils. However, unfertilized ovules never develop into seeds, and fertilization/endosperm initiation clearly stimulates the formation of AEPs in pollinated pistils The inability of AEP-bearing unfertilized ovules to initiate endospermogenesis clearly shows that fertilization is needed for adventitious embryo development Consequently, formation of AEPs is required but is not sufficient for apomictic reproduction in H chrysotrichus. Analysis of the positions of multiple embryos in the endosperm indicated that fertilized ovules are able to develop into seeds even in the absence of a zygotic embryo. The development of AEPs in ovules of H. chrysotrtchus foregoes the stage in which activation of selfed pistil rejection takes place in H unpetiginosus, a species with late-acting self-incompatibility. Our study supports the hypothesis that the self-fertility in H chrysotrichus (and perhaps also in H. ochraceus) resulted from the emergence of pseudogamous apomixis, favored by the physiological peculiarities of the late-acting self-incompatibility and possibly related to polyploidy