Polyembryonyin Non-apomictic Citrus Genotypes

Abstract

Adventitious embryony from nucellar cells is the mechanism leading to apomixis in citrus. However, singular cases of polyembryony have been reported in non-apomictic genotypes as a consequence of 2x×4x hybridisations and in vitro culture of isolated nucellus. The origin of the plants obtained as a consequence of these two processes is still unclear. In this work, we systematically analyzed the genetic structure (ploidy and allelic constitution at SSR locus) of plants obtained from polyembryonic seeds arising from 2x×4x sexual hybridisations or regenerated from nucellus culture in vitro of different non-apomictic citrus genotypes. Histological studies were also conduced to try to identify the initiation process of polyembryony in nonapomictic genotypes. We demonstrate that all plants obtained from the same undeveloped seed in 2x×4x hybridisations resulted from fission of the original zygotic embryo. Also, the plants obtained from in vitro culture of nucellus were recovered by somatic embryogenesis from cells having the same genotype as the zygotic embryos of the same seed. It appears that in non-apomictic citrus, proembryos or embryogenic cells are formed by fission of the original zygotic embryo and that the development of these adventitious embryos, normally hampered, can take place in vivo or in vitro as result of two different mechanisms that prevent the dominance of the initial zygotic embryo.