Preliminary results on embryo rescue for circumventing hybridization barriers in Asparagus

Garden asparagus, Asparagus officinalis, is reproductively isolated from a related ornamental species with potential breeding value, Asparagus densifl orus cv. Sprengeri, by pre- and post-zygotic barriers. The latter barrier o perates at the endosperm level five days after pollination in A. officinali s xA. densiflorus crosses. To try to circumvent this barrier, in vitro embr yo rescue using ovule and ovary cultures was tested. Controlled interspecif ic crosses were made and 2,032 ovules and 826 ovaries were cultured three d ays after pollination under various culture media and incubation conditions . Ovaries cultured for 60 days became red (similar to mature fruits), but s eed formation was incomplete. Transfer of ovules to other media was necessa ry to promote embryo development. The interspecific embryos increased their length from 35 mum at the initiation of culture to 1,900 mum after 120 day s of culture, but seedlings were not obtained. Histological studies reveale d differentiation of protoderm only. The possible causes of the failure of the embryos to complete differentiation and morphogenesis are discussed.