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Zygotic Polyembryony in Interspecific Hybrids of Carica papaya and C. cauliflora


A study of reproductive barriers limiting interspecific hybridization between Carica papaya L. and C. cauliflora Jacq. was undertaken in four reciprocal interspecific crosses using two different lines of each species. Particular attention was focused on determining whether polyembryonic clusters produced in these crosses were of maternal or zygotic origin. Prezygotic barriers were unimportant; pollen tube penetration and zygote formation were similar in intra- and interspecific crosses. Substantial postzygotic disruptions were observed, including disorganized growth and abortion of hybrid embryos and lack of normal endosperm development. In most crosses, disorganized embryos aborted before differentiating into polyembryonic structures. However, crosses employing UH345 (C. cauliflora) as female parent produced some embryos that developed to maturity (6 months), and, in these crosses, embryogenic proliferation from zygotic tissue became evident as early as the beginning of the 3rd month. There was no evidence of somatic embryogenesis from maternal tissues in any cross. Embryos rescued 3 to 6 months after pollination continued embryogenic growth in vitro on basal Murashige and Skoog (MS) medium and germinated on medium containing 0.2 mg BA/liter and 0.5 mg NAA/liter. Zymograms assayed for isocitrate dehydrogenase, malate dehydrogenase, and phosphoglucomutase activity confirmed the zygotic origin of tissues taken from in vitro cultures and recovered plantlets. Vigor, viability, and fertility (< 1% stainable pollen) of hybrids recovered from embryo culture were low. Chemical names used: 6-benzylaminopurine (BA); 1-napthaleneacetic acid (NAA).