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Genetic proof of the occurrence of mono and dizygotic hybrid twins in Citrus rootstocks

Herculano Penna Medina Filho; Rita Bordignon; Rosa Maria Lizana Ballve; Walter José Siqueira

Instituto Agronômico de Campinas, Seção de Genética, Caixa Postal 28, 13001-970 Campinas, SP, Brasil. Send correspondence to H.P.M.F.

ABSTRACT

Nucellar cells which develop within the female gametophyte give rise to adventitious embryos [Strassburger (Jenaische Ztschr. fur Naturw. 12: 647-667, 1878)]. Monozygotic polyembryony by cleavage of the sexual embryo was the explanation of Frost (Current Sci., Special Number, Genetics: 24-27, 1938) for 10 pairs of phenotypically identical twin hybrids from single seeds. A third type of polyembryony was suggested by Bacchi (Bot. Gazet. 105: 221-225, 1943) in Citrus paradise after cytological observations of two gametophytes in a single ovule that would lead to dizygotic, non-identical twins.
This investigation presents genetic proof of the occurrence of the three types of polyembryony by genetic analysis of Pgi-1, Pgm-1, Prxa-1, Got-1 and Got-2 isozyme loci in single seed plants derived from controlled crosses.
Nearly 1000 seedlings were genotyped revealing 633 hybrids. Among them, 12 twin pairs and one triplet were identified: the Citrus sunki x Citrus aurantium triplet and, of the twins (C. sunki x Citrus limonia, C. sunki x C. aurantium, C. sunki x Poncirus trifoliata), 10 monozygotic and two (C. sunki x C. aurantium and C. limonia x C. aurantium) dizygotic. By ascertaining the contribution of the female haploid genotype, it was possible to conclude that genetically distinct female gametes participated in the formation of each plant of the two dizygotic twin pairs, thus proving the reproductive realization of Bacchi's carlier cytological observation and ruling out other mechanisms that would generate non-identical twins.
The seed containing the C. limonia x C. aurantium dizygotics also produced a nucellar plant. This phenomenon, infrequent but nonetheless important, has obvious implications for the evolution of the genus and for citrus breeding.

Keywords: mono; hybrid twins; dizygotic hybrid twins; Citrus rootstocks.

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