Polyembryony

Seeds of polyembryonic mango cultivars characteristically contain several nucellar embryos, and may also contain a zygotic embryo. While nucellar seedlings are preferred as rootstocks for mango because of their uniformity, the breeder, on the other hand, is generally interested in sexual seedlings for the selection of improved rootstocks. Until recently, crosses involving poly-embryonic cultivars as the maternal parents were generally not performed, since reliable methods for identifying zygotic seedlings were not available. The use of polymorphic enzyme systems (isozymes) (Degani et al., 1990, 1992) to identify zygotic seedlings (Schnell and Knight, 1992; Truscott, 1992; Degani et al., 1993) is based on the fact that nucellar seedlings should have the same isozyme alleles as the maternal parent. A variation at a locus coding for an enzyme indicates that the plant has originated by sexual reproduction. Zygotic seedlings arising from self-pollination are distinguished from nucel-lar seedlings by being homozygous at one or more loci at which the female parent is heterozygous. Statistically, when three or four heterozygous loci are examined, up to 88 or 94%, respectively, of the selfed zygotic seedlings are identifiable (Moore and Castle, 1988). Cross-pollination by another cultivar of the same genotype is equivalent to self-pollination. Zygotic seedlings from cross-pollination are distinguishable from those resulting from self-pollination if they express an allele not carried by the female parent.

The frequency of occurrence of zygotic seedlings varies among the poly-embryonic mango cultivars, i.e. 22% with '13-1' (Degani et al., 1993), 20 and 24% with 'Turpentine' (Degani et al., 1993 and Schnell and Knight, 1992, respectively), 2 and 4% with 'Sabre' (Truscott, 1992 and Schnell and Knight,

1992, respectively) and 36 and 64% with 'Madu' and 'Golek', respectively (Schnell and Knight, 1992).

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