Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 298

This sexual or non‐apomictic pathway, as discussed by Harlan and deWet, is believed to offer the greatest opportunity for Tripsacum introgression into maize and represents results of an early attempt to transfer apomixis to maize (Petrov et al. 1979, 1984). In this pathway, a diploid maize (2n = 2x = 20Mz) is crossed with a tetraploid Tripsacum resource (2n = 4x = 72Tr). The resultant F₁ hybrid possesses 10Mz + 36Tr chromosomes. To date, published reports indicate all of these hybrids are pollen sterile and vary considerably in their levels of seed fertility. In this particular pathway, when the 46‐chromosome F₁ is backcrossed to diploid maize, 56‐chromosome individuals result following fertilization of an unreduced egg. As in the prior pathways, this result is generated by a 2n + n mating event. Offspring of the 56‐chromosome individual following a second backcross to maize generally possess 38 chromosomes (20Mz + 18Tr) and resemble those discussed in the 28→38→20 pathway above. The generation of progeny with 38 chromosomes is the result of meiosis in the developing megaspore. In this instance, the maize and Tripsacum complements pair with their homologous sets (Mz–Mz, Tr–Tr). Following a complete occurrence of meiosis I and II divisions, the result is a reduced egg having 10Mz + 18Tr chromosomes, which when backcrossed by a diploid maize, results in progeny having 20Mz + 18Tr chromosomes. Almost exclusively, the 38‐chromosome individuals no longer express any level or form of an apomictic reproductive mechanism, and subsequent backcrossing to maize results in the recovery of individuals possessing 20Mz and a varying number of Tripsacum chromosomes. Upon backcrossing, the 38‐chromosome individuals behave in an identical manner to their 28‐chromosome cousins represented in the 28→38→20 pathway. Generally, Tripsacum introgression by homoeologous pairing and recombination does not occur and genetic transfer of Tripsacum genes to maize is not accomplished.