Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 296

This pathway is the earliest known pathway of maize‐Tripsacum hybridization first reported by Mangelsdorf and Reeves (1939) and repeated by several others. When crossing a diploid maize (2n = 2x = 20Mz) by a diploid Tripsacum (2n = 2x = 36Tr), the F₁ hybrid consists of 10Mz + 18Tr chromosomes. Backcrossing this hybrid by diploid maize typically results in the fertilization of an unreduced egg by the pollen source. This partially apomictic event (a 2n + n mating) results in what is called a B_III derived hybrid (Bashaw and Hignight 1990) and now possesses 20Mz + 18Tr chromosomes. This behavior is also commonly observed in apomictic tetraploid Tripsacum dactyloides (Kindiger and Dewald 1997) which raises an interesting question regarding the potential of diploid Tripsacum to possess, but not utilize the mechanisms of apomictic reproduction. In a subsequent backcross of this 38‐chromosome individual with diploid maize, individuals possessing 20Mz + 1 thru 17Tr. chromosomes are generated. In some instances, the maize constitution can also be slightly aneuploid, 20 + 1 or 2 maize chromosomes. At this point, the predisposition of these individuals is to rapidly lose most if not all of their Tripsacum chromosomes following additional backcrossing. The end result of continued backcrossing with maize is the recovery of maize, often completely lacking any level of Tripsacum genome introgression through homoeologous pairing and/or recombination. Though, in this pathway, Tripsacum introgression is rare, a method for enhancing the opportunity for introgression has been suggested but not pursued (Kindiger and Beckett 1989).