Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 301

One unique attribute found only in the apomictic backcross hybrids, irrespective of their possessing a 38‐chromosome (20Mz + 18Tr) or 39‐chromosome (30Mz + 9Tr) constitution, is the maintenance of their genetic composition. Theoretically, apomictic individuals will reproduce a genetic copy of themselves through the seed they produce. However, studies focused on this behavior in maize‐Tripsacum hybrids have proven this will not necessarily be the case.

First division restitution (FDR) can lead to chromosome doubling in intergeneric hybrids of various grasses, of which many are of polyploid or of complex polyploid origin (Xu and Joppa 1995). Consequently, unreduced gametes bypass the reductional division and are generated only by a normal equational division. This behavior can occur during megasporogenesis or microsporogenesis; however, in the case of pollen sterile apomictic maize‐Tripsacum hybrids, this behavior impacts genetic change by way of the megaspore. Studies utilizing both molecular and phenotypic evaluations have suggested that an FDR event can occur in apomictic Tripsacum dactyloides resulting in genome alterations in an apomictic genotype (Kindiger and Dewald 1996). This behavior has been visually verified by the occasional discovery of major chromosome rearrangements involving Mz6L, Tr16L, Mz2S, and other unknown maize and Tripsacum chromosomes during routine cytogenetic investigations involving both apomictic 38‐ and 39‐chromosome maize‐Tripsacum hybrids. This component of diplosporous apomixis in Tripsacum and maize‐Tripsacum hybrids has not been well studied or addressed.

Evaluation of both apomictic 38‐chromosome and 39‐chromosome individuals that have been increased over several locations for a minimum of 15 years (Figure B6.6), has shown that a low level of genome reorganization does occur, sometimes resulting in genome loss. This behavior is well documented in some of the apomictic Petrov materials and can only be visualized following the passage of time. The generation of a Mz6L‐Tr16L translocation is quite likely a product of this type of behavior (Kindiger and Dewald 1996; Kindiger et al. 1996b). In addition, a long‐term (15+ years) selection program in a 38‐chromosome (20Mz + 18Tr) apomictic genotype has resulted in a pollen sterile, apomictic line that has a near‐perfect resemblance to maize in its plant and ear characteristics (Figure B6.7a,b). Consequently, the occurrence of FDR events in apomictic maize‐Tripsacum hybrid should generate some concern regarding the development of apomictic off‐types which, over time, would increase the genetic non‐uniformity of a particular genotype.

Figure B6.6 A series of 39‐chromosome maize‐Tripsacum (30Mz + 9Tr) hybrids growing at the Japanese National Livestock and Grassland Research Institute, Nishinasuno, Japan.

Figure B6.7 (a) (left). A highly maize‐like 38‐chromosome apomictic maize‐Tripsacum hybrid. This selection has none or few tillers and exhibits a distinct maize phenotype. (b) (right). A top and second ear taken from one of these highly maize‐like apomictic individuals. Note the eight rows on the ear is rarely found in other apomictic maize‐Tripsacum hybrids.