Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 299

Though not addressed in Harlan and de Wet's 1977 research, this pathway is similar enough and worthwhile to discuss with regard to its relevance to apomixis. First published by Petrov and colleagues as early as 1979, and replicated in similar style by others, a diploid or tetraploid maize line is pollinated by a tetraploid, apomictic T. dactyloides clone (Petrov et al. 1979, 1984). If a diploid maize line is utilized, the resultant F₁ 46‐chromosome hybrid possesses 10Mz and 36Tr chromosomes. Upon backcrossing with diploid maize, both apomictic 46‐chromosome and 56‐chromosome (20Mz + 36Tr) individuals can be obtained. The 46‐chromosome offspring are products of apomixis. The 56‐chromosome offspring are products of an unreduced egg being fertilized by the diploid maize pollen source, another 2n + n mating event. Often, these individuals exhibit polyembryony that results in the generation of “twins” being obtained from a single seed (Figure B6.3). In some instances, these polyembryonic events give rise to 46–46 pairs of twins (each apomictic clones of the other), 46–56 “twins,” one arising from a unfertilized reduced egg, the other arising from a fertilized unreduced egg; and in some instances varying combinations of 46‐46‐46 or 46‐46‐56 triplets. Typically, as seedlings, the 56‐chromosome individuals are more vigorous than their 46‐chromosome sibs.

Figure B6.3 Polyembryony expression in germinating seed of an apomictic 46‐chromosome F₁ maize‐Tripsacum hybrid. Note in the pairs, one seedling is often larger and more vigorous than its sib. The larger sib of the pair is often the product of a 2n + n mating event.

Backcrossing the 46‐chromosome individuals by maize, repeats the above cycle. Upon backcrossing the 56‐chromsome individuals with maize, three types of progeny can be observed. Typically, progeny having 56 chromosomes are generated. However, in some instances, 2n + n matings occur, giving rise to individuals possessing 66 chromosomes (30Mz + 36Tr). Occasionally, a reduced egg will be generated and may or may not be fertilized by the available maize pollen. In rare instances of non‐fertilization, a 28‐chromosome individual is generated (10Mz + 18Tr). In instances whereby the maize pollen fertilizes the reduced egg, 38‐chromosome individuals are obtained (20Mz + 18Tr). Generally, individuals possessing 38 chromosomes, rather than 28 chromosomes, are the most common product. What is unique about this pathway is that occasionally, the 38‐chromosome individuals retain all the elements of apomixis which were present in the Tripsacum paternal parent and the F₁ and BC1 individuals. The retention of apomixis to this 38‐chromosome level has been well documented and repeated in several laboratories (Petrov et al. 1979, 1984; Leblanc et al. 1996; Kindiger and Sokolov 1997). In addition, the occurrence of 2n + n matings, polyembryony and variation in apomixis expression is quite similar to that found in apomictic Tripsacum (Kindiger et al. 1996a).

Following the generation and confirmation of apomictic 38‐chromsome individuals (20Mz + 18Tr), it is apparently a difficult and uncommon occurrence to generate and maintain apomixis in backcross generations that have fewer Tripsacum chromosomes. Only one report has been published where apomictic individuals possessing only 9Tr chromosomes were obtained (Kindiger et al. 1996b). Generally, by 2n + n mating events, the 38‐chromosome individuals produce only apomictic 38‐chromosome progeny and 48‐chromsome progeny. Backcrossing the 48‐chromosome individuals results in 48‐chromosome apomictics and 58‐chromosome apomictics. This accumulation of maize genomes continues until a point is achieved where the additional maize genomes eventually shift the individual from an apomictic reproductive mechanism to a traditional sexual mode of reproduction, whence, apomixis is never again attained. This commonly occurs when five or six doses (50–60 maize chromosomes) are present. The result of a 78‐chromosome individual (60Mz + 18Tr) losing apomixis is the return of meiosis and a highly seed‐sterile individual producing an array of highly maize‐like aneuploids with a random set of Tripsacum chromosomes. Backcrossing these individuals, that are typically pollen sterile, generally results in the recovery of diploid maize lines with or without any Tr chromosomes. To date, the apomictic maize‐Tripsacum line possessing 39 chromosomes (30Mz + 9Tr) represents the most advanced level of apomixis transfer to maize. An array of various ear types generated from a series of maize‐Tripsacum hybrids is provided in Figure B6.4.

Figure B6.4 A series of maize‐Tripsacum ear types. Left to right: dent corn; an apomictic 39‐chromosome hybrid; an apomictic 38‐chromosome hybrid (Yudin); an apomictic 56‐chromosome hybrid; two apomictic 46‐chromosome hybrids; and two tetraploid Tripsacum dactyloides.