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.
The 46→56→38 apomictic transfer pathway
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.
Transfer of apomixis from Tripsacum to maize
Apomixis is an asexual mode of reproduction through the seed, and is a genetically controlled process that bypasses female meiosis and fertilization to produce seed identical to the maternal parent. Apomixis is common in polyploid species where reproduction through classic meiotic reproductive events often result in high levels of seed abortion. Apomictic seed development is often viewed as an escape mechanism whereby ovule abortion in polyploid species can be reduced or eliminated through the omission of meiosis. Essentially, three forms of gametophytic apomixis are recognized: diplospory, apospory, and adventitious embryony. In Tripsacum, the dominating form of apomixis is diplosporous pseudogamy of the Antennaria type. In this form of apomixis, the embryo sac originates from the megaspore mother cell either directly by mitosis or following an interrupted meiosis. In addition, an infrequent occurrence of a Taraxacum type of diplosporous pseudogamy has also been observed (Burson et al. 1990).
Tripsacum has been suggested as a model system for the study of apomixis (Bantin et al. 2001). As of this report, the prevailing wisdom suggests that apomixis (at least for Tripsacum) is controlled by no more than one or two genes, likely linked on a particular Tripsacum chromosome (Leblanc et al. 1995; Grimanelli et al. 1998). These results seem to be in agreement with molecular studies focused on understanding apomixis expression in other species (Noyes and Reiseberg 2000; Albertini et al. 2001).
Cytogenetic
