Principles of Plant Genetics and Breeding. George Acquaah

alliances are indeed active and utilizing several molecular approaches to attempt the mapping and actual cloning of the apomixis loci in Tripsacum maize and/or other species. Consequently, research progress in this area has become difficult to monitor following the development of confidentiality agreements, plant patents, commercial‐academic research funding collaborations, etc.

However, some recent information and research progress remain in the public domain. Recent evaluation of several apomictic maize‐Tripsacum hybrids has identified the occurrence of one line that does not possess an intact Tr16 chromosome or the Mz6L‐Tr16L translocation (Figure B6.5a,b). Random amplified polymorphic deoxyribonucleic acid (RAPD) markers previously known to be associated to apomixis (Kindiger et al. 1996b) continue to be present in this germplasm. Cytological analysis of this particular chromosome suggests the chromosome carries the nucleolus organizing region and the Tr16L satellite. This small, iso‐chromosome‐ appearing entity may indeed possess the loci conferring apomixis in this material.

Figure B6.5 (a) (left). The satellite region of Tr16L (arrow) that confers apomixis in the V31 apomictic line. No normal or intact Tr16 is present in this line. (b) (right). An enlargement of the isochromosome with the nucleolus organizing region (NOR) and satellite regions identified.

      Much has been written, scientific and otherwise, on the benefits of apomixis or the potential “sexual revolution” in cereal and crop species. The most obvious benefit of introducing apomixis into crops would be to allow the selection of a particular individual and propagate it indefinitely by seed. Theoretically, in an apomictic system, hybrids could be maintained indefinitely if the first division restitution (FDR) events discussed above did not occur. Most likely, apomixis will be first utilized to stabilize genetic combinations that otherwise could not occur naturally or are difficult or impossible to maintain in nature. The facts are that even if a prototype apomictic system is generated, such as in “apomictic maize” US patent No. 5 710 367, traditional breeding and gene transfer through backcrossing is questionable. Pollen sterility is, so far, the rule for all such hybrids and is likely caused by the presence of the same Tripsacum chromosome detailed in an earlier study (Maguire 1957, 1960). In addition, a near‐obligate level of apomictic seed development does not provide an opportunity to transfer apomixis to other maize germplasm. Also, in this apomictic maize germplasm, seed set remains poor and uncharacterized problems associated with endosperm development persist. Essentially, in the case of the apomictic maize patent, a closed breeding system exists.

       Pitfalls in the development of an apomictic maize

       FDR in apomictic maize‐ Tripsacum hybrids

      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.

       Potential advantages of apomictic hybrid corn

      Utilizing a traditional hybrid corn production methodology, two inbred lines are typically required to produce an F₁ hybrid. At the present time, apomictic hybrids would likely utilize one or both inbred lines that would carry the necessary genes and genetics to develop a true breeding, apomictic F₁ hybrid corn cultivar. Land, labor, and storage space are also required to maintain these inbred lines. If true breeding, apomictic hybrids can be developed, the yearly seed increase of inbreds, the generation of hybrids, the necessary time allowed for such production, land, fertilizer, and required field isolations necessary for producing a hybrid corn line could be omitted. With apomictic hybrid corn, seed generated from that crop would reproduce seed and individuals possessing the identical genetics of the parental hybrid. As such, the development of an apomictic seed crop from an apomictic hybrid would lead to a substantial savings in cost and time to commercial producers and hopefully a decrease in seed price to farmers.

      Under present agricultural