Principles of Plant Genetics and Breeding. George Acquaah

Figure B6.2 Hybrid seed set utilizing Ladyfinger popcorn as the maternal parent when pollinated by tetraploid T. dactyloides. Over 100 F₁ seeds can be readily obtained when an appropriate maize parent is utilized in the cross.

      The F₁ hybrids are completely pollen sterile and microsporogenesis is associated with a varying array of meiotic anomalies (Kindiger 1993) and vary in seed fertility from completely sterile to highly seed fertile (Harlan and de Wet 1977). To date, all seed fertile hybrids generated from tetraploid Tripsacum dactyloides resources exhibit some level of apomictic expression, which following backcrossing with maize, is often lost. The most common or sexual pathway of genomic change in a series of maize‐Tripsacum backcross hybrids has been clearly described by Harlan and de Wet (1977). Comparative genetics and other approaches that may result in the transfer of Tripsacum traits to maize, including apomixis are described below.

       Gene transfer from Tripsacum to maize

      Recent and past research strongly suggests that there is little homeology between the genomes of Tripsacum and maize. Maguire (1962), utilizing a set of recessive phenotypic maize markers, suggested that only maize chromosomes 2, 5, 8, and 9 have a potential for pairing and recombination and for gene introgression with Tripsacum. Additional research has confirmed conservation of loci specific to pistil development between maize and Tripsacum genomes (Kindiger et al. 1995; Li et al. 1997). Maguire (1957, 1960) successfully generated and identified a naturally occurring recombination event between an unknown Tripsacum chromosome and the short arm of maize chromosome 2. Studies using B‐A translocation deletion lines suggested that the Mz9S region could pair and recombine with Tripsacum chromosome 5. Genomic in situ hybridization (GISH) studies have also strongly suggested that only three regions of maize chromosomes have homeology with the Tripsacum genome: the sub‐terminal regions of Mz2S, Mz6L, and Mz8L (Poggio et al. 1999). These regions correspond well with groups of conserved restriction fragment length polymorphism (RFLP) markers identified between maize and Tripsacum genomes (Blakey et al. 1994; Leblanc et al. 1995). As a consequence, few sites are available for Tripsacum introgression into the maize genome and, to date, only two instances are known where verifiable recombination/translocation events have occurred (Maguire 1962; Kindiger et al. 1996b).

      Potential pathways for Tripsacum introgression

The 28→38→20 Non‐apomictic pathway

      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).

       The 28→38 apomictic transfer pathway

      This pathway has been described only once and has been little examined or discussed in the literature. Consequently, some detail regarding the results in this research will be presented. In 1958, Dr. M. Borovsky, Institute of Agriculture, Kishinev, Moldova, performed a series of hybridizations between a diploid popcorn line identified as Risovaia 645 and a sexual diploid (2n = 2x = 36) T. dactyloides clone with the first maize‐Tripsacum hybrids being generated in 1960 (Borovsky 1966; Borovsky and Kovarsky 1967). The F₁ hybrids generated from the experiments possessed 28 chromosomes (10Mz + 18Tr). The F₁ plants were completely male sterile and were highly seed sterile. Backcrossing with diploid maize identified that some of the F₁ hybrids were approximately 1–1.5% seed fertile and resulted in the production of progeny possessing 28 chromosomes (10Mz + 18Tr) and 38 chromosomes (20Mz + 18Tr). When the F₁ was backcrossed to the Tripsacum parent, the fertile F₁'s generated progeny with 28 chromosomes (10Mz + 18Tr) and 46 chromosomes (10Mz − 18Tr + 18Tr). The complete set of backcrosses with maize and Tripsacum resulted in a ratio of approximately 10 (28‐chromosome plants) to one (38‐ or 46‐chromosome plant). Phenotypic observations suggested that the 28‐chromosome progeny were not different from their 28‐chromosome parent while the 38‐ and 46‐chromosome progeny were clearly different. In addition, some seed generated by the 28‐chromosome F₁'s were polyembryonic. Additional evaluations on the 28‐chromosome F₁ and its 28‐chromosome progeny suggested that these F₁ plants and their progeny were apomictic. This early, non‐replicated experiment is to date the only report where a 28‐chromosome F₁ hybrid was maintained by apomixis. Polyembryony was noted, and a diploid sexual Tripsacum was used to generate the interspecific hybrid.

       The 46→56→38 non‐apomictic pathway

      This sexual or non‐apomictic pathway, as discussed by Harlan and deWet, is believed to offer the greatest opportunity for Tripsacum introgression into maize and represents results of an early attempt to transfer apomixis to maize (Petrov et al. 1979, 1984). In this pathway, a diploid maize (2n = 2x = 20Mz) is crossed with a tetraploid Tripsacum resource (2n = 4x = 72Tr). The resultant F₁ hybrid possesses 10Mz + 36Tr chromosomes. To date, published reports indicate all of these hybrids are pollen sterile and vary considerably in their levels of seed fertility. In this particular pathway, when the 46‐chromosome F₁ is backcrossed to diploid maize, 56‐chromosome individuals result 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