PP: Meiotic restitution in wheat polyhaploids (amphihaploids): A potent evolutionary force

Polyploidy is well recognized as a major force in plant speci-ation. Among the polyploids in nature, allopolyploids are pre-ponderant and include important crop plants like bread wheat, Triticum aestivum L. (2n5 6x5 42; AABBDD genomes). Al-lopolyploidy must result through concomitant or sequential events that entail interspecific or intergeneric hybridization and chromosome doubling in the resultant hybrids. To gain insight into the mechanism of evolution of wheat, we ex-tracted polyhaploids of 2 cultivars, Chinese Spring (CS) and Fukuhokomugi (Fuko), of bread wheat by crossing them with maize, Zea mays L. ssp. mays. The derived Ph1-polyhaploids (2n 5 3x 5 21; ABD) showed during meiosis mostly univa-lents, which produced first-division restitution (FDR) nuclei that in turn gave rise to unreduced (2n) male gametes with 21 chromosomes. The haploids on maturity set some viable seed. The mean number of seeds per spike was 1.45 ± 0.161 in CS and 2.3 ± 0.170 in Fuko. Mitotic chromosome prepa-rations from root tips of the derived plantlets revealed 2n 5 42 chromosomes, that is, twice that of the parental polyha-ploid, which indicated that they arose by fusion of unreduced male and female gametes formed by the polyhaploid. The Ph1-induced univalency must have produced 2n gametes and hence bilateral sexual polyploidization and reconstitution of disomic bread wheat. These findings highlight the quantum jump by which bread wheat evolved from durum wheat in nature. Thus, bread wheat offers an excellent example of rapid evolution by allopolyploidy. In the induced polyhaploids (ABD) that are equivalent of amphihaploids, meiotic phenom-ena such as FDR led to regeneration of parental bread wheat, perhaps a simulation of the evolutionary steps that occurred in nature at the time of the origin of hexaploid wheat. Polyploidy—a condition with multiple sets of chromosomes— has long been recognized as a dominant factor in plant spe-ciation, particularly among angiosperms (Leitch and Bennet