Submitted to: Journal of Heredity
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/9/2006
Publication Date: 4/8/2007
Citation: Jauhar, P.P. 2007. Meiotic restitution in wheat polyhaploids (amphihaploids): a potent evolutionary force. Journal of Heredity. 98:188-193. Interpretive Summary: Polyploidy – a condition with multiple genomes (sets of chromosomes) – has played a pivotal role in the formation of many of our most important crop plants, including bread wheat and durum wheat. These cereals resulted from allopolyploidy, i.e., hybridization between wild species, in conjunction with chromosome doubling, some half a million years ago (in the case of durum) and some 10,000 years ago (in the case of bread wheat). Because chromosomes of the wild progenitors are closely related and hence capable of pairing with one another, development of a regulatory mechanism restricting pairing to homologous (identical) chromosomes was necessary for the origin and survival of polyploid wheats. To gain insights into the mechanism of evolution of durum and bread wheat, we recreated some of the steps that occurred in nature. Thus, we produced haploid plants (with half the chromosome number) by crossing durum and bread wheat with maize. These haploids are equivalent of hybrids that were originally formed between wild progenitors thousands of years ago. We demonstrated that these synthetic haploids produced unreduced gametes with 14 chromosomes in case of durum and 21 chromosomes in case of bread wheat, resulting in seed set in both cases. Chromosome studies on the seed-derived plantlets showed precise duplication of chromosomes – 28 of durum or 42 of bread wheat – as a result of fusion of precisely unreduced gametes formed in the parent haploid. Thus, we have demonstrated the reversion of artificially induced haploids to parental durum and bread wheat, essentially a simulation of the evolutionary steps that occurred at the time of origin of these polyploid wheats.
Technical Abstract: Polyploidy – a condition with multiple sets of chromosomes – is recognized as a major force in plant speciation, particularly among the angiosperms. Among the polyploids, allopolyploids are preponderant. Allopolyploidy, resulting from interspecific or intergeneric hybridization accompanied by chromosome doubling, leads to rapid or cataclysmic evolution and has been instrumental in the production of many important cereal, forage, oilseed, and fiber crops. Bread wheat (AABBDD genomes) and its predecessor durum wheat (AABB genomes) offer excellent examples of rapid evolution by allopolyploidy. To gain insights into the mechanism of evolution of wheat, we extracted polyhaploids of bread wheat and durum wheat by crossing them with maize. During meiosis, the Ph1-polyhaploids showed all univalents and produced first-division restitution nuclei that gave rise to unreduced gametes and hence viable seed set. Chromosome studies on the seed-derived plantlets showed precise duplication of chromosomes – 28 of durum or 42 of bread wheat – as a result of fusion of precisely unreduced gametes formed in the parent polyhaploid. Clearly, sexual polyploidization, resulting from functioning of unreduced gametes produced these wheats in nature. Meiotic restitution in interspecific or intergeneric hybrids (amphihaploids), involving wheat and other cereals, is a potent force in the evolution of the Triticeae. In induced polyhaploids, that are equivalent of amphihaploids, we have shown meiotic phenomena such as first-division restitution that lead to reversion to parental durum and bread wheat, essentially a simulation of the evolutionary steps that occurred at the time of origin of these polyploid wheats.