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Hybridization Between Amphidiploids and the Evolution of Polyploids in the Wheat (Aegilops-Triticum) Group

Daniel Zohary and Moshe Feldman
Evolution
Vol. 16, No. 1 (Mar., 1962), pp. 44-61
DOI: 10.2307/2406265
Stable URL: http://www.jstor.org/stable/2406265
Page Count: 18
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Hybridization Between Amphidiploids and the Evolution of Polyploids in the Wheat (Aegilops-Triticum) Group
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Abstract

Several cytological peculiarities of the polyploids in the Aegilops-Triticum group are pointed out, particularly the distribution of "modified genomes" among polyploids and their relationships to unaltered chromosome sets. Attention is drawn to the fact that the polyploids actually consist of three cytological species clusters. Within each cluster species are characterized by an unaltered, common genome while the differential genome (or genomes) in each polyploid are usually modified. It is suggested that these cytological peculiarities hardly conform to a model of simple amphidiploidy. A review of the monographic treatments of the genus Aegilops shows that while diploids are widely separated from one another, specific boundaries on the tetraploid level are often blurred, indicating a wide occurrence of genetic connections between tetraploids sharing a common genome. The presence of such connections was confirmed by field observations of natural hybridization among the various tetraploids of the section Pleionathera. On the basis of these findings, the validity of Kihara's hypothesis that polyploid species of the Aegilops-Triticum group evolved independently and are simple allopolyploids is questioned. Instead, to account for the cytological peculiarities and variation patterns in this group, the hypothesis is advanced that formation of initial amphidiploids was followed in most cases by extensive hybridization between the raw polyploids. Most polyploids within each of the three species clusters are considered to be hybridization derivatives. They are supposed to have evolved from only a few initial amphidiploids which shared a common genome and which most likely originated from the same diploid species which exist today. Since one genome was common, it remained unaltered by hybridization; moreover, it served as a buffer facilitating interamphidiploid gene-flow. Chromosomal substitutions and alterations therefore occurred mainly between unshared chromosome sets, resulting in their differential modification (i.e., production of modified genomes). The evolutionary significance of interpolyploid connections coupled with a breeding behavior of predominant self-pollination, is pointed out. It is proposed that these genetic connections, made possible through polyploidy, are largely responsible for the wide variation ranges marking many polyploid Aegilops species as well as their apparent and recent evolutionary success.

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