| Difference in structural variability of genomes in
Triticum and Aegilops TAIHACHI KAWAHARA Plant Germ-plasm Institute, Faculty of Agriculture, Kyoto Univesity, Mozume, Muko, Kyoto 617, Japan In the tetraploid species of the genus Triticum and Aegilops, only two have the same genomes as their parental species. They are Ae. triuncialis with the CuCuCC genome and Ae. cylindrica with the CCDD genome (Kihara 1949). Other species have one genome homologous to that of a parental species but their second genome is closely related to but not identical with that of the other parental diploids. The latter genome is generally called a modified genome. Zohary & Feldman (1962) proposed that modified genomes have evolved through introgression of chromosome or chromosomal segments between different genomes. In this process of genome modification, the common or pivotal genome, such as A in Triticum and Cu and D in Aegilops, play the role of a genetic buffer. However, an alternative and more simple explanation is that in Triticum and Aegilops, the genomes of several diploid species are stable in their chromosome structure but that those of other diploids are variable. In a tetraploid species, when the combination of genomes is one stable plus one variable genome, structural differentiation or segmental rearrangement would accumulate in the variable genome. Since one genome is stable and therefore acts as a genetic buffer, the chromosome structure of the second genome would change far more rapidly than that of the genomes of diploid species. To verify this hypothesis, several studies have been carried out in tetraploid species of Triticum and Aegilops, by identifying the genomes on which breakpoints of spontaneous reciprocal translocations are located. The frequency of breakpoints would indicate the degree of structural variability of each of the two genomes. The data obtained so far are summarized below. 1. Emmer wheats with the AABB genome. Seven spontaneous reciprocal translocations were found. Of these, four were between the B genome chromosomes, two between the A and the B genome and one between the A genome chromosomes (Kawahara and Tanaka 1983, Kawahara 1984 and unpublished). 2. T.araraticum with the AAGG genome. Kawahara & Tanaka (1983) and Kawahara (1984) identified the translocations in T. araraticum and assumed the genomes involved in each translocation. Of 17 different translocations, 10 were between the G genome chromosomes, five were between the A and the G genome and two were between the A genome chromosomes. 3. Ae. variabilis and Ae. kotschyi with the CuCuSvSv genome. Of seven translocations observed, three were between the Sv genome chromosomes, two were between the Cu and the Sv genome and one was between the Cu genome chromosomes. One breakpoint of the remaining one translocation is assumed to be on a Sv genome chromosome but the other is still unidentified (Kawahara unpublished). |
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