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Pattern of chromosomal rearrangements in T. araraticum.
Since the chromosomes involved in spontaneous translocations have all been identified, we can determine the patterns of chromosomal rearrangements in T. araraticum. Eighteen different translocations were identified from the chromosome pairing of intraspecific hybrids and C-banding. Therefore these translocations are assumed to represent a random sample of entire structural rearrangements. The 4G chromosome was included in 8 translocations, 6G in 6 followed by 3G (4), 1G, 5G and 7G (3). 2G and 5At were involved in two different translocations and 2At, Atl, At2, At3 and At4 in one translocation, respectively. Differences in the number of breakpoints on each chromosome would reflect structural variability of respective chromosomes. Apparently, chromosomes of the G genome are more frequently included in translocations (29 breakpoints) while the At genome chromosomes are included in 7 translocations. The present findings confirm those reported earlier (Badaeva et al. 1994) demonstrating the difference in variability among chromosomes and between the two genomes, At and G. Thus the G genome chromosomes are three to four times more variable than the At genome chromosomes. This may be caused by the higher amount of heterochromatin which increases the probability of chromosome breaks and consequently the frequencies of chromosomal aberrations as was suggested by Badaeva et al. (1994).

Furthermore, such a high variability of the G genome chromosome has great implications in the evolutionary process of this species. Two second genomes of tetraploid wheats, B and G, are assumed to have originated from some species of the section Sitopsis of genus Aegilops, most likely from Ae. speltoides (Sarker and Stebbins 1956, Shands and Kimber 1973, Tanaka et al. 1978, Tsunewaki 1989, Dvorak and Zhang 1990). In the initial stage of tetraploid formation, raw amphidiploid AASS would have formed various progenies with a wide range of chromosomal rearrangements, in which rearrangements including the S genome chromosomes occurred more frequently. From this wide array of recombinants, better adapted types would be selected. The degree of chromosomal rearrangements was so high in S genome that we could not detect high homoeology between S and G genomes. Stable A genome chromosomes would serve as a genetic buffer in this chromosome repattering stage and we can easily detect high homoeology between A genome of diploid wheat and At genome. During this process of chromosome repattering, species specific translocations of 6At-lG-4G (Jiang and Gill 1994) would have been fixed. Thus the G genome chromosomes played a major role in the polyploid formation and adaptation process in T.araraticum.

Fig1

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