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Three types of triple disomic substitution differed in their frequency of distribution. Triple disomic substitution 4G (4D) 5G(5B) 6G(6B) was only found in KS88-9-8. The 3At(3A) 2G(2B) 6G(6B) triple disomic substitution with rec 7A-7At chromosome was represented by two families. Triple disomic substitution 1G(1D) D) 5At(5A) 6G(6B) (Fig. 2a) was the most frequent, and was present in ten families (Table 1).

Some of these substitution types were discovered in previous hybrid generations by Gill et al. (1988). However, they did not identify the rec 7A-7A^t chromosome, the 1G(1D) 6G(6B) double disomic substitution, and the 3A^t(3A) 2G(2B) 6G(6B) triple disomic substitution with rec 7A-7A^t chromosome. The substitution type 1G(1D) 5A^t(5A) 6G(6B) was described as ?G(5A) 6G(6B) in the previous study in which N-banding was used for chromosome identification. The N-banding technique does not permit differentiation between some A, A^t, and D genome chromosomes, including 1D and 5A^t.

The spectrum of substitutions of T. aestivum cv. Wichita x T. araraticum was different from those in hybrids derived from other cultivars (Badaeva et al. 1991). The genotypes of parental forms may have influenced the substitution pattern of their derivatives.

Individual T. araraticum chromosomes differed in the frequency of substitution. Chromosome 6G was the most frequently substituted (24 families). High frequencies of substitution were also found for chromosomes 1G and 5A^t (10 families each), 2G(7 families), and 3A^t (3 families). Substitutions of chromosomes 4G and 5G were present in one family, while substitutions involving other T. araraticum chromosomes were not recovered (Fig. 2b). Although rearrangements, involving A and A^t genome chromosomes, were possible they could not be detected by cytological methods due to the absence of marker bands. These results are in agreement with data on substitutions in T. aestivum x T. timopheevii hybrids (Badaeva et al. 1991). The high frequency of substitutions involving 5A^t and 1G chromosomes in the present material is probably due to the common origin of families with this substitution type.

Based on the results of Badaeva et al. (1991) and present study, we found that some chromosomes have a high frequency of substitution while others are rarely involved in substitutions in different T. aestivum x T. timopheevii cross combinations. We compared these results with data on species- specific chromosomal rearrangements, which occurred during the speciation of the two tetraploid wheat species (Naranjo et al. 1987; Jiang et al. 1994). In durum wheat, the 4A-5A-7B cyclic translocation was discovered, while in Timopheevi wheat a species- specific cyclic translocation included chromosomes 6A^t, 1G and 4G. The chromosomes 4A^t, 5A^t, 6A^t, 1G, 4G, and 7G had a low frequency of substitution. A comparatively high number of 5A^t and 1G substitutions were found in only one cross combination, was due to the common origin of the lines. These data indicated that the frequency of substitutions between two homoeologous chromosomes correlates with the level of their genetic diversity. The T. araraticum accession TA 39 used in this study is characterized by high resistance to leaf rust (04C). Although the derivatives of the crosses with Wichita have not yet been evaluated, some of the lines may have inherited resistance from T. araraticum and they will be useful in breeding programs.

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