The relationship of the D genomes of hexaploid Ae.crassa, Ae.vavilovii and hexaploid wheat

Victor CHAPMAN and T.E. MILLER

Plant Breeding Institute, Cambridge. England

The genome formula M^crDD² which was assigned by KIHARA (1957) to Aegilops crassa Boiss. (2n=6x=42) indicates that two of the three genomes were derived from the D genome of Ae. squarrosa (2n=14). Evidence obtained from the meiotic studies of a haploid (CHAPMAN and MILLER 1977) and from hybrids (SIDDIQUI and JONES 1967) showed that the two D genomes of Ae.crassa are no longer homologous and that neither is completely homologous with the D genome of Ae. squarrosa.

Ae.crassa ssp. vavilovii ZHUK, was raised to species rank as Ae. vavilovii by CHENNAVEERAIAH (1960), when he found by karyotype studies that it differed from Ae. crassa in having only one D genome. The genome formula M^crDS^p was subsequently suggested for Ae. vavilovii by KIHARA and TANAKA (1970).

SIDDIQUI and JONES (1967) pointed out that the confusion between the two species could cause the earlier publications concerning the genomes of Ae. crassa to be less useful. The investigations reported here were intended to compare the genomes of Ae. crassa and Ae. vavilovii and to test the homologous relationships of the D and D² genomes of Ae. crassa and the D genome of Ae. vavilovii with the D genome of hexaploid wheat (Triticum aestivum L. THELL. 2n=6x=42).

Ae. crassa was crossed with Ae. vavilovii and Secale cereale (2n=14). Ae. crassa and Ae. vavilovii were each crossed with T. durum (2n=4x=28) var. Aziziah and with T. aestvum var. Chinese Spring. The Ae. vavilovii was originally kindly supplied by Dr. TANAKA. The analysis of the meiotic chromosome pairing of the hybrids and the Aegilops parents is given in Table 1.

Quadrivalents were seen at meiosis in each of six plants of both Ae. crassa and Ae. vavilovii, and it seems unlikely, therefore, that these resulted from translocation heterozygosity; however, no multivalents were observed in an earlier meiotic study of Ae. vavilovii by CHENNAVEERAIAH (1960). In the hybrid Ae. crassa x Ae. vavilovii the pairing failure resulted in a mean frequency of more than 11 univalents per cell. The difference between the two species is shown again by a comparison of the meiotic chromosome pairing in hybrids of each species with T. durum and with T. aestivum. There was little pairing between the genomes of Ae. vavilovii and wheat in either hybrid, but the pairing in Ae. crassa x T. aestivum was much higher than that in Ae. crassa x T. durum. These results show that Ae. crassa and Ae. vavilovii differ by approximately one genome and that Ae. crassa unlike Ae. vavilovii, has chromosomes which pair with chromosomes of the wheat D genome.

In order to examine further the pairing of Ae. crassa chromosomes with those of the D genome of wheat, hybrids were made between Ae. crassa and each of the seven D genome telocentric lines of Chinese Spring. Meiotic chromosome pairing in each of these hybrids is given in Table 2. In all the hybrids, except those for chromosome 7D, the wheat D genome chromosome was represented by its two telocentrics. In the 7D hybrids only the 7D^s telocentric was present. The mean chromosome pairing was similar in each hybrid, but the frequency of the pairing of the telocentrics, and the configurations in which they paired, varied between hybrids. It can be seen from Table 2 that each of seven chromosomes of Ae. crassa pairs with a different wheat D genome chromosome. Trivalents which included two telocentrics demonstrated that 2D and 6D paired in both arms with the corresponding chromosome of Ae. crassa. For other chromosomes the pairing was limited to one arm. (7D only one arm tested). There weis a maximum of two trivalents per cell in all of the hybrids. In the 3D and 6D hybrids trivalents occurred in which one wheat telocentric was paired. No cells were seen in which the two wheat telocentrics were paired in separate associations although such pairing might have been expected in the 6D hybrid.

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