| I. Research Notes The relationships of the S-genome diploids to polyploid wheats1) G. KIMBER Department of Agronomy, University of Missouri, Columbia, Missouri, U.S.A. In assigning the genome symbols to the diploids in the Triticinae, KIHARA (1949) ascribed the letter S to the species which then constituted the Sitopsis section of the genus Aegilops. This section, the species of which have been given various taxonomic rankings, consists of, at least for the purposes of this paper, three forms: Triticum speltoides (Ae. speltoides), T. longissimum (Ae. longissima and Ae. sharonensis) and T. bicorne (Ae. bicornis). Hybrids between these species had meiotic pairing patterns indicating that they each possessed essentially the same genome. The pairing averaged 6.70 bivalents per cell in the hybrid T. speltoides x T. longissimum (KIMBER, 1961), approximately 7 bivalents per cell in T. speltoides x T. bicorne (KIHARA, 1949) and the equivalent of 6.86 bivalents per cell in T. bicorne x T. longissimum (KIMBER, 1961). A quadrivalent was observed at meiosis in the hybrids involving T. longissimum, and it was ascribed to the presence of a reciprocal translocation. Therefore, in the classical concept of the genome relationships, each of these species should be considered to carry the same basic genome. JENKINS (1929) reported the occurrence of a mode of seven bivalents in hybrids between T. turgidum and T. speltoides; however, some considerable time was to pass before any species of the Sitopsis section came to be accepted as the donor of a genome to the polyploid wheats. In 1940 PATHAK observed similarity between the satellites of the chromosomes of T. speltoides and T. turgidum. SEARS (1956) suggested T. bicorne, and also in 1956, on the basis of morphological evidence, SARKAR and STEBBINS suggested T. speltoides as the donor of the B genome to wheat. RILEY et al. (1958) concluded, on the basis of karyotypic, geographical and synaptic evidence, that T. speltoides had donated the B genome to the polyploid wheats. A conclusive demonstration of the homology of the genome of T. speltoides (or either T. longissimum or T. bicorne) to the B genome of the polyploid wheats was lacking. In the case of the hybrids of T. speltoides to polyploid wheats, considerable homoeologous chromosome pairing was observed (RILEY et al., 1958), and this obscured the homologous chromosome pairing pattern. In the cases of T. longissimum and T. bicorne little, if any, chromosome pairing, homologous or homoeologous takes place in hybrids with polyploid wheat (KIMBER, 1961; RILEY et al., 1958), yet this did not seem to alert workers to the possibility that the Sitopsis section had not contributed the B genome of the polyploid wheats. Recently (KIMBER and ATHWAL, 1972) the recognition of variation in the ability of T. speltoides to affect homoeologous chromosome pairing has led to a reconsideration of the evolution of the polyploid wheats. KIMBER and ATHWAL demonstrated three levels of chromosome pairing in hybrids between T. aestivum and different accessions of T. speltoides. The presence of heteromorphic bivalents in even the low-pairing hybrids was taken as evidence that homoeologous chromosome pairing was occurring and thus homologous chromosome pairing should also be possible. The very low pairing observed in these lines indicates the absence of homologous chromosomes. Confirmation that the low-pairing forms of T. speltoides do not interfere with homologous chromosome pairing was also obtained from the regular bivalent formation observed in a amphiploid derived by colchicine treatment from a low-pairing hybrid (KIMBER and ATHWAL, 1972; KIMBER, unpublished). That the low-pairing form of T. speltoides does not affect homologous chromosome pairing was also supported by a comparison of the meiosis of autotetraploids of the low- and high-pairing T. speltoides lines (LARSEN and KIMBER, 1973). |
| 1) Contribution from the Missouri Agricultural Experiment Station, Journal Series Number 67215. Copied from the Proceedings of the 4th Wheat Genetics Symosium by the kind permission of the Editors and Organizing Committee. |
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