| The effect of the S genome of T. speltoides
on the pairing of homologous chromosomes of T. aestivum1)
J.M. RUBENSTEIN and G. KIMBER Department of Agronomy, Univ. of Missouri Columbia, Mo. 65201, U.S.A. T. speltoides has been considered to have donated the B genome to T. aestivum. Recent work has caused a reconsideration of the evolution of polyploid wheats and has shown that it is doubtful that T. speltoides donated the chromosomes comprising the B genome in T. aestivum (KIMBER and ATHWAL, 1972 ; DVORAK, 1972). KIMBER and ATHWAL observed variation in the amount of pairing in T. aestivum x T. speltoides hybrids. The amount of pairing was classified as high, intermediate, or low and averaged the equivalent of 10.60, 5.65, and 0.70 bivalents per cell, respectively. In the low-pairing hybrid, which formed the basis for the rejection of T. speltoides as the B genome donor, the reduction in chromosome associations may be attributed to a reduction in either homologous or homoeologous pairing, or both. If the low pairing was due to a reduction in homologous associations, then the decrease in bivalent formation might be accounted for by genetic regulation and not be indicative of a lack of homology between the chromosomes of T. speltoides and the B genome of T. aestivum. Some evidence concerning the nature of the reduction in pairing in low-pairing hybrids was given by KIMBER and ATHWAL (1972) by examination of the configurations in a 54-chromosome amphiploid of an F1 hybrid between T. aestivum and T. speltoides, in which chromosome-5B was represented by a pair of telocentric chromosomes for the long arm. The amphiploid showed regular bivalent formation, which indicated that homologous pairing may take place in the presence of the genome of T. speltoides inducing low-pairing in hybrids. Also, in all three types of hybrids of Chinese Spring with T. speltoides, heteromorphic bivalents were observed showing that the pairing of homoeologous chromosomes may occur even in the presence of low-pairing T. speltoides. KIMBER and ATHWAL state, "Since homoeologous chromosome pairing is not completely prohibited in any of the hybrids, it must be concluded that homologous chromosome pairing is not precluded either." While this infers that homologous pairing is not affected, it is necessary to perform a more definitive test of the effect of T. speltoides on homologous pairing. The purpose of this contribution is to report investigations on the effect of the S genome of T. speltoides on the pairing of homologous chromosomes of T. aestivum. To examine the effect an isochromosome of wheat, having arms with identical constituent gene loci and structure, was used to estimate the frequency of homologous pairing. Internal pairing of an isochromosome occurs when the two homologous arms of the chromosome pair with each other to produce a ring univalent. This ring univalent has the advantage of being readily recognizable at meiosis and distinguishable from other wheat chromosomes. Thus, the frequency of pairing of an isochromosome may reflect the frequency of homologous pairing. However, DRISCOLL and DARVEY (1970) have reported differences in the pairing of isochromosomes and homologous chromosomes in the presence of colchicine. By producing hybrids of T. aestivum with the various pairing forms of T. speltoides; high, intermediate, and low, and having one of the wheat chromosomes in an isosomic condition, any effect of T. speltoides on the pairing of homologous chromosomes of wheat might be detected by differences between the hybrids in the frequency of formation of a ring univalent. |
| 1)Contribution from the Missouri Agricultural Experiment Station. Journal Series Number 7398 Approved. |
| --> Next |