| This recognition of the non-homology of the chromosomes of T. speltoides
to those of the B genome of polyploid wheats points to a need for a re-examination
of other pertinent evidence that may help to elucidate the situation. The karyotypic evidence gathered by PATHAK (1940) and RILEY et al. (1958) was, in fact, based on a single accession in each case and thus is open to the possibility of undetected variation in the satellite size existing in these species. WAINES and KIMBER (1973) have shown such variation in the satellites of T. monococcum, and this type of variation may exist in other species of the Triticinae also. In an attempt to demonstrate the nature of the genetic regulation of pairing of T. speltoides, KIMBER (1966) made the hybrid between a form of T. speltoides known to allow a high frequency of multivalent formation in hybrids with polyploid wheat and T. longissimum (Ae. sharonensis), which was characterised by very low-pairing in hybrids with wheat. The F1 was back-crossed to T. speltoides twice, and a segregant, heterozygous for the pairing control, was crossed with T. aestivum. The six F1 plants of this cross segregated four high-pairing to two low-pairing. SEARS (1969) pointed out that the average of 2.9 bivalents per cell in the low-pairing segregants was much lower than that expected if T. speltoides were the donor of the B genome. DVORAK (1972) recorded the pairing frequencies of wheat and Agropyron telocentrics in the presence of T. speltoides with differing abilities to affect homoeologous chromosome pairing. Surprisingly, telocentric 7B of wheat showed a reduced pairing affinity when the frequency of homoeologous pairing was at an intermediate level. DVORAK (loc. cit.) also pointed out that the data of JOHNSON and KIMBER (1967) could not support a closer relationship between the B and S genomes than between the A and S or the D and S. The data of ATHWAL and KIMBER (1972) show a similar lack of homology between the B and the S genomes. RILEY and CHAPMAN (1966) calculated the affinity of the long arms of the chromosomes of homoeologous group 5 of T. aestivum and chromosome 5S of T. speltoides, and they concluded that chromosome 5B showed little affinity to chromosome 5S. Thus, when the pairing behavior of individual chromosomes is considered, there is little evidence that the S genome is homologous to the B genome. The lack of pairing in hybrids between T. longissimum and T. aestivum (KIMBER, 1961) and the obvious genomic similarity between T. longissimum and T. speltoides indicates that the B genome of polyploid wheat could not have been derived from a species so closely related to T. longissimum. In addition to the cytological evidence, recent studies of the electrophoretic banding patterns of seed proteins do not support the concept of T. speltoides as the donor of the B genome to polyploid wheats (JOHNSON and HALL, 1966). At this symposium new evidence has been presented on the relationships of the B and S genomes. RUBENSTEIN and SALLEE (1973) have described the meiotic pairing configurations in various hybrids of T. kotschyi (Ae. variabilis) and T. speltoides, and it is apparent that there is a genome in common between these species. However, when T. kotschyi is crossed with T. aestivum (DRISCOLL and QUINN, 1968), there is virtually no meiotic chromosome pairing, clearly showing that there is little or no chromosome homology between these species. SHANDS and KIMBER (1973) have produced a series of hybrids between T. speltoides, and also between T. turgidum (T. durum) and low-, intermediate- and high-pairing T. speltoides. The T. turgidum hybrids show the same range in pairing behaviors seen in the T. aestivum x T. speltoides crosses described by KIMBER and ATHWAL (1972). The T. timopheevii hybrids, however, are quite different. Even in the hybrid with the low-pairing T. speltoides, where homoeologous chromosome pairing should be minimal, there is the equivalent of approximately seven bivalents. This pairing is taken to represent the homology of the S genome of T. speltoides to the G genome of T. timopheevii. In hybrids between T. timopheevii and T. durum there is apparently one genome in common (SHANDS and KIMBER, 1973), and this is assumed to be the A genome. Consequently, it is improbable that the S genome of T. speltoides is equivalent to the B genome of T. turgidum. |
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