Substitution of rye chromosome 5r for chromosome 5b of wheat and its effect on chromosome pairing.

HEXAPLOID wheat normally exhibits homologous pairing, forming only bivalents at meiosis. However, in the absence of chromosome 5B, homoeologous as well as homologous pairing occurs ( SEARS and OKAMOTO 1958; RILEY and CHAPMAN 1958; RILEY and KEMPANNA 1963). In 5Bdeficient hybrids between wheat and related genera, the level of chromosome pairing is increaesd (RILEY, CHAPMAN and KIMBER 1959). In wheat-rye hybrids, however, the absence of chromosome 5B results in only a relatively small increase in pairing; in fact, the amount of pairing in 5Bdeficient Chinese Spring wheat x King I1 rye hybrids was found to be less than that observed in a nulli-5B haploid of the wheat variety Holdfast (RILEY, CHAPMAN and KIMBER 1959). One interpretation of these data is that the amount of pairing between haploid wheat chromosomes, in the absence of 5B, is reduced by the rye genome. Furthermore, from a study involving a telocentric addition line of rye chromosome 6R and Aegilops speltoides, it was suggested that perhaps no pairing at all takes place between wheat and rye chromosomes, even when the effect of chromosome 5B is suppressed (RILEY and KIMBER 1966). At least some of the rye chromosomes show specific relationships to homoeologous groups of wheat (KATTERMANN 1937; O’MARA 1946; RILEY 1965; SEARS 1968; BARBER, DRISCOLL, ONG and VICKERY 1968). This relationship allows successful genetic substitution of a particular rye chromosome for the wheat chromosomes of the related homoeologous group. The rye chromosome which bears the genetic marker hairy neck ( H p ) is known to be homoeologous with the group-5 chromosomes (KATTERMANN, 1937; O’MARA 1946; SMITH 1963). The research reported in this paper clarifies two aspects of the relationships between wheat and rye chromosomes. The dissimilarity of the effects of the long arms of chromosomes 5R and 5B on chromosome pairing is established quantitatively. Furthermore, it has been demonstrated that, in the absence of chromosome 5B, wheat and rye chromosomes are capable of pairing.