CHROMOSOME TRANSLOCATIONS IDENTIFIED IN VARIETIES OF COMMON WHEAT

Abstract

Introduction Chromosome structural changes in polyploid wheat species appear to have played roles of varying significance in evolution. Pai et nl. (1961) compared total lengths and D N A contents in both diploid and hexaploid species with the summation of corresponding values for the prototypes of their presumed diploid progenitors and concluded that a considerable degree of chromatin elimination had talten place in polyploid wheats subsequent to their origin. Tolerance of loss of chromosome material may be expected to be associated with polyploidy. Kimber and Riley (1963), on the other hand, critically analyzed chromosome pairing under different situations in the absence of the gene(s) on chromosome 5B (V) which, when present, prevents associations between homoeologous chromosomes in common wheat (Triticu7~z aestiwz~m ssp. wlgare (Vill., Host) Mac Key) and concluded that speciation at ?he polyploid level had occurred by gene mutation and sujbstitution and that chromosomal rearrangements had played an insignificant evolutionary role. They also pointed out that identification of the diploid progenitors of hexaploid wheat by genome analysis would not have been possible in the presence of gross chromosonlal alterations. Sears (1953) reported that three varieties of hexaploid wheat differed by single reciprocal translocations from Chinese Spring. O n the basis of nullisomic-tetrasomic compensations which enabled chromosomes to be allocated to seven homoeologous groups, Sears (1954) suggested that Chinese Spring probalbly had the primitive chromosonle structure within which translocations lhad occurred. H e pointed out the lack of critical sensitivity associated with features of the con~pensation test but concluded that homoeologous groups were essentially intact in Chinese Spring since homologies within groups were of a different order from homologies between groups. Sears (1953) indicated that the variety Thatcher had a translocation relative to Chinese Spring, which involved chromosomes 4A ( IV) and 6B (X) ; in the case of Poso (a variety of T . aestiwz~m ssp. compactzun (Host) Mac Key) the chromosomes implicated were 5B and 7B (VII), whilst in the variety Indian chromosomes 3B (111) and 7B had undergone translocation. In addition to the varietal translocations described by Sears, Holdfast and Chinese Spring differ by a reciprocal translocation involving chromosomes 3B and 3D (XVI) (Riley and Kimber, 1961). T h e fact that Chinese Spring appears to possess the primitive chromosonle structure is fortunate in view of the wide use of this variety in studies concerned with monosomic analysis. Meiotic analyses to select monosomic and disomic F1 sibs in each of ?he twenty-one crosses will reveal the existence of structural differences due to translocations in other varieties and enalble the chromosomes involved in such interchanges to be identified. These are thus documented relative to the same primitive base.