MUTATIONS IN POLYPLOID CEREALS: II. THE CYTOGENETICS OF SPELTOID WHEATS

Abstract

The bearded speltoids that are found in beardless varieties of Triticum vulgare are usually segregates from heterozygous speltoids that have arisen through mutational loss of two genes, or gene complexes, that normally determine the glume and, to a lesser extent, the lemma and rachis characteristics that predominantly differentiate T. vulgare from T. spelta, which is possibly one of its ancestors. These "vulgare" genes are epistatic to genes that in their absence determine indurated, sharply and heavily keeled glumes and bearded lemmas closely resembling those of T. spelta. The two "vulgare genes" are both on one chromosome ("C" or "IX"); the hypostatic "spelta" or speltoid genes are probably multiple and have not yet been definitely located.The "vulgare" genes or gene complexes may be lost independently, giving (after segregation) "part-mutations", i.e. beardless speltoids or bearded normals, or they may be lost together, giving the "total mutation" i.e. bearded speltoid. The mutations that produce the "part-mutants" are necessarily segmental changes, but the total mutants may arise through either segmental or whole chromosome loss.Loss of an entire C chromosome gives a β het speltoid which, on selfing, gives normal, het speltoid, and bearded speltoid offspring in a ratio varying about the mode 1:5: few. These β bearded speltoids are dwarf, sterile nullisomics having only 20 pairs of chromosomes, 20II– –, in place of the 21II normal for T. vulgare. The characteristic β Series ratio and its variations are mainly determined by the frequency with which the unpaired C of a β het speltoid is left out of the gynospore nuclei during meiosis and by the lower functioning of 20-chromosome pollen. Zygotic elimination plays a minor role.Deletion of an appreciable interstitial segment, or the whole, of the long arm of C produces γ het speltoids which on selfing give normals, het speltoids, and speltoids in ratios near 1:1: few. If the segment deleted involves both gene complexes the speltoid segregates are bearded and more or less dwarfed and partially sterile. If it does not involve the beard "inhibitor" they are beardless. The γ ratio is determined largely by the lower functioning of pollen carrying the deletion.Deletion of a segment of C too small to be established definitely at metaphase but genetically determinable as of not less than about 30 cross-over units in length gives α het speltoids that segregate normals, het speltoids, and bearded speltoids in ratios approaching 1:2:1 and all segregates are of normal size and fertile. Evidently this deletion does not materially affect pollen functioning.Hybridization and polyploidy have been involved in the evolution of T. vulgare and it is the latter which permits the functioning of gametes bearing deficient chromosome complements and the survival of aberrant types that would be unlikely to exist in a diploid species. The hybrid ancestry accounts for quantitative chromosome changes being able to produce forms that closely resemble ancestral or related species, and also for the fact that crosses between varieties of T. vulgare or of closely related species can produce forms that may be phenotypically indistinguishable (or nearly so) from the mutants.