MUTATIONS IN POLYPLOID CEREALS: IV. A SEGMENTAL CHROMOSOME DEFICIENCY DETERMINING AN α SERIES RATIO IN A SPELTOID WHEAT

The C chromosome of beardless Triticum vulgare carries epistatic inhibitors, about 30 crossover units apart on the longer arm of the chromosome, which prevent the expression of the bearded-speltoid complex of genes. It has earlier been shown that het speltoid mutants may lack one of the two sets of inhibitors through the loss of the entire C chromosome, or its longer arm, or an interstitial segment of this arm. The first determines Series β segregation (1 normal: ca. 5 het speltoids: ca. 0.03 speltoids), the last two Series γ (1: 1: ca. 0.05). A third type of progeny ratio, α, is a distortion of a 1: 2: 1 caused by a greater or lesser deficiency of speltoids and, less markedly, of het speltoids. Some workers regard α ratios as resulting from mutation of the epistatic inhibitors to their recessive state; others as due to chromosome aberration similar to, but less extensive than, that observed in typical γ strains. The latter interpretation is proved correct herein. That deviations from the ratios characteristic of the three Series are determined by the degree of pollen certation superimposed on the proportion in which normal and mutant ovules are formed has been shown by analyses involving natural hybrids and crosses made with other types. The extent of certation in α and γ Series may be measured from their progeny ratios—normal pollen = N + H − Sp: mutant pollen = 2 × Sp. A rough correlation has been established between the length of the mutant C chromosome, percentage conjugation with C, proportion of N: Sp progeny, percentage β het Sp progeny, and the coefficient of certation. The percentage het Sp of Series β expected as secondary mutants in the het Sp progeny of α and γ het Sp approximates to [Formula: see text].