2. Type II erect-type mutants

The arising percentage of the mutants of this type in X₂ was 0.69%. The heterozygous plants were shorter in height and spike-length and they were larger in strawdiameter, spike-width and spike-density, but inferior in germination rate of seeds and seed-fertility to those of the Type I heterozygotes. The homozygotes had more intensified characters than those of the heterozygotes.

The heterozygotes gave normals, heterozygotes, homozygotes and a few of unexpected plants (subnormal and lax-eared) in a ratio 1: 1.4 : 0.3 : 0.03 in the following generation.

The heterozygotes had 2n=42 chromosomes and they included one large chromosome each. The size of this large chromosome was as large as one segment of the long arm of a middle-sized chromosome duplicated. In meiosis of PMC's the 21_II chromosome association including one heteromorphic pair was most frequently observed (79.25%). The heteromorphic pair was frequently separated into two unequal-sized univalents (20.06%). In this heteromorphic pair the smaller partner was of middle size and normal submedian form, but the other one was larger in size, especially longer in the long arm than that of the former. The larger chromosome might have consisted of one whole chromosome and one segment of the long arm of the homologous chromosome. In a few cases the chromosome associations were either 1_III + 19_II + 1_I (0.27%) or 1_IV + 19_II (0.42%). In these associations, too, one heteromorphic pair or two unequal univalents were usually seen.

The homozygotes also had 2n=42 chromosomes, but they included two large chrom_Iosomes resembling the larger one of the heteromorphic pair of heterozygotes. In meiosis of PMC's, 21_II including one large pair were seen most frequently (90.71%), and in a few cells the chromosome associations 20_II +2_I (8.22%), 1_III + 18_II +3_I (0.43%) and 1_IV + 18_II +2_I(0.45%) were observed.

From these cytogenetic results, it is assumed that the larger chromosome of heteromorphic pair may be originated by the single translocation of one segment of the long arm of a chromosome, which involves the region of the erect-type promoting gene (ert) locus. This partial duplication of a chromosome may be the main cause of the occurrence of the Type II erect-type mutants.

Table 1. The quantitative characters of erect-type mutants (in average)

The abnormal segregation ratio of the heterozygotes 1 : 1.4 : 0.3 can be explained by the competition in fertilization between the male normal gametes with 21 chromosomes and the male mutant gametes with 20 + 1 dupl. chromosomes, and by a slight degree of the zygotic elimination of homozygotes. Accordingly, if the actual fertilizing capacity of male normal gametes and male mutant gametes may be about 10 : 4 respectively in rate, and those female gametes are produced in the ratio 1 : 1, the zygotes with 42, 41 + 1 dupl. and 40 + 2 dupl. chromosomes will be produced in a ratio 1 : 1.4 : 0.4 by free combination.

(Received Jan. 8, 1968)

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