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From these cytogenetic findings, it may be inferred that
the chromosomal difference between Type III and Type IV
heterozygous erect-type mutants comes from the fact that the
latter lacks one chromosome which the former possesses.
Therefore, the loss of one chromosome, which carries the
gene or genes for the development of the straw thickness and
leaf-width, may be the main cause of the occurrence of the
thin-short straws and narrow leaves. The erect-type
characters of the spikes, straws and leaves might be mainly
brought by isochromosome formation, which carries an
erect-type gene (ert) or genes in a double dose. The
distinctive diminution of seed garmination rate and seed
fertility of this type mutants might be caused by the
physiological weakness of the plants due to the loss of one
chromosome and duplication of an arm of a chromosome.
Abnormal segregation ratio quasi-normal : quasi-III. er.
het. : III. er. hom. : IV. er. het. : IV. er. hom. :
lax-eared 1 : 5.4 : 2.5 : 0 : 0.6 : 0 can be explained by
the competition in fertilization of male gametes among 21,
20 + 1 iso.,19+1 iso. and 21 chromosome gametes, and some
degree of zygotic elimination of the homozygotes. When the
gametes with 21, 20 + 1 iso., 19+1 iso. and 20 chromosomes
are formed in the ratio 0.2 : 1 : 0.2 : 1 respectively, and
the actual fertilizing capacity ratio of these three kinds
of male gametes are 15 : 8 : 1 : 1 respectively, the zygotes
with 42 (quasi-normal), 41 + 1 iso. (quasi-III. er. het.),
40+2 iso. (III. er. hom.), 40+1 iso.-(IV. er. het.), 39+2
iso.-(IV. er. hom.) and 41-chromosome (1ax-eared) may be
produced in the ratio 3 : 16.6 : 8 : 12.2 : 2.6 : 15=1 : 5.5
: 2.7 : 4.1: 0.9 : 5 respectively, besides very few 39 + 1
iso., 38 +2 iso. and 40 chromosome zygotes. This ratio
closely coincides with the actually obtained ratio 1 : 5.4 :
2.5 : 4 : 0.6 : 5, if some degree or zygotic elimination of
Type IV homozygotes are expected. These explanations,
however, involves some difficult points and it must be
checked further on the results of the crossing experiments
between mutants and normal plants.
Table 2. Chromosome associations
at MI in PMC's of erect-type mutants
Literature Cited
GUSTAFSSON, A. 1947. Mutation in agricultural plants.
Hereditas 33 : 1-100.
HAGBERG, A. 1958. Cytogenetik einiger Gerstenmutanten. Der
Zuchter 28 : 32-36.
UCHIKAWA, I. 1960. Genetic and cytological studies of
X-induced mutants in common wheat. I. On speltoid,
compactoid, lax-eared, short-strawed, dense-eared and
bearded mutants. Mem. Ehime Univ., Sect. II. Ser. B.
4 : 1-52.
UCHIKAWA, I. 1962. Genetic and cytological studies of
X-induced mutants in common wheat. III. Further studies on
dense-eared mutants. Ibid. 4 : 365-387
UCHIKAWA, I. 1966. Genetic and cytological studies of
X-induced mutants in common wheat. VII. On erect-type
mutants. Ibid. 5 : 1-19.
(Received Jan. 8, 1968)
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