| From these cytogenetic facts, the author assumes that Type IV dense-eared
mutants might be originated by duplication of the long arm of a peculiar
chromosome (non IX) bearing the dense - eared promoting or lax - eared suppressing
gene, and that the iso - chromosome might be produced by misdivisions of
one univalent chromosome of a lax - eared mother plant with 41 chromosomes.
In fact the phenomena that two long arms went together to the same pole
were actually observed occasionally at TI and TII PMC's of lax - eared mother
plants. The abnormal segregation ratio of this type may be explained as
follows : if the egg cells with 21-, 20 + 1 iso- and 20 chromosomes are
produced in the ratio 1 : 1 : 0.5 and fertilizing rates in competition of
pollen with 21-, 20 + 1 iso and 20 chromosomes are 10 : 1 : 1 respectively
as was shown in the results of the author's crossing experiments of normals
x heterozygotes and its reciprocal cross and in that of observations of
pollen tetrads of heterozygotes, there may be produced the zygotes with
42 chromosomes (normal), 41 + 1 iso - chromosome (heterozygote), 40 + 2
iso - chromosomes (Short - normal) and 40 chromosomes (homo. lax - eared)
in the ratio 1 : 1.1 : 0.1 : 0.6 : 0.15 : 0.1 by combinations between male
and female gametes. This calculated ratio closely coincides with the ratio
actually obtained, except in the case of homozygous lax - eared zygotes.
Perhaps this kind of zygotes aborted on the way of development. Type V heterozygous dense - eared mutants gave normal, heterozygote, homozygote and lax - eared decendants in the ratio 1 : 0.93 : 0.14 : 0.14 in the following generation, but short - normal plants did not arise in this case. In the cytological observations of these four classes of segregates 42- (21II), 43- (20II + 1III, 66.4%; or 21II + 1I, 30.5%), 44- (20II + 1IV, 51.5%; or 22II, 44.3%) and 41 chromosomes (20II + 1I) were observed. In the case of heterozygotes a few cells showed 20II + 3I (2.8%) and 19II + 1III +2I (0.3%) associations, while in homozygotes 20II + 1III + 1I (2.0%) and 21II + 2I (2.2%) associations were rarely seen, besides above mentioned associations. At TI of PMC's the univalent chromosomes frequently lagged in their behaviors and formed micronuclei at the outside of the daughter nuclei, and then were eliminated. From these cytogenetic results the author assumes that Type V dense-eared mutants have one excess duplicated chromosome in heterozygotes and two in homozygotes. The origin of Type V dense - eared mutants, therefore, may be the simple duplication of a whole chromosome bearing the dense - eared promoting (or lax - eared suppressing) gene, and the duplication may be caused by non - disjunction of the univalent chromosome in meiosis. In fact, non - disjunctions were frequently observed at TI (2.1%) and TII (9.8%) PMC's in the lax - eared mother plants with 41 chromosomes. |
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