Isamu UCHIKAWA
Laboratory of Cytogenetics, Ehime University, Matsuyama, Japan
Misdivisions and non-disjunctions of the C chromosome during meiosis were observed in the offsprings of Beta het speltoids and Type I subcompactoids, as summarized in the following table.
The C chromosome breaks sometimes at first telophase into the long and the short arm, both becoming telocentrics, Ctl and Cts, or iso-chromosomes, Cil and Cis. A comparatively large number of isochromosomes frequently break into telocentrics, Ctl and Cts, at second telophase. However, lagging chromosomes are sometimes observed at this stage. Frequently they are not included in the daughter nuclei and form micronuclei.
The percentages of misdivisions at first telophase are distinctly less than 39.7% found in monosome IX in Chinese Spring wheat by Sears (1952), rather approaching 1.7% found in Swedish wheat by Sanchez-Monge and MacKey (1948). But at second telophase the percentages of misdivisions are slightly lower than 16% observed in Swedish wheat by Sanchez-Monge and MacKey (1948), and again markedly lower than 39.7% observed in Chinese Spring wheat by Sears (1952).
As to the cause of the misdivision, the assumption of Sanchez-Monge (1950) is supported. Namely, when the insertion of the spindle fibre is carried out normally, a normal disjunction may take place, while when an erroneous insertion of the spindle fibre occurs or when the centromere fails to react to the spindle fibre, a misdivision may happen. A contributing factor may also be the elastic repulsion and delta-factor of Oestergren (1943, 1949). Division is also affected by the cytoplasmic current (Carlson, 1938) in some measure.
The percentages of non-disjunctions at first and second telophase are close to 2.5% observed in Swedish wheat by Sanchez-Monge and MacKey (1948). The percentages of the inclusion of two split halves in one nucleus are slightly lower than 19.8% observed in Swedish wheat by Sanchez-Moge and MacKey (1948). The differences are probably due to different wheat varieties used.
The misdivisions and non-disjunctions of the C chromosome in PMC's of Beta het speltoids are supposed to be the main cause of the C-series speltoids and various types of compactoids.