| Results and discussion Samples from the same plant taken on different days showed reasonable agreement in frequency of misdivision. The most samples from one plant were five from an individual with a Red Egyptian monosomic. The chi2 for the misdivision frequencies in these five samples was only 3.04. The chi2's for misdivision frequency in different plants with the same monosome all proved to be insignificantly small (below the 5% point). Therefore all the data for each monosome were combined (Table 1). The percentage of misdivision ranged from 14.6 for the Thatcher monosome to 26.9 for Red Egyptian, and chi2 for the distribution as a whole was a highly significant 23.9. Although the Timstein, Hope, and Chinese chromosomes did not differ significantly among themselves, Thatcher was clearly lower than the other four (beyond the 1 % point of significance), and Red Egyptian was higher (also beyond the 1% point). All the misdivision frequencies were considerably higher than those observed by SANCHEZ-MONGE and MAC KEY (1948) for chromosome 5A but lower than found for 5A by SEARS (1952a). Detailed comparison of the data for 3B with those of SEARS (1952a, Table 1) for 5A requires that some of Sears' categories be lumped together. His first type of class a misdivisions are normal divisions, not misdivisions. The rest of the class a's presumably fit in the "equal with separate arms" class of the present study, while his b misdivisions are "2 isochromosomes" and the c's are "3: 1". If only Chinese Spring is considered, 2 isochromosomes were somewhat more frequent than 3: 1 for 3B (5.7% vs. 4.4%) and also more frequent for 5A (23.0% vs. 13.5%). An even more striking correspondence between 3B and 5A is apparent if all the data are lumped; then 2 isos vs. 3: 1 is 4.1% vs. 4.1% for 3B and 10.7% vs. 12.2% for 5A. Although there were about three times as many misdivisions of 5A as 3B when either the 3: 1 or 2-iso type was considered, the reverse was approximately true for the "equal with separate arms" class. Here the percentages were 11.3 for 3B vs. 3.2 for 5A (Chinese Spring) and 12.2% for 3B vs. 2.5% for 5A (total). Since the studies of 3B and 5A were made by different investigators, it is possible that differences in standards of scoring could account for some of the differences observed. Sometimes it is a matter of judgment whether the two arms of a univalent passing to a pole are attached to the same centromere or not. Consistent differences might therefore be expected in the results obtained by two investigators scoring the same group of cells. However, the comparatively high values for "equal with separate arms" in the present experiment can scarcely all be attributed to differences in scoring. This is a class that should give rise to telocentric chromosomes, and if, therefore, this class were no larger for 3B than for 5A, we would expect to recover telocentrics and isochromosomes in essentially the same ratio for 3B as for 5A. In fact, however, STEINITZ-SEARS (1966) recovered about four times as many 3B telocentrics as isochromosomes, whereas SEARS (1952a) found only about 50% more 5A telocentrics. Besides SEARS' (1952) study of misdivision there has been one other report concerned with the misdivision of monosomes, involving chromosomes 5A, 5B, and 5D from different varieties by MORRIS et al. (1969). They studied misdivision of the monosomes in hybrids of their various varieties with Chinese Spring monosomics and thus not in a pure Chinese background. The rate of misdivision extended over the range 2.4%-23.6% in this material. Chromosome 5D always misdivided with a lower frequency than chromosomes 5A and 5B. |
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