| Misdivision of five different 3B monosomes in Chinese
Spring wheat1) Lotti M. S. SEARS Department of Agronomy, University of Missouri, Columbia, Missouri, U.S.A. That isochromosomes and telocentrics originate through misdivision of univalents at meiosis has been well substantiated for wheat (SEARS 1946, 1952a, 1952b) since it was first suggested by UPCOTT (1937) and DARLINGTON (1939, 1940). SANCHEZ-MONGE and MAC KEY (1948) believed that the second division of meiosis in their varieties of hexaploid wheat, in which they found about 2% of misdivision of chromosome 5A, was the principal time of origin. In his detailed study SEARS (1952) found somewhat more misdivisions at metaphase I than II. In his study chromosome 5A from three different varieties had been introduced into the standard variety Chinese Spring. The frequency for the introduced chromosomes, though lower than for the Chinese 5A (13.7-23.6% vs. 36.0-43.5%), was still considerably higher than that found by SANCHEZ-MONGE and MAC KEY. Telocentrics for the short arm of chromosome 3B, which is conveniently marked with Neatby's virescent gene, are useful for study of the stability of telocentrics (STEINITZ-SEARS 1966). Data are already available in the publication cited on the frequency with which such telocentrics are obtained through misdivision. It is of interest to learn whether the frequencies of misdivision of 3B are of the magnitude expected, and to compare these frequencies with those of 5A. In addition, chromosomes 3B from other varieties are available in Chinese Spring and can be compared with Chinese Spring 3B for rate of misdivision. Some data on MI misdivision of Chinese Spring 3B were reported by STEINITZ-SEARS (1966). Materials and methods Four varietal substitution lines were supplied by E. R. SEARS. These were 'Chinese Spring disomic for chromosome 3B from Thatcher, Timstein, Hope, and Red Egyptian', respectively. Each line was crossed to mono-3B for recovery of off-spring with the substitution chromosome monosomic. For each of the four different 3B chromosomes, several such monosomics were grown, and several fixations were made from each plant. This made it possible to assess the inter-plant variation on one hand, and the day-to-day variation on the other hand. When a univalent divides at MI, it can divide normally-i.e., into two chromatids, each with a long and a short arm. Provided there is enough difference between the two arms, normal division can be easily distinguished from a misdivision resulting in two isochromosomes. Since chromosome 3B was found to have an arm ratio at MI of only 1.4: 1 (as compared to 2.5: 1 for 5A), a misdivision of 3B into two isochromosomes can sometimes be mistakenly recorded as an equal division. In scoring, all doubtful cases of this kind were therefore excluded. Other types of misdivision such as three arms going to one pole and one to the other cannot be mistaken for normal divisions. For this reason when the present results were compared with those Sears obtained for chromosome 5A, special consideration was given to the relative sizes of the 3: 1 classes. Some misdivisions give rise to chromatids deficient in the centromere region, and these chromatids fail to reach the poles in time for inclusion in the telophase nuclei. In the present study they are only assessed by an increase in the frequency of micronuclei in the tetrads, and this increase is presumably too small to be noticed. The offspring resulting from a monosomic will thus show somewhat fewer isochromosomes and telocentrics than expected from the frequency of misdivision (STEINITZ-SEARS 1966). |
| 1) Journal Paper No.6717 of the Missouri Agricultural Experiment Station. Copied from the Proceedings of the IV Wheat Genetics Symposium by the kind permission of the Editors and Organizing Committee. |
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