Micronuclei are formed in the microspores whenever lagging chromosomes fail to be included in the TII nuclei. An important factor determining the frequency of such micronuclei must be the frequency with which univalents divide at MI instead of passing undivided to one pole or the other. If a univalent does not divide and is included in a TI nucleus, it presumably divides normally at the second division and does not lag. If it divides at MI, its halves not only lag at TI and run a risk of not reaching the poles, but also they lag again at TII and are again uncertain of reaching the poles.

Some types of misdivision result in formation of more than two daughter chromosomes from a single univalent. For example, misdivision at MI can result in one arm going toward one pole, one or two arms toward the other pole, and two or one arms left acentric on the metaphase plate. Also, MII misdivision of the two products of MI division of a univalent results in four separate arms, each of which may fail to reach a pole and will then form a micronucleus. Thus the frequency of tetrads with more than two micronuclei may bear some relation to the frequency of misdivision. Unfortunately, however, there are other events that can lead to more than two micronuclei, especially the failure of two homologues to synapse. Such unsynapsed homologues behave as univalents and are both subject to lagging and possible exclusion from the TII nuclei.

In the present data (Table 2) there were differences in percentage of tetrads with micronuclei, but there was, as expected, no consistent relation to frequency of misdivision. Chinese Spring, rather than Thatcher, had the lowest value, and Hope, not Red Egyptian, had the highest. Nor did the frequency of tetrads with two or more micronuclei prove to be a good indicator of misdivision frequency; instead it varied with the percentage of tetrads having micronuclei, albeit showing a greater range.

From the MI data in Table 1 and from SEARS' (1952a) experiment, it appears that the same chromosome from different sources may differ in rate of misdivision. Another interpretation is possible, however. The backcrosses by means of which the chromosomes were transferred to Chinese Spring were not enough to eliminate all genes from the other varieties. It is possible, for example, that the genotype of Thatcher conditions a low rate of misdivision, and that one or more genes for this were transferred to the Thatcher 3B substitution line. A desirable comparison would be of Chinese mono-3B, Thatcher mono-3B, Thatcher 3B monosomic in Chinese, and Chinese 3B monosomic in Thatcher.

Literature cited

DARLINGTON, C. D. 1939. Misdivision and the genetics of the centromere. J. Genet 37: 341-364.

DARLINGTON, C. D. 1940. The origin of isochromosomes. J. Genet. 39: 351-361.

MORRIS, R., J. W. SCHMIDT, V. A. JOHNSON and T. TAIRA 1969. Aneuploid studies at the University of Nebraska. EWAC Newsletter 2: 55-56.

SANCHEZ-MONGE, E. and J. MAC KEY 1948. On the origin of subcompactoids in Triticum vulgare. Genet. 25: 483-520.

SEARS, E. R. 1946. Isochromosomes and telocentrics in Triticum vulgare. Genet 31: 229-230.

SEARS, E. R. 1952a. Misdivision of univalents in common wheat. Chromosoma 4: 535-50.

SEARS, E. R. 1952b. The behavior of isochromosomes and telocentrics in wheat. Chromosoma 4: 551-562.

STEINITZ-SEARS, L. M. 1966. Somatic instability of telocentric chromosomes in wheat and the nature of the centromere. Genetics 54: 241-248.

UPCOTT, M. B. 1937. The external mechanics of the chromosomes. VI. The behavior of the centromere at meiosis. Proc. Roy. Soc. (London)B 124: 336-361.

(Received August 20, 1973)