Nonrandom arrangement of somatic chromosomes of maize and its implication

Somatic association is a comparatively recent aspect of cytological analysis, and several reports regarding nonrandom arrangement of homologous chromosomes at mitotic stages have been forthcoming since the last decade. The present report deals with a pooled analysis of nonrandom organization of satellited chromosomes in twenty-two cultivars of maize.

The materials included six inbreds, CM-104, CM-105, CM-114, CM-115, CM-201 and CM-206; two synthetics, B19 and B23, two composites, Vijay and Diara, and three hybrids, G-5, GS-2 and Globe Hybrid. Also included were two varieties, Golden Bantam and Stowell's Evergreen, and seven races, Nal-Tel (Yucatan-7), Comfite Morocho, Tama Flint, Sikkim Primitive, Warangal Local, Kalimpong Local and Sonada Local. The treatment schedule for mitotic analysis was the same as was detailed in a previous report (see this News Letter).

Satellited chromosomes as mentioned above were used for analyzing the nonrandom organization. Altogether 93 plates were selected, adhering strictly to the following criteria:

(1) All the chromosomes were contained within the perimeter of a figure, whose major-minor axis ratio would not exceed 2:1.

(2) All the chromosome organelles (telomeres, centromeres and satellites) were present and clearly defined.

Distances between the centromeres of the two satellited chromosomes in a cell were measured. To minimize the differences due to the degree of squashing, each distance between the satellited chromosomes was divided by the distance between the two chromosomes farthest apart in the cell concerned. Throughout the report, this standardized distance was referred to as the distance between the satellited chromosomes. The pooled data were classified and statistically analyzed.

The observed distribution was compared with a theoretical distribution of a random pair of points uniformly distributed inside a circle. According to Hammersley and Lord, the distance between such a pair of points has the probability density function for a circle of radius 0.5 (diameter 1-0):

Equation.

The expected and observed frequencies, means and variances were tested by relatively more sensitive parametric tests of significance such as X2 and t tests, rather than by the nonparametric Kolmogorov-Smirnov goodness of fit test, employed in some earlier publications. The results are presented in Table 1.

The expected mean distance and variance were 0.4527 and 0.0451, respectively. The class 0.351 - 0.450 was expected to have the maximum frequency. The mean and variance of the observed data were 0.3892 and 0.0468, respectively. The maximum frequency class was that with a range of 0.251 - 0.350.

The two means were tested for their equality by a t-test, with the resultant t-statistic being 2.832. The observed and expected variances were examined by a X2 test; the X2 value turned out to be 83.99. Similarly, the expected and observed frequency distributions were analyzed. The X2 value was 102.384. All of these tests of significance showed highly significant differences between the expected and observed values. This indicated that the two satellited chromosomes would tend to lie significantly closer together than expected on the basis of random distribution for a pair of points in a circle.

The previous reports differ with regard to the nonrandom arrangement of chromosomes in maize. M. P. Maguire (Chromosoma 21:221-231) indicated premeiotic and somatic association of homologous chromosomes, based on her observations on the relative location of heterochromatic knobs in the interphase nuclei. J. H. Miles (MNL 45:136-139, and Ph.D. Thesis, Indiana University) found that the two chromosomes were not usually associated with each other, but they were associated in the presence of K10. J. D. Horn (MNL 44:198-200 and 45:209-210) presented preliminary data suggesting the association of some, but not all chromosomes. Chromosome 6 did not differ significantly from random expectation. R. G. Palmer (Chromosoma 35:233-246) analyzed the anaphase distributions of chromosomes 1 and 6 in premeiotic mitosis of ameiotic and normal plants and found that they were randomly distributed. J. D. Horn and D. B. Walden (Genetics 88:181-200) proposed affinity distance values among all the chromosomes of the complement. They also suggested primary homologous associations brought about by microtubules and preeminence of the nucleolus. They further reported values for chromosome 6, which were just significantly different from the theoretical value. D. J. deKergommeaux and D. B. Walden (MNL 55:124) further confirmed that the orientation of chromosome 6 is insensitive to an alteration in the microtubules.

This report, which is based on pooled data from several cultivars, shows significantly different values for chromosome 6. This conclusion agrees with that of Horn and Walden, and of deKergomeaux and Walden to a considerable extent. It differs from those of Miles and of Horn. Possibly, the genotypic differences in the materials employed in different studies may account for the observed deviations. It may be concluded that cultivars of maize probably differ with respect to somatic association, which is a process under genetic control. (The present investigation was carried out under the supervision of Prof. A. K. Sharma, at the Department of Botany, University of Calcutta, Calcutta).

Table 1.

J. S. P. Sarma


Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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