The importance of DNA content in the origin and evolution of genotypes has been increasingly realized in recent years. It has, however, been recorded that both increase and decrease in DNA content may be associated with evolution. The increased DNA values have to a great extent been influenced by the large amount of amplified sequences. However, along with amplification there has been both individual duplication and polyploidy contributing towards the DNA increment in the origin of genotypes.
A comprehensive program has been undertaken on the study of somatic chromosomes of maize, with specific reference to intervarietal differences. The present report deals with in situ cytophotometric investigation of DNA content in the somatic chromosomes of 18 cultivars of maize, employing a two-wavelength technique. The two wavelengths selected were 550 nm and 562 nm and the aperture was 6.3. Extinction in the latter wavelength must be two times that in the former, and the relative absorbance values were noted. A minimum of 50 cells at metaphase (4C) were analyzed for each cultivar.
L1 and L2 values were calculated using appropriate conversion formulae. D values for respective proportions of mean L1 and L2 were referred to from Garcia (cited by Sharma and Sharma, 1980, Chromosome Techniques: Theory and Practice, Butterworths, London). The DNA values (m) were estimated by using the formula m = KAL1D), where K is a constant and A is the area of the aperture (pr2). Because K and p are constants, they were omitted for comparative purposes.
Table 1 gives the relevant details of the calculations. A scanning survey of Table 1 indicates that the relative DNA values of the different cultivars studied do not vary much. The minimum value is 0.8689 for Sonada Local, which is with knobless pachytene chromosome complement. The maximum value is 1.1019 for the cultivar Diara. It has knobs and B chromosomes as well. However, this correlation was not consistently observed in the case of other cultivars. In general, inbreds have relatively more DNA content than the knobless outbred races.
Tables 2 and 3 furnish a comparison of the relative DNA values with relevant cytological features, analyzed in the preceding reports of this News Letter. Further observation of Tables 2 and 3 reveals that the relative DNA quantity in different cultivars remains more or less constant, notwithstanding differences in their other cytological features such as knobs, B chromosomes, chromatin length of the haploid complements, etc. (The present investigation was carried out under the supervision of Prof. A. K. Sharma, at the Department of Botany, University of Calcutta, Calcutta.)
J. S. P. Sarma
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