Nuclear DNA content varies between lines of maize and beween maize and teosinte

We have studied variation in 4C nuclear DNA content in the genus Zea by microdensitometry of Feulgen-stained, root-tip prometaphase cells. Our results, summarized in Table 1, showed that:

1) There was highly significant variation in 4C DNA content in maize, ranging from 9.84 pg for the single cross hybrid sweetcorn Seneca 60, to 13.49 pg for a Zapalote Chico accession from southern Oaxaca in Mexico (a difference of 37%.

2) The 4C DNA contents of annual teosintes from Mexico and Northern Guatemala were all within the range recorded for maize. However, the DNA contents of different races of teosinte were significantly different (p < 0.025). The DNA values for these teosintes were, for the most part, similar to those for three "primitive" races of maize from Mexico (i.e. Palomero Toluqueno, Chapalote and Nal-Tel).

3) Z. luxurians (or Guatemala teosinte), a 2n = 20 annual from southern Guatemala, differed from all other 2n = 20 taxa having a much higher 4C DNA content (18.38 pg). This is 871/( higher than the value for Seneca 60 (9.84 pg) and 36% higher than the value for Zapalote Chico (13.49 pg).

4) The 4C DNA content for perennial Z. diploperennis (10.57 pg) was slightly lower than for other "wild" Zea, but within the range recorded for maize. The tetraploid Z. perennis had a DNA content close to twice that of diploid Z. diploperennis.

Differences in DNA content between maize lines have also been found in an independent study by H.J. Price et al. (Texas A & M University). They also showed that variation in DNA content is positively correlated with knob number and percent C-band heterochromatin in the haploid component. Our observations are consistent with theirs since, for example, Seneca 60 has only 6 small heterochromatic bands on C-banded root-tip cells while Zapalote Chico has up to 24, including that of the abnormal K10 chromosome. It also seems likely that variation in DNA content is correlated with differences in the amount of heterochromatin in annual teosintes from Mexico and Northern Guatemala, since higher DNA contents were found in the races with the most heterochromatic knobs.

However, variation in the amount of heterochromatin is probably not the only cause of variation in DNA content in Zea, since: (i) Knobless Tama Flint did not have the lowest DNA content (Table 1), and, (ii) Z. luxurians does not appear to have enough heterochromatin to account for its markedly larger genome.

An interesting feature of the variation in DNA content in maize is its correlation with geographical features. Thus, there appears to be a decrease in both mean knob number and DNA content with increasing latitude of cultivation for maize stocks from the USA. Similarly there is a decrease in knob number (and hence presumably in DNA content) with increasing altitude for maize races grown in Mexico. The fact that such correlations exist suggests that these characters are of adaptive significance. This possibility deserves further study.

One way in which we hope to exploit the variation in DNA content in Zea is to construct hybrids where the chromosomes of both parental sets can be identified on the basis of chromosome size and arm ratio. In such hybrids, the spatial distribution of chromosomes from each known parental set could be investigated in a single cell using the serial thin-section, 3-D reconstruction technique.

Table 1. 4C nuclear DNA content in the genus Zea (N.B. All taxa had 2n = 20 chromosomes except Zea perennis, which had 2n = 40).

D.A. Laurie and M.B. Bennett
 
 


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