The genus Zea exhibits intra- and interspecific variation in DNA content (Price, 1988). Rayburn et al. (1985) found a significant negative correlation between DNA content and latitude and a significant positive correlation between genome size and the amount of heterochromatin in North American inbred and open pollinated lines of maize. Tito et al. (1991) found a positive relationship between genome size and the interval from germination to flowering in several races and lines of maize.
In the present investigation the 2C DNA content of several races from northwestern Argentina is examined. In this report we present information on range of DNA content variation and discuss the origin of this variation.
DNA content was measured in telophase nuclei (2C) of the root apex of germinating seeds. Roots of 0.5-1cm length were fixed in 3:1 (alcohol: acetic acid) for 1-4 days. After fixation roots were rinsed for 30 min in distilled water. Hydrolysis was carried out with HCl 5N at 20 C + 2 for 30 min. The roots were then washed three times in distilled water for 10-15 min and stained for 120 min in Schiff's reagent at pH 2.2. The material was then rinsed three times in SO2 water for 10 min each, kept in distilled water (5-15 min) and squashed in 45% acetic acid. The coverslip was removed after freezing with CO2 and the slide was dehydrated in absolute alcohol and then mounted in Euparal. The amount of Feulgen staining per nucleus, expressed in arbitrary units, was measured at a wavelength of 570nm using the scanning method with a Zeiss Universal Microspectrophotometer (UMSP 30). The DNA content per basic genome, expressed in pg, was calculated using c-tester line as a standard (2C= 5.78pg). The differences in DNA content were tested by an analysis of variance and comparisons between means using Scheffe's method.
There is interracial variation, ranging from 2C= 4.31pg (Altiplano) to 2C=8.8pg (Negro), which represent the lowest and highest values so far obtained and consequently the greatest range. The percent difference (51%) is the biggest found in maize. Other authors report Gaspe flint with 2C= 4.9 pg as the lowest value (Rayburn et al., 1985) and Zapalote chico with 2C=6.74pg (Laurie and Bennett, 1985), SC6 with 2C=6.87pg (Tito et al., 1991), and Tuxpeno with 2C=7.71pg (Guillin et al., 1991) among the highest values. The intraracial variation found in the Argentine races is 11-35% (Table 1).
Two sources of variation of DNA content were detected in these races. One is the presence of B-chromosomes and the other is the polymorphism for repetitive DNA. Porter and Rayburn (1990) did not find a correlation between B-chromosomes and DNA content in twelve maize populations from Arizona. In the races studied in the present work the presence of B-chromosomes (Fig. 1) did not always increase the value of DNA content (Table 1). The DNA content and the determination of chromosome number was made in the same individual. A possible explanation for the observed results is the presence of polymorphism for repetitive DNA in A-chromosomes, which could mask the differences in total DNA content due to variation in the number of B-chromosomes. Differences observed in number of C+ bands in A-chromosomes among individuals of Negro race supported this hypothesis.
Table 1. DNA content and B-chromosomes in six native races of maize
from northwestern Argentina.
|Taxa||DNA (2C) XE.±S.(pg)||No. nucleus||2n|
|"Amarillo de Ocho"||5.49±0.29 bd||20|
|"Capia blanco"||5.24±0.18 b|
|5.86 ±0.32 b|
|"Capia rosado"||5.62±0.18 b||20+1B|
1. Mitotic metaphase. A: "Pisingallo" 2n=20+2B's. B: "Amarillo de ocho"
2n=20+4B's. Arrows show B-chromosomes. The bar=10µm.
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