Sixty-eight individuals from maize from Bolivia were studied. These individuals are from 5 races which represent different altitudinal strata. This fact let us compare differences in frequency of B chromosomes and total DNA content variation in different ecogeographical conditions.
The individuals from races of high altitudes (3200m), Tuimuru, Pisankalla and Jampe Tongo have a higher frequency of individuals with B chromosomes than those from lower altitudes (races Duro Amazónico and Blando Cureño, 200 m).
Individuals with 2n=20+1B (Table 1) have 10ii+1i in all studied cells (Fig. 1A). In 80% of the dyads (T1) studied the B chromosome is included in the nucleus. Individuals with 2 B chromosomes have 10ii+1iiB in 80% of the studied cells (Fig. 1B). In 5% of them the B is not included in the nucleus. Thus, the meiotic behaviour insures the inclusion of B chromosomes in the nucleus in most of the studied cells.
Figure 1. Race Pisankalla. a) Metaphase in individual with one B (10ii+1iB); B) diakinesis in individual with 2 B(10ii+1iiB). Bar=10µm.
The DNA content (2C, pg) was measured using the scanning methods with a Zeiss Universal Microspectrophotometer (UMSP 30), at a wavelength of 570nm. The method (Feulgen stain) was according to Tito et al. (TAB 83:58-64, 1991). The differences in DNA content were tested by an analysis of variance and the comparisons between means by using Scheffe's method. Several authors have found that there is a positive correlation among nuclear DNA content variation and several cellular parameters (chromosome length or volume, heterochromatin, banding pattern, satellite DNA, etc.). Variation has also been found in the nuclear DNA with respect to geographical distribution. Bennett (Jones & Brandham eds., Curr. Chrom. Research p. 151-158, 1976) concluded that there was a DNA amount-latitude cline. A high positive correlation between the northern limit of cultivation of several grasses and DNA amount per diploid genome was found. Rayburn et al. (Am. J. Bot. 72:1610-1617, 1985) observed an opposite cline in Zea mays ssp. mays.
With respect to altitude, Rayburn (Evol. Trends Plants 4:53-57, 1990) described in 12 maize populations from SW U.S. a significant negative correlation between altitude and genome size. These results are in agreement with the negative correlation observed between knob number and altitude in Mexican maize populations.
However, Rayburn and Auger (TAG 79:470-474, 1990) found that U.S.A. populations at 1500m have larger genome size than populations from lower altitudes. Low genome size appears to be a characteristic of maize from elevations above 1800 m (Rayburn 1990 ibid.). These data indicate that adaptation in maize to altitude, with respect to genome size, is very complex.
The variation in nuclear DNA content described could be due to intrachromosomal DNA or supernumerary chromosomal variation. As described above, races from high altitudes have a higher frequency of B's that those of lower altitudes.
In order to analyze DNA content variation independently of B's, we first analyzed DNA content of individuals with 2n=20 chromosomes. Analysis of variance showed that differences between races from higher and lower altitudes were nonsignificant (F=14.96, alpha=0.05). The only significant differences found were between two individuals, one from race Duro Amazónico (2C=7.51 pg) and the other from race Blando Cruceño (2C=5.13pg).
The variation in total DNA content of individuals with B chromosomes (1-3 B's) was within the range of individuals without B's. No significant correlation between B chromosome number and nuclear DNA content was observed. This suggests that intrapopulational variation in the amount of A chromosomal DNA (due to differences in knob number, heterochromatin amount, repeat sequences interspersed with nonrepetitive DNA, etc.) masks the variation due to number and size of B chromosomes.
The authors would like to thank Ing. Agr. Gonzalo Avila, Director del Inst. Fitotéc. de Pairumani, Cochabamba, Bolivia, for supplying seed.
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