B chromosomes in native races of maize from Argentina. II. Genome size and altitude
--Rosato, M; Chiavarino, AM; Naranjo, CA ; Poggio, L

The DNA content of 17 populations (107 individuals) with varying mean frequencies of B-chromosomes (Bs) was determined (Table 1). The materials used are the same that were detailed in part l (Populational frequency and altitude) in this MNL. With the aim of studying the variation in DNA content of A-chromosomes (A-DNA) independently from the variation supplied by the Bs, this estimation was performed in plants without Bs (2n=20) (5 to 6.756pg). In these populations a 36% variation in DNA content of the members of the regular complement was demonstrated. This range of variation is very high compared with the values reported for 32 populations of maize from the USA and Mexico (Laurie and Bennett, Heredity 55:307-313, 1985; Rayburn et al. Am. J. Bot. 72:1610-1617, 1985). It was found that A-DNA content is negatively correlated with altitude of cultivation (r= -0.3139, p=0.0022) (Fig. 1). However, when non-polymorphic populations (VAV 6473 and 6474) were included in the analysis the correlation was not significant (r= -0.1012, p=0.3045) due in this case to their high A-DNA content.

Other authors also observed similar results. Rayburn (Evol. Trends Plant 4:53-57, 1990) found a negative correlation between total DNA content and altitude in 12 american populations of maize growing from 4900 to 7000 ft (1493 to 2134m).

The only exception was that reported by Rayburn and Auger (Theor. Appl. Genet. 79:470-474, 1990), who found an inverse relationship in 12 Indian maize populations collected from 100 to 5300 ft (31 to 1621m).

A significant negative correlation between mean DNA content (measured in 0B individuals) and the mean frequency of Bs per plant was found in the 17 populations (r=-0.3348, p<0.0001) (Figure 2) (Table 1). Interestingly, populations belonging to the same race also showed the same tendency. For instance, populations VAV 6484, 6476 and 6451, belonging to "Amarillo chico" race, with frequency of Bs of 0.20, 0.25 and 0.9 had decreasing values of A-DNA content (6.353, 6.280 and 5.665pg respectively). The same happened when populations VAV 6474, 6473 and 6167 (all from Altiplano race) were compared. The first two, cultivated at 3520m and 3620m, without Bs, had an A-DNA content of 6.5 pg and 6.49 pg whereas the VAV 6167, with a frequency of Bs of 1.29 showed a lower DNA content in the A genome (5.0pg) (Table 1).

A similar phenomenon was found by Longley (J. Agric. Res. 56:177-195, 1938) when analyzing the association between the number of knobs and the occurrence of Bs. He found that Bs are more frequent in plants with fewer than seven knobs in their A chromosomes.

Tito, Poggio and Naranjo (Theor. Appl. Genet. 83:58-64, 1991) observed that higher values of heterochromatin content (equivalent to knobs) are correlated with increasing values of total DNA content in several varieties and lines of maize. As has already been mentioned, the decrease of DNA content in 0B plants could be related to a decrease in heterochromatic bands (knobs).

In conclusion:

-The negative correlation between A-DNA content and altitude of cultivation could be explained if genome size played an adaptive role in relation to environmental variables.
-The positive correlation between frequency of plants with Bs and altitude is closely related to the A-DNA amount.
-The negative correlation between frequency of plants with Bs and A-DNA amount indicates that Bs are better tolerated in populations with lower A-DNA values.
-The results obtained would indicate that there would be a maximum limit to the mass of nuclear DNA or nucleotype (Bennett, New Phytologist 106 (Suppl.):177-200, 1987) so that Bs would be tolerated as long as this maximum limit is not exceeded.
-The clinal variation of A-DNA content and the consequent inverse correlation of B frequencies over an altitudinal gradient could have an adaptive significance, since the populations analyzed were not distributed over a transect (Fig. 1). They were cultivated by aborigines and hardly interbreed with each other. Therefore, historical isolation of populations with high and low A-DNA content by genetic drift at both extremes of an altitudinal gradient, and subsequent gene flow between them, is not a very plausible explanation.

Table 1. Races, altitude of cultivation, mean number of Bs, and mean DNA content of plants without B chromosomes.
Race Population Altitude (m) Mean number of Bs DNA content ±SE (pg) (no. plants)
Blanco VAV 6479 2180 2.615 6.410 ±0.08 (3)
Blanco VAV 6485 2670 1.795 5.800 ±0.09 (6)
Amarillo grande VAV 6480 2420 1.596 6.105 ±0.10 (8)
Altiplano VAV 6167 3000 1.294 5.007 ±0.25 (4)
Harinoso VAV 6475 3240 1.100 6.450 ±0.13 (6)
Capia blanco VAV 6418  2600 1.020 5.767 ±0.14 (6)
Amarillo chico VAV 6451 2000 0.900 5.665 ±0.22(6)
Pisingallo VAV 6313 1600 0.684 6.148 ±0.10 (8)
Capia rosado VAV 6162 2900 0.649 5.741 ±0.22 (3)
Amarillo chico VAV 6476 1690 0.255 6.280 ±0.18 (6)
Blanco y ocho rayas VAV 6481 750 0.255 6.756 ±0.05 (7)
Amarillo chico VAV 6484 2010 0.204 6.353 ±0.19 (7)
Orgullo cuarentón VAV 6482 910 0.122 6.150 ±0.08 (6)
Blanco y ocho rayas VAV 6483 1250 0.111 6.601 ±0.12 (4)
Pichingá VAV 6170 80 0.023 6.172 ±0.10 (4)
Altiplano VAV 6474 3520 0.000 6.514 ±0.05 (7)
Altiplano VAV 6473 3620 0.000 6.493 ±0.20 (6)

Figure 1. Correlation of A-DNA content (in plants without Bs) with altitude.

Figure 2. Correlation of A-DNA content (in plants without Bs) with number of Bs per plant in its population.

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

Return to the MNL 71 On-Line Index
Return to the Maize Genome Database Page