Genome Size and Environmental Correlations in Maize (Zea mays ssp. mays, Poaceae)

Abstract

Variation in the DNA content attributed to sources such as variation in the amount of heterochromatin and occurrence of supernumerary chromosomes (Bs) in native races of maize cultivated at different altitudes (80–3620 m) is discussed. These populations present intra- and interpopulational variation in the DNA content of the regular complement (A-DNA) and the heterochromatic zones (DAPI bands). The mean number of Bs varied from 0 to 2·62 per plant among these populations, showing a positive correlation with the altitude of cultivation. In contrast, both the A-DNA content and the mean number of DAPI bands per plant were negatively correlated with altitude and the mean number of Bs per plant. These clinal variations in A-DNA content and the mean number of DAPI bands, and the inverse correlation of the mean number of Bs per plant over an altitudinal gradient could have an adaptative significance. Analysis of total DNA content and the number of DAPI bands, in individuals with different doses of Bs, indicates that in populations with high A-DNA content the increase in genome size due to Bs could be masked. This phenomenon is associated with the fact that individuals with Bs have a low number of DAPI bands. These results suggest that there is an optimum nucleotype for each population and that Bs are tolerated so long as this nucleotype is not exceeded.