In the maize landraces from Northeastern Argentina inter and even intra-populational genome size variations were detected. Moreover, high variation in number, positions, percentage of heterochromatin as well in size and sequence compositions of knobs were detected. Since knobs would be an important cause of the observed differences in DNA content, the absence of a significant relationship between the percentage of heterochromatin and genome size suggests that other non-coding repetitive DNA sequences contribute to genome size variation. The positive correlations between the length of the vegetative cycle and percentage of heterochromatin found allowed us to attribute an adaptive effect to the heterochromatin, since the vegetative cycle time would be optimized via selection for an appropriate percentage of heterochromatin.

In several genus evolutionary chromosome change involves variation in DNA amount in diploids and genome downsizing in polyploids. The constancy of bimodal karyotypes, even with changes in ploidy level and DNA content per basic genome indicate that the distribution of DNA within the complement is not at random and suggest the presence of mechanisms selecting for constancy, or against changes, in the karyotype morphology.

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