Abstract
In maize, we studied the causes of genome size variation and their correlates with cultivation altitude that suggests the existence of adaptive clines. To discuss the biological role of the genome size variation, we focused on Bolivian maize landraces growing along a broad altitudinal range. These were analyzed together with previously studied populations from altitudinal clines of Northwestern Argentina (NWA). Bolivian populations exhibited numerical polymorphism for B chromosomes (Bs) (from 1 to 5), with frequencies varying from 16.6 to 81.8 and being positively correlated with cultivation altitude. The 2C values of individuals 0B (A-DNA) ranged between 4.73 and 7.71 pg, with 58.33% of variation. The heterochromatic knobs, detected by DAPI staining, were more numerous and larger in individuals 0B than in those with higher doses of Bs. Bolivian and NWA landraces exhibited the same pattern of A-DNA downsizing and fewer and smaller knobs with increasing cultivation altitude, suggesting a mechanistic link among heterochromatin, genome size and phenology. The negative association between the two types of supernumerary DNA (knob heterochromatin and Bs), mainly responsible for the genome size variation, may be considered as an example of intragenomic conflict. It could be postulated that the optimal nucleotype is the result of such conflict, where genome adjustment may lead to an appropriate length of the vegetative cycle for maize landraces growing across altitudinal clines.
Highlights
The knowledge of genome dynamics is essential for understanding the diversification of DNA content among closely related taxa
Northwestern Argentina (NWA) landraces exhibited the same pattern of A-DNA downsizing and fewer and smaller knobs with increasing cultivation altitude, suggesting a mechanistic link among heterochromatin, genome size and phenology
It could be postulated that the optimal nucleotype is the result of such conflict, where genome adjustment may lead to an appropriate length of the vegetative cycle for maize landraces growing across altitudinal clines
Summary
The knowledge of genome dynamics is essential for understanding the diversification of DNA content among closely related taxa. A recent study involving temperate and tropical maize inbred lines grown at sea level revealed that DNA content increases progressively with delayed flowering time [17] All these results suggest that genome downsizing may be associated with the rapid vegetative growth and early flowering observed in short growing seasons, which are typical of cool highland regions [6,8,11,18,19]. This assumption is supported by evidence showing that the percentage of knob heterochromatin of the A-chromosome set (A-HC) is negatively correlated with cultivation altitude, but positively correlated with the vegetative period. Propose the existence of an intranuclear conflict between the knob heterochromatin and the presence of B chromosomes
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