Nuclear Genome Size Variation within Maize and Teosinte.

In higher plants, it has been hypothesized that nuclear DNA content controls life-cycle time through its sequence (direct) and through the physical-mechanical effects of its mass (indirect). These indirect effects—the nucleotype— are thought to influence the duration and rate of mitosis and meiosis and the minimum generation time of a species. Intraspecific variation for nuclear DNA content is known to exist in maize (Zea mays ssp. mays) and has been shown to be correlated with geographical adaptation and amounts of knob heterochromatin, but the extent of its variability and relationship with other genomic properties and phenotypic traits are less known. To that end, we used flow cytometry to investigate the range of nuclear DNA content variation among 27 diverse maize inbred lines (founders of the maize Nested Association Mapping population) that are representative of breeding efforts and worldwide genetic diversity. Significant intraspecific variation for nuclear genome size was found to exist among these 27 inbred lines and within the range of diploid teosintes (Zea mays ssp. parviglumis, Zea mays ssp. huehuetenangensis, Zea mays ssp. mexicana, Zea diploperennis) from Mexico and Guatemala. Nuclear genome size of the NAM founders was shown to be a moderately heritable trait, thus suggesting that genome size variation is partly under genetic control.  In addition, we detected a strong correlation between genome size of the NAM founder lines with adaptive phenotypic traits and population structure, but only a weak correlation with recombination rate in the maize NAM population. These results and possible mechanisms for genome expansion/contraction will be discussed.