Chromatin composition in maize F1 hybrids

Recent studies have demonstrated unexpected nuclear DNA content in F1 hybrids of maize. Flow cytometry was used to determine what portions of the nuclei were affected. Nuclei were isolated from eight maize inbred lines and seven F1 hybrids. The nuclei were stained with PI and were analyzed on a flow cytometer-cell sorter. The PI/DAPI ratios were calculated for each line. From these ratios, the amount of heterochromatin and euchromatin was estimated for different maize inbreds and specific F1 hybrids. The inheritance pattern of both heterochromatin and euchromatin in F1 hybrids was studied. In some F1 hybrids, there was increased heterochromatin amount with no increase in total DNA. In other hybrids, total DNA increase was associated with an increase in the amount of heterochromatin. C-banding analysis was performed to determine if variation in knob number was responsible for this variation. The knob number in each F1 hybrid was as predicted from the parental lines except in two hybrids. In addition, relative abundance of the knob sequence present in the G1 nuclei of F1 hybrids was determined by sorting nuclei and hybridizing with knob DNA sequences. The hybridization results indicated that the amount of knob DNA increases. The increase in knob DNA is not enough, however, to account for the total increase in heterochromatin and/or total nuclear DNA. These results demonstrate the role of heterochromatinization when two parental genomes are combined in a F1 hybrid genome. Chromatin structure changes within hybrid nuclei appear to be necessary for proper organization of the F1 genome.