Chromosomal influences on gene silencing in nucleolar dominance

Nucleolar dominance is an epigenetic phenomenon that involves the silencing of megabases of rRNA genes specific to one species in an interspecific hybrid. Several hundred rRNA genes are grouped together in a region termed the Nucleolar Organizer Region (NOR), which makes up a significant portion of the chromosome. In interspecific hybrids of Arabidopsis, entire NORs from the underdominant species are silenced independent of maternal or paternal influences. Nucleolar dominance could be controlled by the NOR itself or by loci outside of the NOR; evidence for both possibilities has been suggested in past studies. The work presented in here demonstrates that gene silencing due to nucleolar dominance is limited to the NOR and is not part of a larger silencing phenomenon. Secondly, individual underdominant rRNA transgenes that were stably integrated in chromosomal locations outside the NOR are capable of escaping silencing in the hybrid. This suggests that components of the individual genes are not sufficient in nucleolar dominance. Interestingly, an rRNA transgene located in heterochromatin was silenced in the hybrid, suggesting that the heterochromatic environment of rRNA genes may be important. Translocation of an underdominant NOR to another location does not affect nucleolar dominance, indicating that any chromosomal influences specific to the location of the NOR must have been included in the translocation. Telomeres adjoin A. thaliana NORs, but interspecific hybrids made using a telomerase-deficient A. thaliana parent are unaffected for NOR silencing, arguing against telomere effects. Last, a genetic analysis of nucleolar dominance was performed within two ecotypes of the diploid species A. thaliana. Quantitative Trait Loci (QTL) analyses indicate that the NORs themselves are responsible for 50% of the variation, suggesting that an epigenetic mechanism could partly control nucleolar dominance. However, QTL analysis identified an unlinked locus, implicating other genetic influences in the establishment of nucleolar dominance.