Control of Nucleolar Expression in Triticale

Abstract

Ribosomal rRNA genes present in the nucleolar organizing region (NOR) in hexaploid wheat (Triticum aestivum L. em Thell.) are known to be expressed at different levels and some loci exhibit full or partial dominance over others. Nucleolar dominance is best seen in wide-hybrids like hexaploid triticale (X Triticosecale Wittmack) where the wheat loci dominate over the rye (Secale cereale L.) NOR locus. A correlation appears to exist between the methylation of cytosine residues and the expression of a rRNA locus suggesting that a dominant locus would have a much reduced methylation pattern over a recessive locus The effect of rye NOR expression in tetraploid triticales containing various wheat and rye NOR loci was studied. The results showed that when both wheat chromosomes 1B and 6B were present, the rye NOR locus was methylated and suppressed. When either 1B or 6B were present, some minor rye activity was seen, and that a slightly higher degree of activity was observed in the absence of 1B versus 6B. When both 1B and 6B were absent, the rye locus was expressed as in rye. The data indicates that either one of the wheat rRNA loci will suppress rye rRNA loci.