Variation and inheritance of cytosine methylation patterns in wheat at the high molecular weight glutenin and ribosomal RNA gene loci.

Chromosome marking by cytosine methylation has been examined in two gene systems in wheat--at the loci encoding high molecular weight (HMW) glutenin subunits (seed proteins) and ribosomal RNA. Variation in cytosine methylation occurs between progeny in highly inbred lines around the HMW glutenin locus. The variation is inherited through meiosis to F1 and F2 generations but occasionally a new variant arises. Specific cytosine residues lose their methyl group in the seed, the organ where the genes are expressed. Within the multigene family of ribosomal RNA genes, several subsets of genes can be defined based upon the cytosine methylation patterns. High activity of a ribosomal RNA gene locus is correlated with loss of methylation at specific cytosine residues, especially in the promoter and upstream regulatory regions. A model is described in which the subset of genes selected to be used are those to which specific regulatory proteins and transcription complexes bind most favourably. Binding of such proteins inhibits cytosine methylation and so marks the subset of genes for expression in subsequent cell generations. Examples are described where new types of RNA genes are introduced via sexual crosses that result in changes to the methylation patterns of the ribosomal RNA genes. The processes determining the changes begin, it is believed, in the fertilised egg.