CYTOSINE METHYLATION AND NUCLEOLAR DOMINANCE IN CEREAL HYBRIDS

Authors: Guill, Katherine; O'DELL, M - JOHN INNES INSTITUTE; FLAVELL, R - JOHN INNES INSTITUTE; Gustafson, J

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary: Rye and wheat, as individual species, produce quality foods and forage for man and beast alike. Each crop species has its own set of problems and attributes. Rye has excellent disease resistance, but poor bread making qualities. Wheat has excellent bread and pasta potential though it can be rather susceptible to molds, fungal infection, and pest infestation. The hybridization of rye and wheat, producing a species commonly referred to a triticale, has been accomplished by simple crossing procedures. Unfortunately, the parental contributions, especially in quality attributes, are not always fully expressed in hybrid individuals. In fact, the wheat parent is more evident than the rye parent in triticale offspring. This research found wheat maintained dominance over rye in a particular region of the nucleus that played a role in gene expression. Wheat is able to out-compete rye for factors acting at the DNA level that are required for gene transcription. Small-cereal-grain breeders and scientists armed with knowledge of the mechanism for gene expression will be able to apply this to their work when making hybrid selections or studying ribosomal RNA gene expression.

Technical Abstract: Agronomically useful expression of alien genomes, genes, or gene complexes, when placed into a host background, is often less than parental expression. Rye nucleolus organizer regions (NORs), the sites of ribosomal RNA genes, are suppressed in wheat-rye hybrids. Wheat and wheat-rye hybrid genetic stocks containing differing numbers of wheat and rye nucleolus organizers, as well as addition lines and rye-barley hybrids were used in Southern hybridization experiments to determine the cause of nucleolar dominance and suppression in cereal hybrids. With knowledge of restriction endonuclease sites that isolate a large region of the spacer unit (the region of DNA between ribosomal genes) and the addition of a methylation sensitive enzyme, HpaII, an indirect method of assaying NOR expression was established. HpaII will not recognize the CCGG (C = cytosine, G = guanine) restriction site if either C has a methyl group attached. The results indicated less cleavage by the HpaII enzyme on the rye NOR sequences when major NORs from other cereals were present. The reduction in the number of rye rRNA genes containing an unmethylated CCGG site in the promoter was associated with the suppression of the rye nucleolus. These results were consistent with a model in which promoter and upstream regulatory repeats of ribosomal RNA genes compete for limited concentrations of regulatory proteins and where genes, methylated at key binding sites, fail to attract these regulatory proteins and thus remain inactive.