RIBOSOMAL RNA GENE STRUCTURE AND FUNCTION IN ALLOTETRAPLOID Arabidopsis AND Brassica

¹ Department of Soil and Crop Sciences Texas A&M University College Station, TX 77843-2474

² Biology Department, Washington University, St. Louis, MO 63130

Polyploidization is an important evolutionary process for plant speciation. It is estimated that over 80% of flowering plants and over 90% of species in the grass family are polyploid. Most important crops are polyploid (e.g., canola, cotton, maize, oat, potato, soybean, tobacco, and wheat). The common occurrence of polyploids in nature probably reflects an advantage of having redundant genes, freeing some gene copies from the constraints of natural selection, therefore allowing accumulation of new mutations that improve fitness. Consistent with this notion, polyploids are more widely distributed over more habitats than their diploid progenitors. Most polyploids are considered to be genetic diploid because the number of genes expressed in the polyploids could be reduced to that expressed in the diploid progenitor. Little is known, however, about changes in expression of parental genes and their roles in polyploid evolution. Allopolyploidy, one of the most common forms of polyploidy, is resulted from interspecific hybridization, in which two or more sets of genomes from different progenitor species are brought together in a single nucleus. Using Arabidopsis and its related species, Brassica as systems, we are interested in understanding how genomic changes occur during the process of interspecific hybridization and polyploidization and what is the consequence of these changes. We have found that repetitive rRNA genes are differentially expressed in polyploid plants; only one set of parental rRNA genes is expressed, the other set is silenced. This phenomenon is known as nucleolar dominance. The genes are silenced not by alterations in DNA sequence but by epigenetic regulation associated with chromatin. The silenced genes can be reactivated by changes in DNA methylation and histone acetylation; suggesting a role of chromatin in differential expression of parental rRNA genes in allotetraploid plants.