A CHANGE OF PLOIDY CAN MODIFY EPIGENETIC SILENCING

A silent transgene in if Arabidopsis thaliana was reactivated in an ou tcross but not upon selfing of hemizygous plants. This result could on ly be explained by assuming a genetic difference between the transgene -free gametes of the wild-type and hemizygous transgenic plants, respe ctively, and led to the discovery of ploidy differences between the pa rental plants. To investigate whether a change of ploidy by itself can indeed influence gene expression, we performed crosses of diploid or tetraploid plants with a strain containing a single copy of a transgen ic resistance gene in an active state. We observed reduced gene expres sion of the transgene in triploid compared with diploid hybrids. This led to loss of the resistant phenotype at various stages of seedling d evelopment in part of the population. The gene inactivation was revers ible. Thus, an increased number of chromosomes can result in a new typ e of epigenetic gene inactivation, creating differences in gene expres sion patterns. We discuss the possible impact of this finding for gene tic diploidization in the light of widespread, naturally occurring pol yploidy and polysomaty in plants.