Phenotypic instability and rapid gene silencing in newly formed arabidopsis allotetraploids

Allopolyploid hybridization serves as a major pathway for plant evolution, but in its early stages it is associated with phenotypic and genomic instab ilities that are poorly understood. We have investigated allopolyploidizati on between Arabidopsis thaliana (2n = 2x = 10; n, gametic chromosome number ; x, haploid chromosome number) and Cardaminopsis arenosa (2n = 4x = 32). T he variable phenotype of the allotetraploids could not be explained by cyto logical abnormalities. However, we found suppression of 20 of the 700 genes examined by amplified fragment length polymorphism of cDNA. Independent re verse transcription-polymerase chain reaction analyses of 10 of these 20 ge nes confirmed silencing in three of them, suggesting that similar to 0.4% o f the genes in the allotetraploids are silenced. These three silenced genes were characterized. One, called K7, is repeated and similar to transposons . Another is RAP2.1, a member of the large APETALA2 (AP2) gene family, and has a repeated element upstream of its 5' end. The last, L6, is an unknown gene close to ALCOHOL DEHYDROGENASE on chromosome 1. CNG DNA methylation of K7 was less in the allotetraploids than in the parents, and the element va ried in copy number. That K7 could be reactivated suggests epigenetic regul ation. L6 was methylated in the C. arenosa genome. The present evidence tha t gene silencing accompanies allopolyploidization opens new avenues to this area of research.