Autopolyploidization affects transcript patterns and gene targeting frequencies in Physcomitrella

Qualitative changes in gene expression after an autopolyploidization event, a pure duplication of the whole genome, might be relevant for a different regulation of molecular mechanisms between angiosperms growing in a life cycle with a dominant diploid sporophytic stage and the haploid-dominant bryophytes. Whereas angiosperms repair DNA double strand breaks (DSB) preferentially via non-homologous end joining (NHEJ), in bryophytes homologous recombination (HR) is the main DNA-DSB repair pathway facilitating the precise integration of foreign DNA into the genome via gene targeting (GT). Here, we studied the influence of ploidy on gene expression patterns and GT efficiency in the moss Physcomitrella using haploid plants and autodiploid plants, generated via an artificial duplication of the whole genome. Single cells (protoplasts) were transfected with a GT construct and material from different time-points after transfection was analysed by microarrays and SuperSAGE sequencing. In the SuperSAGE data, we detected 3.7% of the Physcomitrella genes as differentially expressed in response to the whole genome duplication event. Among the differentially expressed genes involved in DNA-DSB repair was an upregulated gene encoding the X-ray repair cross-complementing protein 4 (XRCC4), a key player in NHEJ. Analysing the GT efficiency, we observed that autodiploid plants were significantly GT suppressed (p<0.001) attaining only one third of the expected GT rates. Hence, an alteration of global transcript patterns, including genes related to DNA repair, in autodiploid Physcomitrella plants correlated with a drastic suppression of HR.