Somatic transposition and meiotically driven elimination of an active helitron family in Pleurotus ostreatus

Helitrons constitute a superfamily of DNA transposons that were discovered in silico and are widespread in most eukaryotic genomes. They are postulated to mobilize through a “rolling-circle” mechanism, but the experimental evidence of their transposition has been described only recently. Here, we present the inheritance patterns of HELPO1 and HELPO2 helitron families in meiotically derived progeny of the basidiomycete Pleurotus ostreatus. We found distorted segregation patterns of HELPO2 helitrons that led to a strong under-representation of these elements in the progeny. Further investigation of HELPO2 flanking sites showed that gene conversion may contribute to the elimination of such repetitive elements in meiosis, favouring the presence of HELPO2 vacant loci. In addition, the analysis of HELPO2 content in a reconstructed pedigree of subclones maintained under different culture conditions revealed an event of helitron somatic transposition. Additional analyses of genome and transcriptome data indicated that P. ostreatus carries active RNAi machinery that could be involved in the control of transposable element proliferation. Our results provide the first evidence of helitron mobilization in the fungal kingdom and highlight the interaction between genome defence mechanisms and invasive DNA.