Recovery of sexual reproduction enables strain in-breeding in the industrial fungus Trichoderma reesei

Trichoderma reesei, an industrial producer of enzymes for food, feed and biorefinery purposes, is heterothallic. Since all industrial producer strains are derived from the single MAT1-2 isolate QM6a, strain improvement by crossing is not possible and simple switching of the mating type does not enable sexual reproduction. We have therefore used a systems biology approach to identify genes that enable sexual reproduction in the QM6a industrial strain line. To this end, T. reesei QM6a was crossed with the MAT1-1 wild-type strain H. jecorina CBS999.97, and the descendants were backcrossed 8-times in two lineages with T. reesei QM6a to obtain mating competent MAT1-1 strains with a minimal set of CBS999.97 specific genes. Two of the backcrossed strains (A8_02 and A8_11) were sequenced with Illumina technology at 400x coverage. Genes putatively involved in fruiting body formation were identified with comparative genome analysis.