Abstract
BackgroundFilamentous fungi are frequently used as production platforms in industrial biotechnology. Most of the strains involved were known as reproducing exclusively asexually thereby preventing the application of conventional strain breeding techniques. In the last decade, evidence was obtained that a number of these imperfect fungi possess a sexual life cycle, too. Trichoderma reesei, an industrial producer of enzymes for food, feed and biorefinery purposes, is heterothallic and takes a special position among industrially utilized species as all industrial strains are derived from the single MAT1-2 isolate QM6a. Consequently, strain improvement by crossing is not feasible within this strain line as this necessitates a MAT1-1 mating partner. Simply switching the mating type in one of the mating partners to MAT1-1, however, is not sufficient to produce a genotype capable of sexual reproduction with QM6a MAT1-2.ResultsWe have used a systems biology approach to identify genes restoring sexual reproduction in the QM6a strain line. To this end, T. reesei QM6a was crossed with the MAT1-1 wild-type strain CBS999.97. The descendants were backcrossed 8-times in two lineages with QM6a to obtain mating competent MAT1-1 strains with a minimal set of CBS999.97 specific genes. Comparative genome analysis identified a total of 73 genes of which two—encoding an unknown C2H2/ankyrin protein and a homolog of the WD-protein HAM5—were identified to be essential for fruiting body formation. The introduction of a functional ham5 allele in a mating type switched T. reesei QM6a allowed sexual crossing with the parental strain QM6a.ConclusionThe finding that Trichoderma reesei is generally capable of undergoing sexual reproduction even under laboratory conditions raised hope for the applicability of classical breeding techniques with this fungus as known for plants and certain yeasts. The discovery that the wild-type isolate QM6a was female sterile, however, precluded any progress along that line. With the discovery of the genetic cause of female sterility and the creation of an engineered fertile strain we now provide the basis to establish sexual crossing in this fungus and herald a new era of strain improvement in T. reesei.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0311-2) contains supplementary material, which is available to authorized users.
Highlights
Filamentous fungi are frequently used as production platforms in industrial biotechnology
Linke et al Biotechnol Biofuels (2015) 8:155 because—in contrast to plants and yeasts, for which strain breeding by crossing has been an essential strategy for strain improvement—the majority of them were isolated in their asexual form and their sexual form and life cycle had been unknown for many years
Comparative genome analysis of these strains revealed cumulative sequence differences between them and T. reesei QM6a of a magnitude ranging from 21,000 to 141,000 single nucleotide variant (SNV) in all exon regions preventing the straightforward identification of genes responsible for female sterility
Summary
Filamentous fungi are frequently used as production platforms in industrial biotechnology. Trichoderma reesei, an industrial producer of enzymes for food, feed and biorefinery purposes, is heterothallic and takes a special position among industrially utilized species as all industrial strains are derived from the single MAT1-2 isolate QM6a. Hypocrea jecorina) is a major industrial enzyme producer, of cellulases and hemicellulases that are used for applications in the food, feed and biorefinery businesses [8,9,10,11]. This fungus too was for a long time considered to be asexual. Heterothallic species require two compatible mating partners each of them carrying a different mating type locus—either MAT1-1 or MAT1-2—which are required for successful sexual reproduction [13, 14]
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