title: Restoration of female fertility in Trichoderma reesei QM6a provides the basis for inbreeding in this industrial cellulase producing fungus.
abstract: BACKGROUND
  Filamentous 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.
  
  
  RESULTS
  We 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.
  
  
  CONCLUSIONS
  The 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.
authors.affiliation: ACIB GmbH, c/o Institute of Chemical Engineering, Technische Universität Wien, Gumpendorferstraße 1a, 1060 Vienna, Austria.
authors.forename: Rita
authors.initials: R
authors.lastname: Linke
authors.affiliation: Bioinformatics, Institute for Knowledge Discovery, Graz University of Technology, Petersgasse 14/V, 8010 Graz, Austria ; Core Facility Bioinformatics, ACIB GmbH, Petersgasse 14/V, 8010 Graz, Austria.
authors.forename: Gerhard G
authors.initials: GG
authors.lastname: Thallinger
authors.affiliation: AB Enzymes GmbH, Feldbergstrasse 78, 64293 Darmstadt, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Haarmann
authors.affiliation: AB Enzymes GmbH, Feldbergstrasse 78, 64293 Darmstadt, Germany.
authors.forename: Jasmin
authors.initials: J
authors.lastname: Eidner
authors.affiliation: AB Enzymes GmbH, Feldbergstrasse 78, 64293 Darmstadt, Germany.
authors.forename: Martina
authors.initials: M
authors.lastname: Schreiter
authors.affiliation: AB Enzymes GmbH, Feldbergstrasse 78, 64293 Darmstadt, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Lorenz
authors.affiliation: ACIB GmbH, c/o Institute of Chemical Engineering, Technische Universität Wien, Gumpendorferstraße 1a, 1060 Vienna, Austria ; Research Division Biotechnology and Microbiology, Institute of Chemical Engineering, Technische Universität Wien, Gumpendorferstraße 1a, 1060 Vienna, Austria.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Seiboth
authors.affiliation: ACIB GmbH, c/o Institute of Chemical Engineering, Technische Universität Wien, Gumpendorferstraße 1a, 1060 Vienna, Austria ; Research Division Biotechnology and Microbiology, Institute of Chemical Engineering, Technische Universität Wien, Gumpendorferstraße 1a, 1060 Vienna, Austria.
authors.forename: Christian P
authors.initials: CP
authors.lastname: Kubicek
keywords: Cellulase; Comparative genomics; Female fertility; Strain breeding; Trichoderma reesei