Trichoderma reesei meiosis generates segmentally aneuploid progeny with higher xylanase-producing capability.
BACKGROUND
Hypocrea jecorina is the sexual form of the industrial workhorse fungus Trichoderma reesei that secretes cellulases and hemicellulases to degrade lignocellulosic biomass into simple sugars, such as glucose and xylose. H. jecorina CBS999.97 is the only T. reesei wild isolate strain that is sexually competent in laboratory conditions. It undergoes a heterothallic reproductive cycle and generates CBS999.97(1-1) and CBS999.97(1-2) haploids with MAT1-1 and MAT1-2 mating-type loci, respectively. T. reesei QM6a and its derivatives (RUT-C30 and QM9414) all have a MAT1-2 mating type locus, but they are female sterile. Sexual crossing of CBS999.97(1-1) with either CBS999.97(1-2) or QM6a produces fruiting bodies containing asci with 16 linearly arranged ascospores (the sexual spores specific to ascomycetes). This sexual crossing approach has created new opportunities for these biotechnologically important fungi.


RESULTS
Through genetic and genomic analyses, we show that the 16 ascospores are generated via meiosis followed by two rounds of postmeiotic mitosis. We also found that the haploid genomes of CBS999.97(1-2) and QM6a are similar to that of the ancestral T. reesei strain, whereas the CBS999.97(1-1) haploid genome contains a reciprocal arrangement between two scaffolds of the CBS999.97(1-2) genome. Due to sequence heterozygosity, most 16-spore asci (>90%) contain four or eight inviable ascospores and an equal number of segmentally aneuploid (SAN) ascospores. The viable SAN progeny produced higher levels of xylanases and white conidia due to segmental duplication and deletion, respectively. Moreover, they readily lost the duplicated segment approximately two weeks after germination. With better lignocellulosic biomass degradation capability, these SAN progeny gain adaptive advantages to the natural environment, especially in the early phase of colonization.


CONCLUSIONS
Our results have not only further elucidated T. reesei evolution and sexual development, but also provided new perspectives for improving T. reesei industrial strains.
Taiwan International Graduate Program in Molecular and Cellular Biology, Academia Sinica, Taipei, 115 Taiwan ; Institute of Life Sciences, National Defense Medical Center, Taipei, 115 Taiwan ; Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan.
Yu-Chien
YC
Chuang
Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan ; Institute of Genome Sciences, National Yang-Ming University, Taipei, 112 Taiwan.
Wan-Chen
WC
Li
Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan.
Chia-Ling
CL
Chen
Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan.
Paul Wei-Che
PW
Hsu
Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan.
Shu-Yun
SY
Tung
Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan ; Present address: Department of Forest Sciences, University of Helsinki, Helsinki, Finland.
Hsiao-Che
HC
Kuo
Austrian Institute of Technology, Health and Environment Department, Bioresources, University and Research Center, UFT Campus Tulln, Tulln/Donau, 3430 Austria.
Monika
M
Schmoll
Taiwan International Graduate Program in Molecular and Cellular Biology, Academia Sinica, Taipei, 115 Taiwan ; Institute of Molecular Biology, Academia Sinica, Taipei, 115 Taiwan.
Ting-Fang
TF
Wang
Aneuploidy; Conidia pigmentation; Genome evolution; Hypocrea jecorina; Lignocellulosic biomass; Meiosis; Sexual development; Trichoderma reesei; Xylanase