Presentation Information
[P02-162]Retrotransposon-mediated loss of sexual reproduction in the katsuobushi fungus, Aspergillus chevalieri
○Kentaro Hiramatsu1, Kazuki Mori2, Chihiro Kadooka3, Kayu Okutsu4, Yumiko Yoshizaki1,4, Kazunori Takamine1,4, Kosuke Tashiro2, Masatoshi Goto1,5, Hisanori Tamaki1,4, Taiki Futagami1,4 (1. United Grad. Sch. Agric. Sci., Kagoshima Univ. (Japan), 2. Grad. Sch. Agric., Kyushu Univ. (Japan), 3. Fac. Biotechnol. Life Sci., Sojo Univ. (Japan), 4. Fac. Agric., Kagoshima Univ. (Japan), 5. Fac. Agric., Saga Univ. (Japan))
Keywords:
Aspergillus chevalieri,sexual reproduction,transcription factor,katsuobushi,HMG-box
Katsuobushi, a dried bonito, is a traditional Japanese fermented food. A particular type of katsuobushi, known as karebushi, is produced through repeated cycles of cultivating xerophilic fungi on the surface of bonito fillets. These processes reduce the moisture content and refine the flavor of karebushi.
Aspergillus chevalieri is one of the predominant fungi growing on the surface of karebushi. This species includes two types that differ in their life cycles: a teleomorphic type, which produces cleistothecia (sexual fruiting bodies), and an anamorphic type, which produces conidia (asexual spores) under same culture conditions. This contrasts with the model fungus Aspergillus nidulans, which switches between sexual and asexual development depending on environmental factors such as oxygen availability and light.
In addition, we observed that the teleomorphic strain of A. chevalieri was cultured on minimal agar medium, it initially observed yellow mycelium. However, upon prolonged incubation, it partially formed conidia on yellow mycelium. We isolated the conidia from these cultures, and subcultured on minimal agar medium, consistently exhibited an anamorphic phenotype. This observation suggests that the teleomorphic strain may have lost the ability of sexual reproduction. In this study, we aimed to understand why A. chevalieri lost the ability of sexual reproduction.
To identify the mechanism, we sequenced the genome of the anamorphic mutant using a PacBio long-read sequencer. Comparative genomic analysis of the teleomorphic strain and its anamorphic mutant revealed the insertion of an approximately 5.8-kb retrotransposon upstream of a gene encoding an HMG-box transcription factor in the anamorphic mutant. Disruption of this transcription factor gene in the teleomorphic strain resulted in the acquisition of an anamorphic phenotype, whereas complementation restored the teleomorphic phenotype. These findings indicate that this transcription factor is required for sexual reproduction in A. chevalieri. In addition, the expression level of the transcription factor gene in the retrotransposon-inserted anamorphic mutant was reduced to approximately 15% of that in the teleomorphic strain.
Furthermore, the expression levels of the mating-type genes MAT1-1 and MAT1-2 were reduced in the anamorphic mutant, suggesting that this transcription factor regulates MAT genes expression. In addition, single disruptants of MAT1-1 or MAT1-2, as well as the double disruptant, did not show an anamorphic phenotype; however, the number of cleistothecia were reduced, and the cleistothecia were sterile. These results suggest that the HMG-box transcription factor gene also regulates the genes involved in sexual reproduction in addition to MAT genes.
Aspergillus chevalieri is one of the predominant fungi growing on the surface of karebushi. This species includes two types that differ in their life cycles: a teleomorphic type, which produces cleistothecia (sexual fruiting bodies), and an anamorphic type, which produces conidia (asexual spores) under same culture conditions. This contrasts with the model fungus Aspergillus nidulans, which switches between sexual and asexual development depending on environmental factors such as oxygen availability and light.
In addition, we observed that the teleomorphic strain of A. chevalieri was cultured on minimal agar medium, it initially observed yellow mycelium. However, upon prolonged incubation, it partially formed conidia on yellow mycelium. We isolated the conidia from these cultures, and subcultured on minimal agar medium, consistently exhibited an anamorphic phenotype. This observation suggests that the teleomorphic strain may have lost the ability of sexual reproduction. In this study, we aimed to understand why A. chevalieri lost the ability of sexual reproduction.
To identify the mechanism, we sequenced the genome of the anamorphic mutant using a PacBio long-read sequencer. Comparative genomic analysis of the teleomorphic strain and its anamorphic mutant revealed the insertion of an approximately 5.8-kb retrotransposon upstream of a gene encoding an HMG-box transcription factor in the anamorphic mutant. Disruption of this transcription factor gene in the teleomorphic strain resulted in the acquisition of an anamorphic phenotype, whereas complementation restored the teleomorphic phenotype. These findings indicate that this transcription factor is required for sexual reproduction in A. chevalieri. In addition, the expression level of the transcription factor gene in the retrotransposon-inserted anamorphic mutant was reduced to approximately 15% of that in the teleomorphic strain.
Furthermore, the expression levels of the mating-type genes MAT1-1 and MAT1-2 were reduced in the anamorphic mutant, suggesting that this transcription factor regulates MAT genes expression. In addition, single disruptants of MAT1-1 or MAT1-2, as well as the double disruptant, did not show an anamorphic phenotype; however, the number of cleistothecia were reduced, and the cleistothecia were sterile. These results suggest that the HMG-box transcription factor gene also regulates the genes involved in sexual reproduction in addition to MAT genes.
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