The fungal sexual revolution continues: indications of sexuality in the citric acid producing fungus Aspergillus niger

Abstract

Sexual reproduction is an important proliferation mechanism which provides advantages to a species by promoting genetic variation. Interestingly, for a large group of fungal species, including the industrially relevant Aspergillus niger, this mode of reproduction has not been described yet. In recent years, strong indications of the sexual potential of A. niger have accumulated 1. These include the ability of certain strains to produce pre-mature sexual structures (sclerotia) and the presence of a mating type MAT1-1 locus in the commonly used A. niger strains, such as ATCC 1015 and CBS 513.88. Here we show additional evidence of the sexual potential of A. niger. Formation of sclerotia could be induced in the progenitor of the industrial citric acid producing strains of A. niger, ATCC 1015, and in pyrG mutants derived from it 2. The capability of ATCC 1015 to form sclerotia, known to act as sexual structures in related organisms, represents a step forward towards the discovery of a sexual cycle in this important industrial species. To find the second mating type locus, we performed genome sequencing of the A. niger neotype strain CBS 554.65 and identified the missing MAT1-2 locus 3. A nucleotide alignment showed a different orientation of the MAT1-1 locus of ATCC 1015 compared to the MAT1-2 locus of CBS 554.65. While the genomic context of the MAT1-2 locus in CBS 554.65 is similar to the one of other MAT1-2 A. niger strains and other Aspergillus species, the region comprising the MAT1-1 locus is inverted in all sequenced strains of A. niger. These observations suggest the occurrence of genetic flipping or switching events at the MAT1-1 locus of A. niger, which might have a direct impact on its sexuality. These results provide new insights in the mating system of A. niger and pave the way for the discovery of a sexual cycle in a species long thought to be asexual. 1. Ellena, V., Sauer, M. & Steiger, M. G. Fungal Biol. Biotechnol. 7, 17 (2020). 2. Ellena, V., Bucchieri, D., Arcalis, E., Sauer, M. & Steiger, M. G. Fungal Biol. 125, 485–494 (2021). 3. Ellena, V., Seekles, S. J., Ram, A. F. J. & Steiger, M. G. BMC Genomics 1–17 (2021).