Towards a sexual cycle in Aspergillus niger

Valeria Ellena 1,2 Michael Sauer 1,2 Matthias Steiger 1,2
1ACIB GmbH, Austrian Centre of Industrial Biotechnology, Vienna, Austria
2Department of Biotechnology, University of Natural Resources and Life Sciences, Vienna, Austria

In recent years an increasing number of filamentous fungi previously considered asexual, such as Aspergillus fumigatus, have been shown to possess a cryptic sexual cycle, triggered only under highly specific conditions. However, for around 20% of the known fungal species, including Aspergillus niger, a sexual cycle has not been found yet. A. niger is a relevant industrial microorganism, currently used as a versatile cell factory for the production of organic acids and enzymes. The discovery of a sexual cycle in A. niger would not only broaden the current biological knowledge of this organism but also allow to unravel new strategies for strain improvement.
In this study, we report some strong indications of the sexual potential of A. niger.
First, sclerotia-like structures were observed in the sequenced A. niger strain ATCC 1015, containing the MAT1-1 gene, when plated in combination with an opposite mating type A. niger strain. Sclerotia are considered pre-mature sexual structures which can mature to cleistothecia, the latter being an important requirement for the occurrence of a sexual cycle in Aspergilli. The observed sclerotia-like structures are often formed at the contact zones between the colonies derived from the two strains, suggesting an interaction between them. Also, the typical presence of liquid droplets on top of the formed structures can be often observed.
Second, a reduction of the normal asexual conidiation can be observed when the two opposite mating type strains are plated together on the same plate. This reduction is even more drastic when sclerotia-like structures are formed. Overall, our findings strongly indicate that steps towards a complete sexual cycle of A. niger can be achieved.