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Colony sectorization of Metarhizium anisopliae is a sign of ageing

¹ Department of Entomology, 4112 Plant Science Building, University of Maryland, College Park, MD 20742-4454, USA
² School of Biological Sciences, University of Wales Swansea, Swansea SA2 8PP, UK

Correspondence
Raymond J. St Leger
stleger{at}umd.edu

Spontaneous phenotypic degeneration resulting in sterile sectors is frequently observed when culturing filamentous fungi on artificial medium. Sterile sectors from two different strains of the insect pathogenic fungus Metarhizium anisopliae were investigated and found to contain reduced levels of cAMP and destruxins (insecticidal peptides). Microarray analysis using slides printed with 1730 clones showed that compared to wild-type, sterile sectors down-regulated 759 genes and upregulated 27 genes during growth in Sabouraud glucose broth or on insect cuticle. The differentially expressed genes are largely involved in cell metabolism (18·8 %), cell structure and function (13·6 %) and protein metabolism (8·8 %). Strong oxidative stress was demonstrated in sectorial cultures using the nitro blue tetrazolium assay and these cultures show other syndromes associated with ageing, including mitochondrial DNA alterations. However, genes involved in deoxidation and self-protection (e.g. heat-shock proteins, HSPs) were also upregulated. Further evidence of physiological adaptation by the degenerative sectorial cultures included cell-structure reorganization and the employment of additional signalling pathways. In spite of their very similar appearance, microarray analysis identified 181 genes differentially expressed between the two sectors, and the addition of exogenous cAMP only restored conidiation in one of them. Most of the differentially expressed genes were involved in catabolic or anabolic pathways, but the latter included genes for sporulation. Compared to the mammalian ageing process, sectorization in M. anisopliae showed many similarities, including similar patterns of cAMP production, oxidative stress responses and the involvement of HSPs. Thus, a common molecular machinery for ageing may exist throughout the eukaryotes.

Microarray data demonstrating the transcriptional variations of Metarhizium anisopliae sectors grown in different media are available in Supplementary Table S1 with the online version of this paper.

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