A RID-like cytosine methyltransferase homologue controls sexual development in the fungusPodospora anserina
Authors: Grognet P , Timpano H , Carlier F , Aït-Benkhali J , Berteaux-Lecellier V , Debuchy R , Bidard F , Malagnac F 
Published: March 2019

DNA methyltransferases are ubiquitous enzymes conserved in bacteria, plants and opisthokonta. These enzymes, which methylate cytosines, are involved in numerous biological processes, notably development. In mammals and higher plants, methylation patterns established and maintained by the cytosine DNA methyltransferases (DMTs) are essential to zygotic development. In fungi, some members of an extensively conserved fungal-specific DNA methyltransferase class are both mediators of the Repeat Induced Point mutation (RIP) genome defense system and key players of sexual reproduction. Yet, no DNA methyltransferase activity of these purified RID (RIP deficient) proteins could be detectedin vitro. These observations led us to explore how RID-like DNA methyltransferase encoding genes would play a role during sexual development of fungi showing very little genomic DNA methylation, if any.

To do so, we used the model ascomycete fungusP. anserina. We identified thePaRidgene, encoding a RID-like DNA methyltransferase and constructed knocked-out ΔPaRiddefective mutants. Crosses involvingP. anserinaΔPaRidmutants are sterile. Our results show that, although gametes are readily formed and fertilization occurs in a ΔPaRidbackground, sexual development is blocked just before the individualization of the dikaryotic cells leading to meiocytes. Complementation of ΔPaRidmutants with ectopic alleles ofPaRid, including GFP-tagged, point-mutated, inter-specific and chimeric alleles, demonstrated that the catalytic motif of the putative PaRid methyltransferase is essential to ensure proper sexual development and that the expression of PaRid is spatially and temporally restricted. A transcriptomic analysis performed on mutant crosses revealed an overlap of the PaRid-controlled genetic network with the well-known mating-types gene developmental pathway common to an important group of fungi, the Pezizomycotina.

Author Summary

Sexual reproduction is considered to be essential for long-term persistence of eukaryotic species. Sexual reproduction is controlled by strict mechanisms governing which haploids can fuse (mating) and which developmental paths the resulting zygote will follow. In mammals, differential genomic DNA methylation patterns of parental gametes, known as ‘DNA methylation imprints’ are essential to zygotic development, while in plants, global genomic demethylation often results in female-sterility. Although animal and fungi are evolutionary related, little is known about epigenetic regulation of gene expression and development in multicellular fungi. Here, we report on a gene of the model fungusPodospora anserina, encoding a protein called PaRid that looks like a DNA methyltrasferase. We showed that expression of the catalytically functional version of the PaRid protein is required in the maternal parental strain to form zygotes. By establishing the transcriptional profile ofPaRidmutant strain, we identified a set of PaRid direct and/or indirect target genes. Half of them are also targets of a mating-type transcription factor known to be a major regulator of sexual development. So far, there was no other example of identified RID targets shared with a well-known developmental pathway that is common to an important group of fungi, the Pezizomycotina