DNA methylation is linked to the monoallelic expression ofMRP3, a diatom mating type determining gene

Abstract

AbstractDiatoms are unicellular microalgae widely distributed in aquatic ecosystems. In diatoms, sexual reproduction is needed to counteract cell miniaturization imposed by the rigid silica shell, and only small cells, below a species-specific size threshold, are competent for sex.We performed a genome-wide Enzymatic Methyl-seq analysis in the heterothallic diatomPseudo-nitzschia multistriatacomparing cells of different size and opposite mating type (MT) to investigate potential epigenetic controls in life cycle transitions. We found an imprinting-like pattern of methylation at the sex locus: alleles of the gene responsible for the specification of the MT+,MRP3, are hypermethylated in MT- and differentially methylated in MT+, with transcription occurring only on the MT+ hypomethylated variant. The methylation pattern is overall stable over theP. multistriatalife cycle. Absence of methylation inMRP3is necessary for its expression but not sufficient, since large non-sexual cells have the same methylation profile of small sexual cells but do not expressMRP3, suggesting that additional controls are involved in the mechanism of sex determination.The methylation profile of the mating type determining geneMRP3of the diatomPseudo-nitzschia multistriatais different between opposite mating types. Lack of methylation is necessary but not sufficient forMRP3expression in the sexually competent MT+ strains.Highlights- The mating type locus is differentially methylated in MT+ and MT-- DNA methylation is linked to MRP3 expression- DNA methylation does not play a role in the acquisition of sexual competence