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

Abstract

Diatoms 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 diatom Pseudo-nitzschia multistriata comparing 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 the P. multistriata life cycle. Absence of methylation in MRP3 is necessary for its expression but not sufficient, since large non-sexual cells have the same methylation profile of small sexual cells but do not express MRP3, suggesting that additional controls are involved in the mechanism of sex determination.