Contrary to most animals that inherit mitochondrial DNA maternally, several species of bivalves have a fundamentally different mode of transmission, known as Doubly Uniparental Inheritance (DUI). This unique system is characterized by the presence of two distinct mitochondrial genomes: a genome M transmitted by males and found in sperm cells, and a genome F transmitted by females, present in all tissues in females, and in somatic tissues in males. In freshwater mussels, some closely related gonochoric (separate sexes) species have the DUI system, while others hermaphroditic species have lost the mitochondrial male genome. Our project will aim to determine the relationship between the DUI system and its potential role in sex determination.