Doubly Uniparental Inheritance: two mitochondrial genomes, one precious model for organelle DNA inheritance and evolution.

Eukaryotes have exploited several mechanisms for organelle uniparental inheritance, so that this feature arose and evolved independently many times in their history. Metazoans’ mitochondria commonly experience Strict Maternal Inheritance (SMI), i.e. they are only transmitted by females. However, the most noteworthy exception comes from some bivalve mollusks, in which two mitochondrial lineages (together with their genomes) are inherited, one through females (F), the other through males (M). M and F genomes show up to 30% sequence divergence. This inheritance mechanism is known as Doubly Uniparental Inheritance (DUI), since both sexes inherit uniparentally their mitochondria. Here we review what we know about this unusual system and we propose a model for evolution of DUI that might account for its origin as sex determination mechanism. Moreover, we propose DUI as a choice model to address many aspects that should be of interest to a wide range of biological sub-fields, such as mitochondria inheritance, mtDNA evolution and recombination, genomic conflicts, evolution of sex, developmental biology, etc. Actually, as researches proceed, mitochondria appear to have acquired a central role in many fundamental processes of life, which are not only in their metabolic activity as cellular power plants, such as cell signaling, fertilization, development, differentiation, ageing, apoptosis and sex determination. A function of mitochondria in the origin and maintenance of sex has been also proposed.