Elsa Froufe

Mitogenomes are defined as compact and structurally stable over aeons. This perception results from a vertebrate-centric vision, where few types of mtDNA rearrangements are described. Here, we bring a new light to the involvement of mitochondrial replication in the strand asymmetry of the vertebrate mtDNA. Using several species of deep-sea hatchetfish (Sternoptychidae) displaying distinct mtDNA structural arrangements, we unravel the inversion of the coding direction of protein-coding genes (PCGs). This unexpected change is coupled with a strand asymmetry nucleotide composition reversal and is shown to be directly related to the strand location of the Control Region (CR). An analysis of the fourfold redundant sites of the PCGs (greater than 6000 vertebrates), revealed the rarity of this phenomenon, found in nine fish species (five deep-sea hatchetfish). Curiously, in Antarctic notothenioid fishes (Trematominae), where a single PCG inversion (the only other record in fish) is coupled with the inversion of the CR, the standard asymmetry is disrupted for the remaining PCGs but not yet reversed, suggesting a transitory state. Our results hint that a relaxation of the classic vertebrate mitochondrial structural stasis promotes disruption of the natural balance of asymmetry of the mtDNA. These findings support the long-lasting hypothesis that replication is the main molecular mechanism promoting the strand-specific compositional bias of this unique and indispensable molecule.

The basic knowledge of freshwater bivalves in the Unionida in some regions of the world is still limited, hindering potential conservation efforts, including in Vietnam. A subset of these mussels, the freshwater bivalve tribe Anodontini, is especially difficult to properly identify morphologically due to intraspecific shell similarity. This study aims to define the species of Anodontini in Vietnam and describe their evolutionary relationships and distributions by estimating phylogenies and analyzing collected specimens. The Anodontini are represented in Vietnam by five species divided among three genera: Sinanodonta, Cristaria, and Pletholophus. Sinanodonta woodiana, a large species complex, is represented in Vietnam by Sinanodonta jourdyi. Cristaria is confirmed to include the widespread Cristaria plicata and substantiates the validity of Cristaria truncata. Finally, Pletholophus is here recognized as distinct from Cristaria, containing two species in Vietnam, Pletholophus tenuis, and a species new to science. Our study is an important baseline for future studies on Vietnamese freshwater mussels and highlights the importance of surveys, molecular work, and taxonomic expertise to describe the biodiversity of understudied regions.