Evaluating the utility of the female-specific mitochondrial f-orf gene for population genetic, phylogeographic and systematic studies in freshwater mussels (Bivalvia: Unionida)

Did doubly uniparental inheritance (DUI) of mtDNA originate as a cytoplasmic male sterility (CMS) system?

Animal and plant species exhibit an astonishing diversity of sexual systems, including environmental and genetic determinants of sex, with the latter including genetic material in the mitochondrial genome. In several hermaphroditic plants for example, sex is determined by an interaction between mitochondrial cytoplasmic male sterility (CMS) genes and nuclear restorer genes. Specifically, CMS involves aberrant mitochondrial genes that prevent pollen development and specific nuclear genes that restore it, leading to a mixture of female (male-sterile) and hermaphroditic individuals in the population (gynodioecy). Such a mitochondrial-nuclear sex determination system is thought to be rare outside plants. Here, we present one possible case of CMS in animals. We hypothesize that the only exception to the strict maternal mtDNA inheritance in animals, the doubly uniparental inheritance (DUI) system in bivalves, might have originated as a mitochondrial-nuclear sex-determination system. We document and explore similarities that exist between DUI and CMS, and we propose various ways to test our hypothesis.

A proposed method for analyzing molecular signatures to detect hermaphroditism in freshwater mussels: a case study using the eastern floater (Pyganodon cataracta)

Many freshwater mussels (order Unionida) have an unusual system of doubly uniparental inheritance (DUI) of mitochondrial (mt) DNA. In species with DUI, males possess a female-transmitted (F-type) mt genome and a male-transmitted (M-type) mt genome. These genomes contain non-canonical open reading frame (orf) genes, referred to as f-orf and m-orf, present in F and M mt genomes, respectively. These genes have been implicated in sexual development in Unionida. When gonochoric species become hermaphroditic, which has happened several times in Unionida, they lose their M-type mt genome and f-orf genes evolve dramatically. Resulting F-ORF proteins are highly divergent in terms of primary nucleotide sequence, inferred amino acids, and hydrophobic properties; these genes (and proteins) are referred to as hermaphroditic orfs or h-orfs (and H-ORFs). We investigated patterns of hydrophobicity divergence for H-ORF proteins in hermaphrodites versus F-ORF proteins in closely related gonochoric species against cytochrome c oxidase subunit 1 (cox1) divergences. This approach was used to assess whether cryptic hermaphrodites can be detected. Although we did not detect evidence for the recent transition of any populations of eastern floaters (Pyganodon cataracta (Say, 1817)) to hermaphroditism, our analyses demonstrate that molecular signatures in mtDNA can be used to detect hermaphroditism in freshwater mussels.

Molecular and morphological signatures for extreme environmental adaptability of the invasive mussel Brachidontes pharaonis (Fischer, 1870)

Brachidontes pharaonis (Bivalvia:Mytilidae) is one of the most successful Lessepsian migrants. Its extensive populations' expansion and phenotypic plasticity might reshape the Mediterranean biodiversity. Individuals of B. pharaonis were collected from various sites in the Mediterranean Sea and the Red Sea in Egypt. Species-specific primers for Cytochrome Oxidase Subunit 1 gene were designed. They were applied for analysis of mussel's population genetics and assessment of its aquatic environmental DNA (eDNA) abundance. Morphological, allometric and morphometric characteristics were also described. The newly designed primers could efficiently detect the species presence, abundance, and genetic diversity. The Northern Red Sea and north-westward populations exhibited higher nucleotide diversities than southwards. Phylogeny and principal coordinates' analysis (PCoA) detected three geographical categories for B. pharaonis: one of the Indian Ocean, other of the Middle Red Sea and southwards, and the other extends from the Northern Red Sea to the westernmost part of the Mediterranean. Intraspecific differences in the shell shape, colour, and biometrics were noted. The shells were significantly smaller and lighter in rocky habitats than in sandy ones. The morphometric indices and allometry were significantly different between rocky and sandy environments. In general, B. pharaonis genetic and morphological features appeared to contribute much to the species success in versatile habitats.