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Mitochondrial (mt) function depends critically on optimal interactions between components encoded by mt and nuclear DNAs. mitochondrial DNA (mtDNA) inheritance (SMI) is thought to have evolved in animal species to maintain mito-nuclear complementarity by preventing the spread of selfish mt elements thus typically rendering mtDNA heteroplasmy evolutionarily ephemeral. Here, we show that mtDNA intraorganismal heteroplasmy can have deterministic underpinnings and persist for hundreds of millions of years. We demonstrate that the only exception to SMI in the animal kingdom, that is, the doubly uniparental mtDNA inheritance system in bivalves, with its three-way interactions among egg mt-, sperm mt- and nucleus-encoded gene products, is tightly associated with the maintenance of separate male and female sexes (dioecy) in freshwater mussels. Specifically, this mother-through-daughter and father-through-son mtDNA inheritance system, containing highly differentiated mt genomes, is found in all dioecious freshwater mussel species. Conversely, all hermaphroditic species lack the paternally transmitted mtDNA (=possess SMI) and have heterogeneous macromutations in the recently discovered, novel protein-coding gene (F-orf) in their maternally transmitted mt genomes. Using immunoelectron microscopy, we have localized the F-open reading frame (ORF) protein, likely involved in specifying separate sexes, in mitochondria and in the nucleus. Our results support the hypothesis that proteins coded by the highly divergent maternally and paternally transmitted mt genomes could be directly involved in sex determination in freshwater mussels. Concomitantly, our study demonstrates novel features for animal mt genomes: the existence of additional, lineage-specific, mtDNA-encoded proteins with functional significance and the involvement of mtDNA-encoded proteins in extra-mt functions. Our results open new avenues for the identification, characterization, and functional analyses of ORFs in the intergenic regions, previously defined as "noncoding," found in a large proportion of animal mt genomes.

This is the first American Fisheries Society conservation assessment of freshwater gastropods (snails) from Canada and the United States by the Gastropod Subcommittee (Endangered Species Committee). This review covers 703 species representing 16 families and 93 genera, of which 67 species are considered extinct, or possibly extinct, 278 are endangered, 102 are threatened, 73 are vulnerable, 157 are currently stable, and 26 species have uncertain taxonomic status. Of the entire fauna, 74% of gastropods are imperiled (vulnerable, threatened, endangered) or extinct, which exceeds imperilment levels in fishes (39%) and crayfishes (48%) but is similar to that of mussels (72%). Comparison of modern to background extinction rates reveals that gastropods have the highest modern extinction rate yet observed, 9,539 times greater than background rates. Gastropods are highly susceptible to habitat loss and degradation, particularly narrow endemics restricted to a single spring or short stream reaches. Compilation of this review was hampered by a paucity of current distributional information and taxonomic uncertainties. Although research on several fronts including basic biology, physiology, conservation strategies, life history, and ecology are needed, systematics and curation of museum collections and databases coupled with comprehensive status surveys (geographic limits, threat identification) are priorities.

Doubly uniparental inheritance (DUI) of mitochondrial DNA in marine mussels (Mytiloida), freshwater mussels (Unionoida), and marine clams (Veneroida) is the only known exception to the general rule of strict maternal transmission of mtDNA in animals. DUI is characterized by the presence of genderassociated mitochondrial DNA lineages that are inherited through males (male-transmitted or M types) or females (female-transmitted or F types), respectively. This unusual system constitutes an excellent model for studying basic aspects of mitochondrial DNA inheritance and the evolution of mtDNA genomes in general.Here we compare published mitochondrial genomes of unionoid bivalve species with DUI, with an emphasis on characterizing unassigned regions, to identify regions of the F and M mtDNA genomes that could (i) play a role in replication or transcription of the mtDNA molecule and/or (ii) determine whether a genome will be transmitted via the female or the male gamete. Our results reveal the presence of one Fspecific and one M-specific open reading frames (ORFs), and we hypothesize that they play a role in the transmission and/or gender-specific adaptive functions of the M and F mtDNA genomes in unionoid bivalves. Three major unassigned regions shared among all F and M unionoid genomes have also been identified, and our results indicate that (i) two of them are potential heavy-strand control regions (O H ) for regulating replication and/or transcription and that (ii) multiple and potentially bidirectional light-strand origins of replication (O L ) are present in unionoid F and M mitochondrial genomes. We propose that unassigned regions are the most promising candidate sequences in which to find regulatory and/or genderspecific sequences that could determine whether a mitochondrial genome will be maternally or paternally transmitted.

BackgroundDoubly uniparental inheritance (DUI) is an atypical system of animal mtDNA inheritance found only in some bivalves. Under DUI, maternally (F genome) and paternally (M genome) transmitted mtDNAs yield two distinct gender-associated mtDNA lineages. The oldest distinct M and F genomes are found in freshwater mussels (order Unionoida). Comparative analyses of unionoid mitochondrial genomes and a robust phylogenetic framework are necessary to elucidate the origin, function and molecular evolutionary consequences of DUI. Herein, F and M genomes from three unionoid species, Venustaconcha ellipsiformis, Pyganodon grandis and Quadrula quadrula have been sequenced. Comparative genomic analyses were carried out on these six genomes along with two F and one M unionoid genomes from GenBank (F and M genomes of Inversidens japanensis and F genome of Lampsilis ornata).ResultsCompared to their unionoid F counterparts, the M genomes contain some unique features including a novel localization of the trnH gene, an inversion of the atp8-trnD genes and a unique 3'coding extension of the cytochrome c oxidase subunit II gene. One or more of these unique M genome features could be causally associated with paternal transmission. Unionoid bivalves are characterized by extreme intraspecific sequence divergences between gender-associated mtDNAs with an average of 50% for V. ellipsiformis, 50% for I. japanensis, 51% for P. grandis and 52% for Q. quadrula (uncorrected amino acid p-distances). Phylogenetic analyses of 12 protein-coding genes from 29 bivalve and five outgroup mt genomes robustly indicate bivalve monophyly and the following branching order within the autolamellibranch bivalves: ((Pteriomorphia, Veneroida) Unionoida).ConclusionThe basal nature of the Unionoida within the autolamellibranch bivalves and the previously hypothesized single origin of DUI suggest that (1) DUI arose in the ancestral autolamellibranch bivalve lineage and was subsequently lost in multiple descendant lineages and (2) the mitochondrial genome characteristics observed in unionoid bivalves could more closely resemble the DUI ancestral condition. Descriptions and comparisons presented in this paper are fundamental to a more complete understanding regarding the origins and consequences of DUI.

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