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.

Population genetics of sex determination in Mytilus mussels: reanalyses and a model.

Large variations in offspring sex ratio have been reported in Mytilus mussels, which show doubly uniparental inheritance of mitochondria (DUI). Here, we reanalyzed the published sex ratio data, using simple population genetics concepts and logistic regression. Contrary to previous studies that detected only maternal effects, we found both paternal and maternal effects on the offspring sex ratio. We propose that sex in Mytilus is controlled by a pair of nuclear sex ratio alleles expressed in the mother and by minor sex-determining genes inherited from the father and also possibly from the mother.

Widespread and persistent invasions of terrestrial habitats coincident with larval feeding behavior transitions during snail-killing fly evolution (Diptera: Sciomyzidae).

BACKGROUND: Transitions in habitats and feeding behaviors were fundamental to the diversification of life on Earth. There is ongoing debate regarding the typical directionality of transitions between aquatic and terrestrial habitats and the mechanisms responsible for the preponderance of terrestrial to aquatic transitions. Snail-killing flies (Diptera: Sciomyzidae) represent an excellent model system to study such transitions because their larvae display a range of feeding behaviors, being predators, parasitoids or saprophages of a variety of mollusks in freshwater, shoreline and dry terrestrial habitats. The remarkable genus Tetanocera (Tetanocerini) occupies five larval feeding groups and all of the habitat types mentioned above. This study has four principal objectives: (i) construct a robust estimate of phylogeny for Tetanocera and Tetanocerini, (ii) estimate the evolutionary transitions in larval feeding behaviors and habitats, (iii) test the monophyly of feeding groups and (iv) identify mechanisms underlying sciomyzid habitat and feeding behavior evolution. RESULTS: Bayesian inference and maximum likelihood analyses of molecular data provided strong support that the Sciomyzini, Tetanocerini and Tetanocera are monophyletic. However, the monophyly of many behavioral groupings was rejected via phylogenetic constraint analyses. We determined that (i) the ancestral sciomyzid lineage was terrestrial, (ii) there was a single terrestrial to aquatic habitat transition early in the evolution of the Tetanocerini and (iii) there were at least 10 independent aquatic to terrestrial habitat transitions and at least 15 feeding behavior transitions during tetanocerine phylogenesis. The ancestor of Tetanocera was aquatic with five lineages making independent transitions to terrestrial habitats and seven making independent transitions in feeding behaviors. CONCLUSIONS: The preponderance of aquatic to terrestrial transitions in sciomyzids goes against the trend generally observed across eukaryotes. Damp shoreline habitats are likely transitional where larvae can change habitat but still have similar prey available. Transitioning from aquatic to terrestrial habitats is likely easier than the reverse for sciomyzids because morphological characters associated with air-breathing while under the water's surface are lost rather than gained, and sciomyzids originated and diversified during a general drying period in Earth's history. Our results imply that any animal lineage having aquatic and terrestrial members, respiring the same way in both habitats and having the same type of food available in both habitats could show a similar pattern of multiple independent habitat transitions coincident with changes in behavioral and morphological traits.

Novel protein genes in animal mtDNA: a new sex determination system in freshwater mussels (Bivalvia: Unionoida)?

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.

Pyganodon (Bivalvia: Unionoida: Unionidae) phylogenetics: a male- and female-transmitted mitochondrial DNA perspective.

Species boundaries, evolutionary relationships and geographic distributions of many unionoid bivalve species, like those in the genus Pyganodon, remain unresolved in Eastern North America. Because unionoid bivalves are one of the most imperiled groups of animals in the world, understanding the genetic variation within and among populations as well as among species is crucial for effective conservation planning. Conservation of unionoid species is indispensable from a freshwater habitat perspective but also because they possess a unique mitochondrial inheritance system where distinct gender-associated mitochondrial DNA lineages coexist: a female-transmitted (F) mt genome and a male-transmitted (M) mt genome that are involved in the maintenance of separate sexes (=dioecy). In this study, 42 populations of Pyganodon sp. were sampled across a large geographical range and fragments of two mitochondrial genes (cox1 and cox2) were sequenced from both the M- and F-transmitted mtDNA genomes. Our results support the recency of the divergence between P. cataracta and P. fragilis. We also found two relatively divergent F and M lineages within P. grandis. Surprisingly, the relationships among the P. grandis specimens in the F and M sequence trees are not congruent. We found that a single haplotype in P. lacustris has recently swept throughout the M genotype space leading to an unexpectedly low diversity in the M lineage in that species. Our survey put forward some challenging results that force us to rethink hybridization and species boundaries in the genus Pyganodon. As the M and F genomes do not always display the same phylogeographic story in each species, we also discuss the importance of being careful in the interpretation of molecular data based solely on maternal transmitted mtDNA genomes. The involvement of F and M genomes in unionoid bivalve sex determination likely played a role in the genesis of the unorthodox phylogeographic patterns reported herein.

Mitochondrial phylogenomics of the Bivalvia (Mollusca): searching for the origin and mitogenomic correlates of doubly uniparental inheritance of mtDNA.

BACKGROUND: Doubly 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). RESULTS: Compared 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). CONCLUSION: The 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.

The unusual system of doubly uniparental inheritance of mtDNA: isn't one enough?

Mitochondria possess their own genetic material (mitochondrial DNA or mtDNA), whose gene products are involved in mitochondrial respiration and oxidative phosphorylation, transcription, and translation. In animals, mitochondrial DNA is typically transmitted to offspring by the mother alone. The discovery of 'doubly uniparental inheritance' (DUI) of mtDNA in some bivalves has challenged the paradigm of strict maternal inheritance (SMI). In this review, we survey recent advances in our understanding of DUI, which is a peculiar system of cytoplasmic DNA inheritance that involves distinct maternal and paternal routes of mtDNA transmission, a novel extension of a mitochondrial gene (cox2), recombination, and periodic 'role-reversals' of the normally male and female-transmitted mitochondrial genomes. DUI provides a unique opportunity for studying nuclear-cytoplasmic genome interactions and the evolutionary significance of different modes of mitochondrial inheritance.

Extreme primary and secondary protein structure variability in the chimeric male-transmitted cytochrome c oxidase subunit II protein in freshwater mussels: evidence for an elevated amino acid substitution rate in the face of domain-specific purifying selection.

BACKGROUND: Freshwater unionoidean bivalves, and species representing two marine bivalve orders (Mytiloida and Veneroida), exhibit a mode of mtDNA inheritance involving distinct maternal (F) and paternal (M) transmission routes concomitant with highly divergent gender-associated mtDNA genomes. Additionally, male unionoidean bivalves have a approximately 550 bp 3' coding extension to the cox2 gene (Mcox2e), that is apparently absent from all other metazoan taxa. RESULTS: Our molecular sequence analyses of MCOX2e indicate that both the primary and secondary structures of the MCOX2e region are evolving much faster than other regions of the F and M COX2-COX1 gene junction. The near N-terminus approximately 2/3 of the MCOX2e region contains an interspecifically variable number of predicted transmembrane helices (TMH) and interhelical loops (IHL) whereas the C-terminus approximately 1/3 is relatively conserved and hydrophilic while containing conserved functional motifs. MCOX2e displays an overall pattern of purifying selection that leads to the preservation of TMH/IHL and C-terminus tail sub-regions. However, 14 amino acid positions in the MCOX2e TMH/IHL sub-region might be targeted by diversifying selection, each representing a site where there exists interspecific variation for the constituent amino acids residing in a TMH or IHL. CONCLUSION: Our results indicate that Mcox2e is unique to unionoidean bivalves, likely the result of a single insertion event that took place over 65 MYA and that MCOX2e is functional. The predicted TMH number, length and position variability likely stems from substitution-based processes rather than the typically implicated insertion/deletion events. MCOX2e has relatively high rates of primary and secondary structure evolution, with some amino acid residues potentially subjected to site-specific positive selection, yet an overall pattern of purifying selection leading to the preservation of the TMH/IHL and hydrophilic C-terminus tail subregions. The more conserved C-terminus tail (relative to the TMH/IHL sub-region of MCOX2e) is likely biologically active because it contains functional motifs. The rapid evolution of primary and secondary structure in MCOX2e, combined with the action of both positive and purifying selection, provide supporting evidence for the hypothesis that MCOX2e has a novel reproductive function within unionoidean bivalves. All tolled, our data indicate that unionoidean bivalve MCOX2 is the first reported chimeric animal mtDNA-encoded protein.

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

Freshwater mussels (order: Unionida) represent one of the most critically imperilled groups of animals; consequently, there exists a need to establish a variety of molecular markers for population genetics and systematic studies in this group. Recently, two novel mitochondrial protein-coding genes were described in unionoids with doubly uniparental inheritance of mtDNA. These genes are the f-orf in female-transmitted mtDNA and the m-orf in male-transmitted mtDNA. In this study, whole F-type mitochondrial genome sequences of two morphologically similar Lampsilis spp. were compared to identify the most divergent protein-coding regions, including the f-orf gene, and evaluate its utility for population genetic and phylogeographic studies in the subfamily Ambleminae. We also tested whether the f-orf gene is phylogenetically informative at the species level. Our preliminary results indicated that the f-orf gene could represent a viable molecular marker for population- and species-level studies in freshwater mussels.

Genome Survey of the Freshwater Mussel Venustaconcha ellipsiformis (Bivalvia: Unionida) Using a Hybrid De Novo Assembly Approach.

Freshwater mussels (Bivalvia: Unionida) serve an important role as aquatic ecosystem engineers but are one of the most critically imperilled groups of animals. Here, we used a combination of sequencing strategies to assemble and annotate a draft genome of Venustaconcha ellipsiformis, which will serve as a valuable genomic resource given the ecological value and unique "doubly uniparental inheritance" mode of mitochondrial DNA transmission of freshwater mussels. The genome described here was obtained by combining high-coverage short reads (65× genome coverage of Illumina paired-end and 11× genome coverage of mate-pairs sequences) with low-coverage Pacific Biosciences long reads (0.3× genome coverage). Briefly, the final scaffold assembly accounted for a total size of 1.54 Gb (366,926 scaffolds, N50 = 6.5 kb, with 2.3% of "N" nucleotides), representing 86% of the predicted genome size of 1.80 Gb, while over one third of the genome (37.5%) consisted of repeated elements and >85% of the core eukaryotic genes were recovered. Given the repeated genetic bottlenecks of V. ellipsiformis populations as a result of glaciations events, heterozygosity was also found to be remarkably low (0.6%), in contrast to most other sequenced bivalve species. Finally, we reassembled the full mitochondrial genome and found six polymorphic sites with respect to the previously published reference. This resource opens the way to comparative genomics studies to identify genes related to the unique adaptations of freshwater mussels and their distinctive mitochondrial inheritance mechanism.

Evolution of sex-dependent mtDNA transmission in freshwater mussels (Bivalvia: Unionida).

Doubly uniparental inheritance (DUI) describes a mode of mtDNA transmission widespread in gonochoric freshwater mussels (Bivalvia: Palaeoheterodonta: Unionida). In this system, both female- and male-transmitted mtDNAs, named F and M respectively, coexist in the same species. In unionids, DUI is strictly correlated to gonochorism and to the presence of the atypical open reading frames (ORFans) F-orf and M-orf, respectively inside F and M mtDNAs, which are hypothesized to participate in sex determination. However, DUI is not found in all three Unionida superfamilies (confirmed in Hyrioidea and Unionoidea but not in Etherioidea), raising the question of its origin in these bivalves. To reconstruct the co-evolution of DUI and of ORFans, we sequenced the mtDNAs of four unionids (two gonochoric with DUI, one gonochoric and one hermaphroditic without DUI) and of the related gonochoric species Neotrigonia margaritacea (Palaeoheterodonta: Trigoniida). Our analyses suggest that rearranged mtDNAs appeared early during unionid radiation, and that a duplicated and diverged atp8 gene evolved into the M-orf associated with the paternal transmission route in Hyrioidea and Unionoidea, but not in Etherioidea. We propose that novel mtDNA-encoded genes can deeply influence bivalve sex determining systems and the evolution of the mitogenomes in which they occur.

Ancient androdioecy in the freshwater crustacean Eulimnadia.

Among the variety of reproductive mechanisms exhibited by living systems, one permutation--androdioecy (mixtures of males and hermaphrodites)--is distinguished by its rarity. Models of mating system evolution predict that androdioecy should be a brief stage between hermaphroditism and dioecy (separate males and females), or vice versa. Herein we report evidence of widespread and ancient androdioecy in crustaceans in the genus Eulimnadia, based on observations of over 33,000 shrimp from 36 locations from every continent except Antarctica. Using phylogenetic, biogeographical and palaeontological evidence, we infer that androdioecy in Eulimnadia has persisted for 24-180 million years and has been maintained through multiple speciation events. These results suggest that androdioecy is a highly successful aspect of the life history of these freshwater crustaceans, and has persisted for orders of magnitude longer than predicted by current models of this rare breeding system.

Evidence for a fourteenth mtDNA-encoded protein in the female-transmitted mtDNA of marine Mussels (Bivalvia: Mytilidae).

BACKGROUND: A novel feature for animal mitochondrial genomes has been recently established: i.e., the presence of additional, lineage-specific, mtDNA-encoded proteins with functional significance. This feature has been observed in freshwater mussels with doubly uniparental inheritance of mtDNA (DUI). The latter unique system of mtDNA transmission, which also exists in some marine mussels and marine clams, is characterized by one mt genome inherited from the female parent (F mtDNA) and one mt genome inherited from the male parent (M mtDNA). In freshwater mussels, the novel mtDNA-encoded proteins have been shown to be mt genome-specific (i.e., one novel protein for F genomes and one novel protein for M genomes). It has been hypothesized that these novel, F- and M-specific, mtDNA-encoded proteins (and/or other F- and/or M-specific mtDNA sequences) could be responsible for the different modes of mtDNA transmission in bivalves but this remains to be demonstrated. METHODOLOGY/PRINCIPAL FINDINGS: We investigated all complete (or nearly complete) female- and male-transmitted marine mussel mtDNAs previously sequenced for the presence of ORFs that could have functional importance in these bivalves. Our results confirm the presence of a novel F genome-specific mt ORF, of significant length (>100aa) and located in the control region, that most likely has functional significance in marine mussels. The identification of this ORF in five Mytilus species suggests that it has been maintained in the mytilid lineage (subfamily Mytilinae) for ∼13 million years. Furthermore, this ORF likely has a homologue in the F mt genome of Musculista senhousia, a DUI-containing mytilid species in the subfamily Crenellinae. We present evidence supporting the functionality of this F-specific ORF at the transcriptional, amino acid and nucleotide levels. CONCLUSIONS/SIGNIFICANCE: Our results offer support for the hypothesis that "novel F genome-specific mitochondrial genes" are involved in key biological functions in bivalve species with DUI.

Characterization of a mitochondrial ORF from the gender-associated mtDNAs of Mytilus spp. (Bivalvia: Mytilidae): identification of the "missing" ATPase 8 gene.

Bivalve species are characterized by extraordinary variability in terms of mitochondrial (mt) genome size, gene arrangement and tRNA gene number. Many species are thought to lack the mitochondrial protein-coding gene atp8. Of these species, the Mytilidae appears to be the only known taxon with doubly uniparental inheritance of mtDNA that does not possess the atp8 gene. This raises the question as to whether mytilids have completely lost the ATP8 protein, whether the gene has been transferred to the nucleus or whether they possess a highly modified version of the gene/protein that has led to its lack of annotation. In the present study, we re-investigated all complete (or nearly complete) F and M mytilid mt genomes previously sequenced for the presence of conserved open reading frames (ORFs) that might code for ATP8 and/or have other functional importance in these bivalves. We also revised the annotations of all available complete mitochondrial genomes of bivalves and nematodes that are thought to lack atp8 in an attempt to detect it. Our results indicate that a novel mytilid ORF of significant length (i.e., the ORF is >85 amino acids in length), with complete start and stop codons, is a candidate for the atp8 gene: (1) it possesses a pattern of evolution expected for a protein-coding gene evolving under purifying selection (i.e., the 3rd>1st>2nd codon pattern of evolution), (2) it is actively transcribed in Mytilus species, (3) it has one predicted transmembrane helix (as do other metazoan ATP8 proteins), (4) it has conserved functional motifs and (5), comparisons of its amino acid sequence with ATP8 sequences of other molluscan or bivalve species reveal similar hydropathy profiles. Furthermore, our revised annotations also confirmed the mt presence of atp8 in almost all bivalve species and in one nematode species. Our results thus support recognizing the presence of ATPase 8 in most bivalves mt genomes (if not all) rather than the continued characterization of these genomes as lacking this gene.

Comparative mitochondrial genomics of freshwater mussels (Bivalvia: Unionoida) with doubly uniparental inheritance of mtDNA: gender-specific open reading frames and putative origins of replication.

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 gender-associated 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 F-specific 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 gender-specific sequences that could determine whether a mitochondrial genome will be maternally or paternally transmitted.

THE EXTENT OF INTROGRESSION OUTSIDE THE CONTACT ZONE BETWEEN NOTROPIS CORNUTUS AND NOTROPIS CHRYSOCEPHALUS (TELEOSTEI: CYPRINIDAE).

The cyprinid fishes, Notropis cornutus and N. chrysocephalus, hybridize in a long, narrow zone in the midwestern United States. To quantify the extent of introgression of genetic markers outside of this zone, samples were collected along transects starting near the region of contact (as defined by morphological characters), followed by samples progressively more distant. Diagnostic allozymic and mitochondrial DNA (mtDNA) restriction site markers were used to estimate the extent of introgression outside of the zone, while polymorphic allozyme and mtDNA markers were used to evaluate the potential for gene flow among populations within transects. Analysis of populations from the northern transect provided evidence for differentiation of populations for some of the markers; however, on average, enough gene flow has occurred to overcome substantial differentiation. Introgressed mtDNA and allozyme haplotypes were rare and found only in the population closest to the contact zone. The rarity of introgressed alleles in the more northern populations is consistent with the recent origin of these populations after the Wisconsin glaciation (less than 12,000 years bp) and/or selection maintaining the northern boundary of the contact zone. Analysis of populations from the southern transect revealed evidence for population subdivision but no evidence for introgression at the diagnostic allozyme loci; however, nearly all individuals from this transect possessed introgressed mtDNA haplotypes, with samples furthest from the contact zone exhibiting the highest frequencies of introgression. Patterns of variation for one of the polymorphic allozyme markers (Est-A) and introgressed mtDNAs were highly correlated, suggesting that allozymic heterogeneity at this locus is also the result of introgression. The most likely explanation for these data is that these introgressed haplotypes are indicators of a more southern position of the contact zone during the Pleistocene, with the contact zone shifting northward with the recession of the glacial front. Such movement implicates selection in the maintenance of distributional limits of these species, and hence, the width and position of the contact zone.

MULTIPLE ORIGINS OF GENDER-ASSOCIATED MITOCHONDRIAL DNA LINEAGES IN BIVALVES (MOLLUSCA: BIVALVIA).

Previous studies have shown that marine mussels (genus Mytilus) and a freshwater mussel (Pyganodon grandis) contain two distinct gender-associated mitotypes, which is a characteristic feature of the phenomenon of doubly uniparental inheritance (DUI) of mitochondrial DNA (mtDNA). Here we present evidence for the presence of distinct male (M) and female (F) mitotypes in three other bivalve species, the mytilid Geukensia demissa, and the unionid species P. fragilis and Fusconaia flava. Nucleotide sequences of a segment of the COI gene from the M and F mitotypes from each of the three mytilid species (M. edulis, M. trossulus, G. demissa) and three unionid species (P. grandis, P. fragilis, F. flava) were used for phylogenetic analysis. The analysis suggests three independent origins of M and F mitotypes for the six species examined; one for the three unionid species, one for the two Mytilus species, and one for Geukensia. The first of these F/M divergence events, while of uncertain age, predates the divergence of the two unionid genera and is likely older than either of the two F/M divergence events in the mytilid taxa. The most parsimonious explanation of multiple F/M divergence events is that they represent independent origins of DUI. Another possibility is that, in a given taxon, an F or M mitotype assumes the role of the opposite mitotype (by virtue of a mechanism that remains to be clarified) and subsequently was fixed within its new gender. The fixation of a mtDNA lineage derived from a mitotype of switched function would reset the divergence of the gender-associated lineages to zero, thereby mimicking a de novo split of F and M lineages from a preexisting mtDNA genome that was not gender specific. Further broad-scale taxonomic studies of the occurrence of distinct M and F mitotypes may allow for the evaluation of the latter hypothesis.