Quantitation of the male and female types of mitochondrial DNA in a blue mussel, Mytilus galloprovincialis, using real-time polymerase chain reaction assay

The absence of canonical respiratory complex I subunits in male-type mitogenomes of three Donax species

Bivalves are an extraordinary class of animals in which species with a doubly uniparental inheritance (DUI) of mitochondrial DNA have been described. DUI is characterized as a mitochondrial homoplasmy of females and heteroplasmy of male individuals where F-type mitogenomes are passed to the progeny with mother egg cells and divergent M-type mitogenomes are inherited with fathers sperm cells. However, in most cases only male individuals retain divergent mitogenome inherited with spermatozoa. Additionally, in many of bivalves, unique mitochondrial features, like additional genes, gene duplication, gene extensions, mitochondrial introns, and recombination, were observed. In this study, we sequenced and assembled male-type mitogenomes of three Donax species. Comparative analysis of mitochondrial sequences revealed a lack of all seven NADH dehydrogenase subunits as well as the presence of three long additional open reading frames lacking identifiable homology to any of the existing genes.

The extremely divergent maternally- and paternally-transmitted mitochondrial genomes are co-expressed in somatic tissues of two freshwater mussel species with doubly uniparental inheritance of mtDNA

Freshwater mussel species with doubly uniparental inheritance (DUI) of mtDNA are unique because they are naturally heteroplasmic for two extremely divergent mtDNAs with ~50% amino acid differences for protein-coding genes. The paternally-transmitted mtDNA (or M mtDNA) clearly functions in sperm in these species, but it is still unknown whether it is transcribed when present in male or female soma. In the present study, we used PCR and RT-PCR to detect the presence and expression of the M mtDNA in male and female somatic and gonadal tissues of the freshwater mussel species Venustaconcha ellipsiformis and Utterbackia peninsularis (Unionidae). This is the first study demonstrating that the M mtDNA is transcribed not only in male gonads, but also in male and female soma in freshwater mussels with DUI. Because of the potentially deleterious nature of heteroplasmy, we suggest the existence of different mechanisms in DUI species to deal with this possibly harmful situation, such as silencing mechanisms for the M mtDNA at the transcriptional, post-transcriptional and/or post-translational levels. These hypotheses will necessitate additional studies in distantly-related DUI species that could possess different mechanisms of action to deal with heteroplasmy.

Co-expressed mitochondrial genomes: recently masculinized, recombinant mitochondrial genome is co-expressed with the female-transmitted mtDNA genome in a male Mytilus trossulus mussel from the Baltic Sea

BACKGROUND

Few exceptions have been described from strict maternal inheritance of mitochondrial DNA in animals, including sea mussels (Mytilidae), clams (Donacidae, Veneridae and Solenidae) and freshwater mussels (Unionoidae) order. In these bivalves mitochondria and their DNA are transferred through two separate routes. The females inherit only the maternal mitochondrial DNA whereas the males inherit maternal as well as paternal mitochondrial DNA, which is usually present only in gonads and sperm. The mechanism controlling this phenomenon is unclear but leads to the existence of two separate mitochondrial DNA lineages in a single species. The lineages are usually well differentiated: up to 20-50% divergence in nucleotide sequence. Occasionally, a maternal mitochondrial DNA can invade the paternal transmission route, eventually replacing the diverged M-type and lowering the divergence. Such role reversal (masculinization) event has happened recently in the Mytilus population of the Baltic Sea which consists of M. edulis × M. trossulus hybrids, but the functional status of the resulting mitochondrial genome was unknown.

RESULTS

In this paper we sequenced transcripts from one specimen that was identified as male carrying both the female mitochondrial genome and a recently masculinized mitochondrial genome. Additionally, the analysis of the control region has showed that the recently masculinized, recombinant genome, not only has an M-type control region and all coding regions derived from the F-type, but also is transcriptionally active along side the maternally inherited F-type genome. In the comparative analysis, the two genomes exhibit different substitution patterns, typical for the M vs. F genome comparisons. The genetic distances and ratios of non-synonymous substitutions also suggest that one of the genomes is transitioning from the maternal to the paternal inheritance mode, consistent with its recent masculinization.

CONCLUSION

We have shown, for the first time, that the recently masculinized mitochondrial genome is active and that it accumulates excess of non-synonymous substitutions across its coding sequence. This suggests, that, under certain cytonuclear incompatibility conditions, masculinization may serve to restore the endangered functionality of the paternally inherited genome. This is also another example of a mitochondrial genome in which the recombination in the control region predated its transition from paternal to maternal transmission route.

The largest unassigned regions of the male- and female-transmitted mitochondrial DNAs in Musculista senhousia (Bivalvia Mytilidae)

Musculista senhousia is a marine mussel with doubly uniparental inheritance (DUI) of mitochondria. In this study we analyzed the largest unassigned region (LUR) of its female- and male-transmitted mitochondrial genomes, described their fine characteristics and searched for shared features. Our results suggest that both LURs contain the control region of their respective mitochondrial genomes. The female-transmitted control region is duplicated in tandem, with the two copies evolving in concert. This makes the F-mtDNA of M. senhousia the first Bivalve mitochondrial genome with this feature. We also compared M. senhousia control regions to that of other Mytilidae, and demonstrated that signals for basic mtDNA functions are retained over evolutionary times even among the fast-evolving mitochondrial genomes of DUI species. Finally, we discussed how similarities between female and male LURs may be explained in the context of DUI evolution and if the duplicated female control region might have influenced the DUI system in this species.

Strict sex-specific mtDNA segregation in the germ line of the DUI species Venerupis philippinarum (Bivalvia: Veneridae)

Doubly Uniparental Inheritance (DUI) is one of the most striking exceptions to the common rule of standard maternal inheritance of metazoan mitochondria. In DUI, two mitochondrial genomes are present, showing different transmission routes, one through eggs (F-type) and the other through sperm (M-type). In this paper, we report results from a multiplex real-time quantitative polymerase chain reaction analysis on the Manila clam Venerupis philippinarum (formerly Tapes philippinarum). We quantified M- and F-types in somatic tissues, gonads, and gametes. Nuclear and external reference sequences were used, and the whole experimental process was designed to avoid any possible cross-contamination. In most male somatic tissues, the M-type is largely predominant: This suggests that the processes separating sex-linked mitochondrial DNAs (mtDNAs) in somatic tissues are less precise than in other DUI species. In the germ line, we evidenced a strict sex-specific mtDNA segregation because both sperm and eggs do carry exclusively M- and F-types, respectively, an observation that is in contrast with a previous analysis on Mytilus galloprovincialis. More precisely, whereas two mtDNAs are present in the whole gonad, only the sex-specific one is detected in gametes. Because of this, we propose that the mtDNA transmission is achieved through a three-checkpoint process in V. philippinarum. The cytological mechanisms of male mitochondria segregation in males and degradation in females during the embryo development (here named Checkpoint #1 and Checkpoint #2) are already well known for DUI species; a Checkpoint #3 would act when primordial germ cells (PGCs) are first formed and would work in both males and females. We believe that Checkpoint #3 is a mere variation of the "mitochondrial bottleneck" in species with standard maternal inheritance, established when their PGCs separate during embryo cleavage.

Role-reversal of gender-associated mitochondrial DNA affects mitochondrial function in Mytilus edulis (Bivalvia: Mytilidae)

Mussel species of the genus Mytilus possess an unusual system of mitochondrial DNA (mtDNA) transmission termed doubly uniparental inheritance. They are characterized by the presence of two highly divergent gender-associated mtDNA genomes (often with>20 and>10% divergences in DNA and amino acid sequences, respectively) that are inherited either maternally (F mtDNA) or paternally (M mtDNA). Females are typically homoplasmic for the F mtDNA and males are heteroplasmic with the F mtDNA being most common in all tissues except the gonad that is dominated by the M mtDNA. Collectively, males are polymorphic for two classes of M mtDNAs known as the "standard male" and "recently masculinized" M types (SM and RM, respectively). The coding portions of the RM mtDNA genome differ from the SM mtDNA by as much as the maternally inherited F mtDNA genome differs from the SM type. Because the SM and RM types exhibit considerable amino acid sequence divergence, we hypothesized that these differences could affect mitochondrial respiratory chain complex enzyme activities. To test this hypothesis, the activity of the major mitochondrial respiratory chain complexes (complexes II, I+III and IV) as well as the activity of citrate synthase were measured on gonad samples from males containing either the SM or RM mtDNA. Our data demonstrate that the mitochondrial subunits encoded by the RM mtDNA are associated with higher enzymatic activities than the gene products of the SM mtDNA.

Inheritance of two M type mitochondrial DNA from sperm and unfertilized eggs to offspring in Mytilus galloprovincialis

In Mytilus mussels, paternal mitochondrial DNA (mtDNA) from sperm is known to be transmitted to offspring. This phenomenon is called doubly uniparental inheritance (DUI). Under DUI, sperm mtDNA (M type) is inherited only by males. Female mussels receive maternal mtDNA (F type). However, in our previous study, we showed female and unfertilized eggs have both F and M types. We hypothesized that the two M types both from sperm and unfertilized eggs were transmitted to offspring. To test the hypothesis, we examined the number of M type haplotypes in mature M. galloprovincialis. The M type in larvae was compared with those of the parents. Cross experiments were carried out to test the inheritance of M type. In six of 20 mature mussels, two M types were detected by sequence analysis and polymerase chain reaction-restriction fragment length polymorphism. In cross experiments of larval samples from five of 12 crosses, double peak wave was observed by single nucleotide polymorphisms analysis. In these larval samples, the higher peak wave was identical to the parental M type. Larvae received much more paternal M type than the maternal ones. We demonstrated that two M types from sperm and unfertilized eggs were transmitted to offspring in M. galloprovincialis.

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.