Segregation of mitochondrial genomes in a heteroplasmic lineage with Leber hereditary optic neuroretinopathy.

Relatively little is known about the factors maintaining mitochondrial DNA (mtDNA) sequence diversity in humans. A detailed understanding of the transmission genetics of mtDNA has been partly hampered by the lack of evidence for heteroplasmic individuals. Among families with Leber hereditary optic neuroretinopathy, we found a maternal lineage with individuals heteroplasmic for a single nucleotide change, and we were able to follow the segregation of polymorphic mitochondrial genomes over 3 generations. The results show that rapid segregation can occur but also that the level of heteroplasmy can be maintained from one generation to another. In this family the disease phenotype is associated with the mtDNA sequence change, confirming the involvement of the mutation in the disease.     

The plant-specific ssDNA binding protein OSB1 is involved in the stoichiometric transmission of mitochondrial DNA in Arabidopsis

Plant mitochondrial genomes exist in a natural state of heteroplasmy, in which substoichiometric levels of alternative mitochondrial DNA (mtDNA) molecules coexist with the main genome. These subgenomes either replicate autonomously or are created by infrequent recombination events. We found that Arabidopsis thaliana OSB1 (for Organellar Single-stranded DNA Binding protein1) is required for correct stoichiometric mtDNA transmission. OSB1 is part of a family of plant-specific DNA binding proteins that are characterized by a novel motif that is required for single-stranded DNA binding. The OSB1 protein is targeted to mitochondria, and promoter-beta-glucuronidase fusion showed that the gene is expressed in budding lateral roots, mature pollen, and the embryo sac of unfertilized ovules. OSB1 T-DNA insertion mutants accumulate mtDNA homologous recombination products and develop phenotypes of leaf variegation and distortion. The mtDNA rearrangements occur in two steps: first, homozygous mutants accumulate subgenomic levels of homologous recombination products; second, in subsequent generations, one of the recombination products becomes predominant. After the second step, the process is no longer reversible by backcrossing. Thus, OSB1 participates in controlling the stoichiometry of alternative mtDNA forms generated by recombination. This regulation could take place in gametophytic tissues to ensure the transmission of a functional mitochondrial genome.     

Linking inter-individual variability to endocrine disruptors: insights for epigenetic inheritance

Endocrine disrupting chemicals (EDCs) can induce a myriad of adverse health effects. An area of active investigation is the multi- and transgenerational inheritance of EDC-induced adverse health effects referring to the transmission of phenotypes across multiple generations via the germline. The inheritance of EDC-induced adverse health effects across multiple generations can occur independent of genetics, spurring much research into the transmission of underlying epigenetic mechanisms. Epigenetic mechanisms play important roles in the development of an organism and are responsive to environmental exposures. To date, rodent studies have demonstrated that acquired epigenetic marks, particularly DNA methylation, that are inherited following parental EDC exposure can escape embryonic epigenome reprogramming. The acquired epimutations can lead to subsequent adult-onset diseases. Increasing studies have reported inter-individual variations that occur with epigenetic inheritance. Factors that underlie differences among individuals could reveal previously unidentified mechanisms of epigenetic transmission. In this review, we give an overview of DNA methylation and posttranslational histone modification as the potential mechanisms for disease transmission, and define the requirements for multi- and transgenerational epigenetic inheritance. We subsequently evaluate rodent studies investigating how acquired changes in epigenetic marks especially DNA methylation across multiple generations can vary among individuals following parental EDC exposure. We also discuss potential sources of inter-individual variations and the challenges in identifying these variations. We conclude our review discussing the challenges in applying rodent generational studies to humans.

     

Mitochondrial DNA Transmission Genetics in Crickets

This paper presents the results of a single generation study of the transmission genetics of mitochondrial DNA in the field cricket Gryllus firmus. In this species, individuals heteroplasmic for at least two different-sized mitochondrial genomes can be collected easily from natural populations. The frequencies of mtDNA size variants in heteroplasmic females and samples of their offspring were estimated by densitometry of autoradiographs. The variance in mitochondrial genotype frequencies among the offspring of heteroplasmic females indicates that, through genetic drift, fixation would take several hundred animal generations. Differences between the observations and data on mtDNA transmission in yeast and cows are discussed in light of the differences in organelle sampling regime and early developmental events in these species. Our data also show shifts in genotype frequencies in the transmission from mother to offspring that suggest a bias in favor of smaller genomes. The nature of mtDNA size variation in natural populations of crickets is discussed in reference to a mutation-selection balance.     

The occurrence of mountain hare mitochondrial DNA in wild brown hares

If interspecific hybrids are fertile and backcross to either parental species, transmission of mitochondrial DNA over the species barrier can occur. To investigate if such transmission has occurred between the brown hare Lepus europeus Pall and the mountain hare L. timidus L. in Scandinavia, an analysis of genetic variation in mitochondrial DNA from 36 hares, collected from 15 localities, was performed. Sequence divergence of mtDNA between species was estimated at 8 ± 1% (SD). Intraspecific mtDNA sequence divergence varied between 0.09 and 0.38% in brown hares and 0.10 and 1.44% in mountain hares. In six out of 18 brown hares examined, two different haplotypes of mountain hare origin were detected, demonstrating a transmission of mtDNA haplotypes from mountain hares to brown hares. The results indicate that interspecific hybridization between the two species occurs in wild populations.     

Transmission, inheritance and replication of mitochondrial DNA in mammals: implications for reproductive processes and infertility

The mitochondrial genome contributes key proteins to the electron-transfer chain, which through oxidative phosphorylation, generates the vast majority of cellular ATP. This maternally inherited genome is transmitted to subsequent generations through the oocyte. Its transmission, inheritance and replication are strictly regulated so that fully mature cells can be appropriately populated with mitochondrial DNA once they mature into adult cells. However, gametes do not always acquire the appropriate numbers of mitochondrial DNA copy; this often renders them inappropriate for successful fertilisation outcome. Furthermore, the number of assisted reproductive technologies that can overcome problems associated with infertility and that can provide enhanced genetic outcomes for the offspring is increasing. However, such techniques could also have a detrimental effect on offspring survival. If we are to introduce these technologies into in vitro fertilisation clinics and animal production, then we first need to validate their use carefully.     

Transmission of human mitochondrial DNA along the paternal lineage in transmitochondrial mice

Previously we obtained heteroplasmic mice carrying murine and human mitochondrial DNA (mtDNA). Even the fourth generation of such mice had human mtDNA in their organs, hence, they were used to study the possibility of paternal mtDNA transmission. A lineage was obtained in which human mtDNA was transmitted by males to the progeny in four successive generations. This is the first observation of such a continuous paternal transmission of mtDNA. Persistence of paternal mtDNA in several successive generations of animals suggests that mechanisms aimed at elimination of paternally inherited mtDNA species are not as strict as has been postulated.     

Effect of hydroxyurea treatment on transmission and recombination of mitochondrial genes in Saccharomyces cerevisiae: a new method to modify the input of mitochondrial genes in crosses.

Summary The effects of hydroxyurea, an inhibitor of DNA synthesis, on the transmission and recombination of mitochondrial genes conferring resistance to different antibiotics in Saccharomyces cerevisiae have been studied.Under the conditions used hydroxyurea does not induce respiratory deficient mutants in treated cultures. Homosexual and heterosexual crosses have been performed in which one of the parents was grown for several generations in a medium containing 10^-1 M hydroxyurea prior to mating and the other parent was untreated. In all cases, the transmission of mitochondrial alleles originating from the hydroxyurea treated parent increased. In homosexual crosses the increase of transmission is coordinated for all the alleles. In heterosexual crosses, there is a differential increase of transmission of the different alleles depending on the distance of the marker considered from .All the observed effects can be easily interpreted according to the predictions of the model proposed by Dujon et al. (1974), if one assumes that hydroxyurea treatment increases the input of mitochondrial alleles of the treated parent without major modifications of the frequency of recombination. For each cross the relative copy number of mitDNA of the treated parent has been calculated and the increase so found is in good agreement with the modification of the overall frequency of recombinants observed. Effects of hydroxyurea on input can be interpreted if one assumes that hydroxyurea has a differential effect on mitochondrial and nuclear DNA synthesis: nuclear DNA synthesis would be inhibited while mitochondrial DNA synthesis would still continue.In conclusion, we propose a new powerful genetic tool for the study of the transmission and recombination of mitochondrial genes which offers the possibility of experimentally controlling input, without inducing rho^- mutation.     

Molecular insights into the transgenerational inheritance of stress memory

There is accumulating evidence to show that environmental stressors can regulate a variety of phenotypes in descendants through germline-mediated epigenetic inheritance. Studies of model organisms exposed to environmental cues (e.g., diet, heat stress, toxins) indicate that altered DNA methylations, histone modifications, or non-coding RNAs in the germ cells are responsible for the transgenerational effects. In addition, it has also become evident that maternal provision could provide a mechanism for the transgenerational inheritance of stress adaptations that result from ancestral environmental cues. However, how the signal of environmentally-induced stress response transmits from the soma to the germline, which may influence offspring fitness, remains largely elusive. Small RNAs could serve as signaling molecules that transmit between tissues and even across generations. Furthermore, a recent study revealed that neuronal mitochondrial perturbations induce a transgenerational induction of the mitochondrial unfolded protein response mediated by a Wnt-dependent increase in mitochondrial DNA levels. Here, we review recent work on the molecular mechanism by which parental experience can affect future generations and the importance of soma-to-germline signaling for transgenerational inheritance.     

Inter- and intragenerational transmission of a human mitochondrial DNA heteroplasmy among 13 maternally-related individuals and differences between and within tissues in two family members

The transmission of a C16,291C/T heteroplasmy in the HV1 region of human mitochondrial DNA (mtDNA) was examined in buccal cells from 13 maternally-related individuals across three generations and in additional tissues (hair, blood, or finger nails) from three members of this family. The ratio of C:T at nucleotide position (np) 16,291 showed wide intra- and intergenerational variation as well as tissue variation within individuals. Our results demonstrate that one or two sequence differences between samples in the mtDNA does not warrant an exclusion. To avoid false exclusions especially when comparing mtDNA from hair samples, we recommend the analysis of as many samples as possible in order to minimize the possibility that the detection of a rare polymorphism in a single sample would be considered an exclusion when it is really a match. The observation that the transmission of a mtDNA heteroplasmy from one individual to her offspring is likely to differ among the first-generation offspring and between that generation and subsequent generations lends further credence to the bottleneck theory of inheritance of human mtDNA.     

Replication of bromodeoxyuridylate-substituted mitochondrial DNA in yeast.

The DNA of several strains of Saccharomyces cerevisiae was labeled by growing the culture in medium supplemented with thymidylate and bromodeoxyuridylate. It was thus possible to follow the course of mitochondrial DNA replication in density shift experiments by determining the buoyant density distribution of unreplicated and replicated DNAs in analytical CsCl gradients. DNA replication was followed for three generations after transfer of cultures from light medium to heavy medium and heavy medium to light medium. Under both conditions, the density shifts observed for mitochondrial DNA were those expected for semiconservative, nondispersive replication. This was further confirmed by analysis of the buoyant density of alkali-denatured hybrid mitochondrial DNA. With this method, no significant recombination between replicated and unreplicated DNA was detected after three generations of growth.     

Transmissible mitochondrial hypovirulence in a natural population of Cryphonectria parasitica

A cytoplasmically transmissible hypovirulence syndrome has been identified in virus-free strains of the chestnut blight fungus Cryphonectria parasitica isolated from healing cankers on American chestnut trees in southwestern Michigan. The syndrome is associated with symptoms of fungal senescence, including a progressive decline in the growth potential and abundance of conidia, and elevated levels of respiration through the cyanide-insensitive alternative oxidase pathway. Conidia from senescing mycelia exhibited varying degrees of senescence ranging from normal growth to death soon after germination. Cytoplasmic transmission of hypovirulence between mycelia occurred by hyphal contact and coincided with the transfer of a specific restriction fragment length polymorphism from the mitochondrial DNA (mtDNA) of the donor strains into the mtDNA of virulent recipients. The transmission of the senescence phenotype was observed not only among vegetatively compatible strains but also among incompatible strains. Hypovirulence was present in isolates from the same location with different nuclear genotypes as identified by DNA fingerprinting. This study confirms that mitochondrial hypovirulence can occur spontaneously and spread within a natural population of a phytopathogenic fungus.     

Selfish Little Circles: Transmission Bias and Evolution of Large Deletion-Bearing Mitochondrial DNA in Caenorhabditis briggsae Nematodes

Selfish DNA poses a significant challenge to genome stability and organismal fitness in diverse eukaryotic lineages. Although selfish mitochondrial DNA (mtDNA) has known associations with cytoplasmic male sterility in numerous gynodioecious plant species and is manifested as petite mutants in experimental yeast lab populations, examples of selfish mtDNA in animals are less common. We analyzed the inheritance and evolution of mitochondrial DNA bearing large heteroplasmic deletions including nad5 gene sequences (nad5Δ mtDNA), in the nematode Caenorhabditis briggsae. The deletion is widespread in C. briggsae natural populations and is associated with deleterious organismal effects. We studied the inheritance patterns of nad5Δ mtDNA using eight sets of C. briggsae mutation-accumulation (MA) lines, each initiated from a different natural strain progenitor and bottlenecked as single hermaphrodites across generations. We observed a consistent and strong drive toward higher levels of deletion-bearing molecules in the heteroplasmic pool of mtDNA after ten generations of bottlenecking. Our results demonstrate a uniform transmission bias whereby nad5Δ mtDNA accumulates to higher levels relative to intact mtDNA in multiple genetically diverse natural strains of C. briggsae. We calculated an average 1% per-generation transmission bias for deletion-bearing mtDNA relative to intact genomes. Our study, coupled with known deleterious phenotypes associated with high deletion levels, shows that nad5Δ mtDNA are selfish genetic elements that have evolved in natural populations of C. briggsae, offering a powerful new system to study selfish mtDNA dynamics in metazoans.     

Vertical and horizontal transmission drive bacterial invasion

A huge variety of Arthropod species is infected with endosymbiotic Wolbachia bacteria that manipulate their host’s reproduction to invade populations. In addition to vertical transmission from mother to offspring through the egg cytoplasm, it has been demonstrated through phylogenetic analyses and natural transfer experiments that horizontal transmission of Wolbachia (i.e. contagion) can occur between Arthropod hosts. More recently, factors influencing horizontal transfer have also been explored. While it is clear that horizontal transmission between species plays a major role in the evolutionary history of Wolbachia infections among insects, its role in the spread of a new infection through a host population, notably through within‐species transfers, remained unknown. In this issue of Molecular Ecology, Kraaijeveld et al. (2011) present the first evidence that horizontal transmission played a key role in the early spread of parthenogenesis‐inducing Wolbachia through the parasitoid wasp Leptopilina clavipes. To support their finding, the authors studied genetic variation in three types of markers, including host nuclear DNA, mitochondrial DNA and Wolbachia DNA. Specifically, they examined potential associations between their diversity patterns. No diversity was detected in Wolbachia genes, indicating that a single Wolbachia strain must have infected and spread through L. clavipes. In addition, a correlation between substantial variation in mitochondrial and nuclear genotypes suggested that horizontal transmission played an important role in the current clonal genetic variation in this wasp. Such horizontal transmission could be facilitated by a specific host ecology (e.g. parasitoid wasps sharing the same host resource) and potentially impact co‐evolution between host and symbiont.     

Mitochondrial protein synthesis is required for maintenance of intact mitochondrial genomes in Saccharomyces cerevisiae.

The genes of Saccharomyces cerevisiae coding for the mitochondrial threonine and tryptophan tRNA synthetases and for a putative mitochondrial ribosomal protein have been cloned. These, and the previously cloned gene for a mitochondrial elongation factor, were used to disrupt or partially delete the wild-type chromosomal copies of the genes in the respiratory-competent strain W303. In each case, inactivation of a gene whose product is required for mitochondrial protein synthesis causes an instability in mitochondrial DNA. Although intact mitochondrial genomes are rapidly and quantitatively eliminated in the protein synthesis defective strains, specific rho- genomes can be maintained stably over many generations. These results indicate that mitochondrial protein synthesis is required for the propagation of wild-type mitochondrial DNA in yeast.     

Theoretical and empirical aspects of gene-culture coevolution

Cultural transmission can be roughly defined as the transfer of information between individuals by social learning (see below). Genes used to be, and for most species still are, the only means available for the accurate transfer of information across generations. In species where cultural transmission has developed, notably the human, interactions can occur between the two inheritance systems. Gene-culture coevolution refers to the evolutionary phenomena that arise from these interactions. As we shall see, these interactions can take various forms.     

Analysis of paternal transmission of mitochondrial DNA in Drosophila

It has previously been shown that paternal mitochondrial DNA (mtDNA) can be detected in later generations in Drosophila. To further analyze the paternal transmission of mtDNA, the progeny of two intraspecific and three interspecific crosses were examined in the frequency of the paternal transmission of mtDNA, using closely related species of the melanogaster species subgroup. Types of mtDNA in the progeny of the individual backcrosses of F(1) females were analyzed by selective amplification of paternal mtDNA. More than 100 F(1) females were examined for each backcross. The same type of mtDNA as that of the paternal mtDNA was detected in approximately 20-60% of the backcrosses. The present results indicate that paternal leakage occurs in the intraspecific crosses as well as in the interspecific crosses in Drosophila.     

Mitochondrial gene segregation in mammals: is the bottleneck always narrow?

Summary The segregation of a heteroplasmic silent polymorphism in the mitochondrial ND6 gene has been followed in a human maternal lineage comprising eight individuals and spanning three generations. Heteroplasmy persisted in all eight maternally related family members. More importantly, the frequencies of the two alleles showed relatively little variation among individuals or between generations. In contrast to the findings in other mammalian lineages, the present results indicate relatively slow mitochondrial gene segregation. A narrow bottleneck in the number of mitochondrial DNA (mtDNA) molecules, which occurs at some stage of oogenesis, has been advanced to explain rapid mammalian mitochondrial gene segregation. It is suggested here that the segregation of mitochondrial genes may be more complex than initially envisaged, and that models need to be developed that account for both rapid and slow segregation. One possibility, which reconciles both physical and genetic studies of mammalian mtDNA, is that the unit of mitochondrial segregation is the organelle itself, each containing multiple mtDNA molecules.     

Biparental mitochondrial DNA inheritance in the parasitic trematode Schistosoma mansoni

The maternal inheritance of mitochondrial DNA (mtDNA) in eukaryotic organisms occurs because of the selective destruction of paternal mtDNA molecules that may be present in the zygote. The elimination of sperm mtDNA is less efficient in interspecific crosses, and biparental inheritance of mtDNA has been observed in a variety of species. Because interspecific crosses are likely to be extremely rare in nature, parental inheritance of mtDNA has been deemed of little relevance to population genetics. The mtDNA of the parasitic trematode Schistosoma mansoni was examined for its utility in addressing epidemiological questions related to the transmission and spread of schistosomiasis. Prior to embarking on such experiments, we sought to confirm the mode of inheritance of this molecule using the highly polymorphic mtDNA minisatellite as a marker. In 3 separate crosses, mtDNA apparently identical to paternal DNA was observed in some individuals of the F2 and F3 generations. These observations thus suggest the intraspecific paternal inheritance of mtDNA across multiple generations in Schistosoma mansoni.     

The strength and timing of the mitochondrial bottleneck in salmon suggests a conserved mechanism in vertebrates

In most species mitochondrial DNA (mtDNA) is inherited maternally in an apparently clonal fashion, although how this is achieved remains uncertain. Population genetic studies show not only that individuals can harbor more than one type of mtDNA (heteroplasmy) but that heteroplasmy is common and widespread across a diversity of taxa. Females harboring a mixture of mtDNAs may transmit varying proportions of each mtDNA type (haplotype) to their offspring. However, mtDNA variants are also observed to segregate rapidly between generations despite the high mtDNA copy number in the oocyte, which suggests a genetic bottleneck acts during mtDNA transmission. Understanding the size and timing of this bottleneck is important for interpreting population genetic relationships and for predicting the inheritance of mtDNA based disease, but despite its importance the underlying mechanisms remain unclear. Empirical studies, restricted to mice, have shown that the mtDNA bottleneck could act either at embryogenesis, oogenesis or both. To investigate whether the size and timing of the mitochondrial bottleneck is conserved between distant vertebrates, we measured the genetic variance in mtDNA heteroplasmy at three developmental stages (female, ova and fry) in chinook salmon and applied a new mathematical model to estimate the number of segregating units (N(e)) of the mitochondrial bottleneck between each stage. Using these data we estimate values for mtDNA Ne of 88.3 for oogenesis, and 80.3 for embryogenesis. Our results confirm the presence of a mitochondrial bottleneck in fish, and show that segregation of mtDNA variation is effectively complete by the end of oogenesis. Considering the extensive differences in reproductive physiology between fish and mammals, our results suggest the mechanism underlying the mtDNA bottleneck is conserved in these distant vertebrates both in terms of it magnitude and timing. This finding may lead to improvements in our understanding of mitochondrial disorders and population interpretations using mtDNA data.