Elimination of paternal mitochondrial DNA in intraspecific crosses during early mouse embryogenesis

Nuclear Transfer: Preservation of a Nuclear Genome at the Expense of Its Associated mtDNA Genome(s)

Nuclear transfer technology has uses across theoretical and applied applications, but advances are restricted by continued poor success rates and health problems associated with live offspring. Development of reconstructed embryos is dependent upon numerous interlinking factors relating both to the donor cell and the recipient oocyte. For example, abnormalities in gene expression following somatic cell nuclear transfer (SCNT) have been linked with an inability of the oocyte cytoplasm to sufficiently epigenetically reprogram the nucleus. Furthermore, influences on the propagation of mitochondria and mitochondrial DNA (mtDNA) could be of great importance in determining the early developmental potential of NT embryos and contributing to their genetic identity. mtDNA encodes some of the subunits of the electron transfer chain, responsible for cellular ATP production. The remaining subunits and those factors required for mtDNA replication, transcription and translation are encoded by the nucleus, necessitating precise intergenomic communication. Additionally, regulation of mtDNA copy number, via the processes of mtDNA transcription and replication, is essential for normal preimplantation embryo development and differentiation. Unimaternal transmission following natural fertilization usually results in the presence of a single identical population of mtDNA, homoplasmy. Heteroplasmy can result if mixed populations of mtDNA genomes co-exist. Many abnormalities observed in NT embryos, fetuses, and offspring may be caused by deficiencies in OXPHOS, perhaps resulting in part from heteroplasmic mtDNA populations. Additionally, incompatibilities between the somatic nucleus and the cytoplast may be exacerbated by increased genetic divergence between the two genomes. It is important to ensure that the nucleus is capable of sufficiently regulating mtDNA, requiring a level of compatibility between the two genomes, which may be a function of evolutionary distance. We suggest that abnormal expression of factors such as TFAM and POLG in NT embryos will prematurely drive mtDNA replication, hence impacting on early development.

Human therapeutic cloning (NTSC)

Human therapeutic cloning or nuclear transfer stem cells (NTSC) to produce patient-specific stem cells, holds considerable promise in the field of regenerative medicine. The recent withdrawal of the only scientific publications claiming the successful generation of NTSC lines afford an opportunity to review the available research in mammalian reproductive somatic cell nuclear transfer (SCNT) with the goal of progressing human NTSC. The process of SCNT is prone to epigenetic abnormalities that contribute to very low success rates. Although there are high mortality rates in some species of cloned animals, most surviving clones have been shown to have normal phenotypic and physiological characteristics and to produce healthy offspring. This technology has been applied to an increasing number of mammals for utility in research, agriculture, conservation, and biomedicine. In contrast, attempts at SCNT to produce human embryonic stem cells (hESCs) have been disappointing. Only one group has published reliable evidence of success in deriving a cloned human blastocyst, using an undifferentiated hESC donor cell, and it failed to develop into a hESC line. When optimal conditions are present, it appears that in vitro development of cloned and parthenogenetic embryos, both of which may be utilized to produce hESCs, may be similar to in vitro fertilized embryos. The derivation of ESC lines from cloned embryos is substantially more efficient than the production of viable offspring. This review summarizes developments in mammalian reproductive cloning, cell-to-cell fusion alternatives, and strategies for oocyte procurement that may provide important clues facilitating progress in human therapeutic cloning leading to the successful application of cell-based therapies utilizing autologous hESC lines.

Mitochondria-related male infertility

Approximately 15% of human couples are affected by infertility, and about half of these cases of infertility can be attributed to men, through low sperm motility (asthenozoospermia) or/and numbers (oligospermia). Because mitochondrial genome (mtDNA) mutations are identified in patients with fertility problems, there is a possibility that mitochondrial respiration defects contribute to male infertility. To address this possibility, we used a transmitochondrial mouse model (mito-mice) carrying wild-type mtDNA and mutant mtDNA with a pathogenic 4,696-bp deletion (DeltamtDNA). Here we show that mitochondrial respiration defects caused by the accumulation of DeltamtDNA induced oligospermia and asthenozoospermia in the mito-mice. Most sperm from the infertile mito-mice had abnormalities in the middle piece and nucleus. Testes of the infertile mito-mice showed meiotic arrest at the zygotene stage as well as enhanced apoptosis. Thus, our in vivo study using mito-mice directly demonstrates that normal mitochondrial respiration is required for mammalian spermatogenesis, and its defects resulting from accumulated mutant mtDNAs cause male infertility.

Complete elimination of maternal mitochondrial DNA during meiosis resulting in the paternal inheritance of the mitochondrial genome in Chlamydomonas species

The non-Mendelian inheritance of organellar DNA is common in most plants and animals. In the isogamous green alga Chlamydomonas species, progeny inherit chloroplast genes from the maternal parent, as paternal chloroplast genes are selectively eliminated in young zygotes. Mitochondrial genes are inherited from the paternal parent. Analogically, maternal mitochondrial DNA (mtDNA) is thought to be selectively eliminated. Nevertheless, it is unclear when this selective elimination occurs. Here, we examined the behaviors of maternal and paternal mtDNAs by various methods during the period between the beginning of zygote formation and zoospore formation. First, we observed the behavior of the organelle nucleoids of living cells by specifically staining DNA with the fluorochrome SYBR Green I and staining mitochondria with 3,3'-dihexyloxacarbocyanine iodide. We also examined the fate of mtDNA of male and female parental origin by real-time PCR, nested PCR with single zygotes, and fluorescence in situ hybridization analysis. The mtDNA of maternal origin was completely eliminated before the first cell nuclear division, probably just before mtDNA synthesis, during meiosis. Therefore, the progeny inherit the remaining paternal mtDNA. We suggest that the complete elimination of maternal mtDNA during meiosis is the primary cause of paternal mitochondrial inheritance.

Molecular phylogeny of silkmoths reveals the origin of domesticated silkmoth, Bombyx mori from Chinese Bombyx mandarina and paternal inheritance of Antheraea proylei mitochondrial DNA

Molecular phylogeny of some of the economically important silkmoths was derived using three mitochondrial genes, 12S rRNA, 16S rRNA, and COI, and the control region (CR). Maximum likelihood (ML) analyses showed two distinct clades, one consisting of moths from Bombycidae family and the other from Saturniidae family. The mitochondrial CR showed length polymorphisms with indels. The ML analyses for complete mitochondrial genome sequences of Bombyx mori (strains Aojuku, C108, Backokjam, and Xiafang), Japanese and Chinese strains of B. mandarina (Japanese mandarina and Chinese mandarina) and, Antheraea pernyi revealed two distinct clades, one comprising of B. mori strains and the other with B. mandarina, and A. pernyi forming an outgroup. Pairwise distances revealed that all of the strains of B. mori studied are closer to Chinese than to Japanese mandarina. Phylogenetic analyses based on whole mitochondrial genome sequences, the finding of a tandem triplication of a 126bp repeat element only in Japanese mandarina, and chromosome number variation in B. mandarina suggest that B. mori must have shared its recent common ancestor with Chinese mandarina. Another wild species of the Bombycidae family, Theophila religiosa, whose phylogenetic status was not clear, clustered together with the other bombycid moths in the study. Analysis of the interspecific hybrid, A. proylei gave evidence for paternal inheritance of mitochondrial DNA.

Transmission of mitochondrial DNA in pigs and progeny derived from nuclear transfer of Meishan pig fibroblast cells

In embryos derived by nuclear transfer (NT), fusion, or injection of donor cells with recipient oocytes caused mitochondrial heteroplasmy. Previous studies have reported varying patterns of mitochondrial DNA (mtDNA) transmission in cloned calves. Here, we examined the transmission of mtDNA from NT pigs to their progeny. NT pigs were created by microinjection of Meishan pig fetal fibroblast nuclei into enucleated oocytes (maternal Landrace background). Transmission of donor cell (Meishan) mtDNA was analyzed using 4 NT pigs and 25 of their progeny by PCR-mediated single-strand conformation polymorphism (PCR-SSCP) analysis, PCR-RFLP, and a specific PCR to detect Meishan mtDNA single nucleotide polymorphisms (SNP-PCR). In the blood and hair root of NT pigs, donor mtDNAs were not detected by PCR-SSCP and PCR-RFLP, but detected by SNP-PCR. These results indicated that donor mtDNAs comprised between 0.1% and 1% of total mtDNA. Only one of the progeny exhibited heteroplasmy with donor cell mtDNA populations, ranging from 0% to 44% in selected tissues. Additionally, other progeny of the same heteroplasmic founder pig were analyzed, and 89% (16/18) harbored donor cell mtDNA populations. The proportion of donor mtDNA was significantly higher in liver (12.9 +/- 8.3%) than in spleen (5.0 +/- 3.9%), ear (6.7 +/- 5.3%), and blood (5.8 +/- 3.7%) (P < 0.01). These results demonstrated that donor mtDNAs in NT pigs could be transmitted to progeny. Moreover, once heteroplasmy was transmitted to progeny of NT-derived pigs, it appears that the introduced mitochondrial populations become fixed and maternally-derived heteroplasmy was more readily maintained in subsequent generations.

N/A

N/A

Mitochondrial DNA and ageing

The accumulation of mitochondrial DNA mutations has been proposed as a potential mechanism in the physiological processes of ageing and age-related disease. Although mitochondria have long been anticipated as a perpetrator of ageing, there was little experimental evidence to link these changes directly with the cellular pathology of ageing. Recently, considerable progress in understanding basic mitochondrial genetics and in identifying acquired mtDNA mutations in ageing has been made. Furthermore, the creation of mtDNA-mutator mice has provided the first direct evidence that accelerating the mtDNA mutation rate can result in premature ageing, consistent with the view that loss of mitochondrial function is a major causal factor in ageing. This review will, therefore, focus on recent developments in ageing research related to the role played by mtDNA.

Transmission of mitochondrial DNA following assisted reproduction and nuclear transfer

Mitochondria are the organelles responsible for producing the majority of a cell's ATP and also play an essential role in gamete maturation and embryo development. ATP production within the mitochondria is dependent on proteins encoded by both the nuclear and the mitochondrial genomes, therefore co-ordination between the two genomes is vital for cell survival. To assist with this co-ordination, cells normally contain only one type of mitochondrial DNA (mtDNA) termed homoplasmy. Occasionally, however, two or more types of mtDNA are present termed heteroplasmy. This can result from a combination of mutant and wild-type mtDNA molecules or from a combination of wild-type mtDNA variants. As heteroplasmy can result in mitochondrial disease, various mechanisms exist in the natural fertilization process to ensure the maternal-only transmission of mtDNA and the maintenance of homoplasmy in future generations. However, there is now an increasing use of invasive oocyte reconstruction protocols, which tend to bypass mechanisms for the maintenance of homoplasmy, potentially resulting in the transmission of either form of mtDNA heteroplasmy. Indeed, heteroplasmy caused by combinations of wild-type variants has been reported following cytoplasmic transfer (CT) in the human and following nuclear transfer (NT) in various animal species. Other techniques, such as germinal vesicle transfer and pronuclei transfer, have been proposed as methods of preventing transmission of mitochondrial diseases to future generations. However, resulting embryos and offspring may contain mtDNA heteroplasmy, which itself could result in mitochondrial disease. It is therefore essential that uniparental transmission of mtDNA is ensured before these techniques are used therapeutically.

Mitochondrial dynamics and aging: Mitochondrial interaction preventing individuals from expression of respiratory deficiency caused by mutant mtDNA

In mammalian cells, there is an extensive and continuous exchange of mitochondrial DNA (mtDNA) and its products between mitochondria. This mitochondrial complementation prevents individuals from expression of respiration deficiency caused by mutant mtDNAs. Thus, the presence of mitochondrial complementation does not support the generally accepted mitochondrial theory of aging, which proposes that accumulation of somatic mutations in mtDNA is responsible for age-associated mitochondrial dysfunction. Moreover, the presence of mitochondrial complementation enables gene therapy for mitochondrial diseases using nuclear transplantation of zygotes.

Heteroplasmy and paternally oriented shift of the organellar DNA composition in barley-wheat hybrids during backcrosses with wheat parents

Mitochondrial (mt) and chloroplast (ct) genome inheritance was studied in barley-wheat hybrids, as were their progenies obtained from backcrosses with different common wheat cultivars, by monitoring the composition of 4 mtDNA (coxI, a 5'-flanking region of cob, nad3-orf156, and 5'-upstream region of 18S/5S) and 2 ctDNA (simple-sequence repeat locus downstream of trnS and a 3'-flanking region of rbcL) loci. In male sterile F1 and BC1 plants, maternal barley mtDNA fragments were mainly detected and very low levels of paternal wheat fragments were occasionally detected by PCR in coxI, a 5'-flanking region of cob and nad3-orf156, whereas a 5'-upstream region of 18S/5S showed clear heteroplasmy, containing both maternal and paternal copies, with maternal copies prevailing. Plants showing such heteroplasmic mtDNA composition remained either semisterile or became completely sterile in the later backcross generations. Only maternal ctDNA copies were detected in these plants. In 3 stable, self-fertile, and vigourous lines obtained in the advanced backcross generations and possessing recombinant wheat nuclear genome, however, only mt- and ctDNA copies of wheat parents were detected; thus, the original alloplasmic condition appeared to be lost. Our results suggest that transmission followed by selective replication of the paternal wheat organellar DNA leads to a paternally oriented shift of the organellar DNA composition in barley-wheat hybrids, which correlates with the restoration of fertility and plant vigour. These 2 processes seem to be related to nucleocytoplasmic compatibility and to be under the control of the nuclear genome composition.

Generation of trans-mitochondrial mice carrying homoplasmic mtDNAs with a missense mutation in a structural gene using ES cells

Generation of various kinds of trans-mitochondrial mice, mito-mice, each carrying mtDNAs with a different pathogenic mutation, is required for precise investigation of the pathogenesis of mitochondrial diseases. This study used two respiration-deficient mouse cell lines as donors of mtDNAs with possible pathogenic mutations. One cell line expressed 45-50% respiratory activity due to mouse mtDNAs with a T6589C missense mutation in the COI gene (T6589C mtDNA) and the other expressed 40% respiratory activity due to rat (Rattus norvegicus) mtDNAs in mouse cells. By cytoplasmic transfer of these mtDNAs to mouse ES cells, we isolated respiration-deficient ES cells. We obtained chimeric mice and generated their F(6) progeny carrying mouse T6589C mtDNAs by its female germ line transmission. They were respiration-deficient and thus could be used as models of mitochondrial diseases caused by point mutations in mtDNA structural genes. However, chimeric mice and mito-mice carrying rat mtDNAs were not obtained, suggesting that significant respiration defects or some deficits induced by rat mtDNAs in mouse ES cells prevented their differentiation to generate mice carrying rat mtDNAs.

Active digestion of sperm mitochondrial DNA in single living sperm revealed by optical tweezers

In almost all eukaryotes, mitochondrial (mt) genes are transmitted to progeny mainly from the maternal parent. The most popular explanation for this phenomenon is simple dilution of paternal mtDNA, because the paternal gametes (sperm) are much smaller than maternal gametes (egg) and contribute a limited amount of mitochondria to the progeny. Recently, this simple explanation has been challenged in several reports that describe the active digestion of sperm mtDNA, down-regulation of mtDNA replication in sperm, and proteolysis of mitochondria triggered by ubiquitination. In this investigation, we visualized mt nucleoids in living sperm by using highly sensitive SYBR green I vital staining. The ability to visualize mt nucleoids allowed us to clarify that the elimination of sperm mtDNA upon fertilization is achieved through two steps: (i) gradual decrease of mt nucleoid numbers during spermatogenesis and (ii) rapid digestion of sperm mtDNA just after fertilization. One notable point is that the digestion of mtDNA is achieved before the complete destruction of mitochondrial structures, which may be necessary to avoid the diffusion and transmission of potentially deleterious sperm mtDNA to the progeny.

Gene therapy for progeny of mito-mice carrying pathogenic mtDNA by nuclear transplantation

Pathogenic mutations in mtDNAs have been shown to be responsible for expression of respiration defects and resultant expression of mitochondrial diseases. This study directly addressed the issue of gene therapy of mitochondrial diseases by using nuclear transplantation of zygotes of transmitochondria mice (mito-mice). Mito-mice expressed respiration defects and mitochondrial diseases due to accumulation of mtDNA carrying a large-scale deletion (DeltamtDNA). Second polar bodies were used as biopsy samples for diagnosis of mtDNA genotypes of mito-mouse zygotes. Nuclear transplantation was carried out from mito-mouse zygotes to enucleated normal zygotes and was shown to rescue all of the F(0) progeny from expression of respiration defects throughout their lives. This procedure should be applicable to patients with mitochondrial diseases for preventing their children from developing the diseases.

mtDNA inheritance in the mosquitoes of Anopheles stephensi

The inheritance of mtDNA was tested in malaria vector mosquitoes of Anopheles stephensi strains using PCR-RFLP analysis for its utility in addressing epidemiological questions related to the transmission and spread of malaria. Reciprocal crosses were made between two haplotypes with distinct mtDNA restriction fragment length polymorphism (RFLP) profiles through 20 consecutive generations. All of the progenies produced by these crosses had the mtDNA haplotype of the female parent suggesting that, if it occurs, paternal inheritance of mtDNA in An. stephensi is rare.

Exclusive paternal expression and novel alternatively spliced variants of epsilon-sarcoglycan mRNA in mouse brain

Mutations of SGCE encoding epsilon-sarcoglycan cause myoclonus-dystonia. SGCE is paternally expressed; however, 5-10% of patients show maternal inheritance of the disease. We found Sgce was exclusively paternally expressed in mice by using a novel polymorphism marker. The result was confirmed in Sgce heterozygous knockout mice. This finding suggests that maternally inherited myoclonus-dystonia may not result from maternal expression of SGCE. Furthermore, we report a new family of alternatively spliced Sgce mRNA expressed in the brain coding for different C-terminal sequences possessing a PDZ-binding motif. Our results provide a better basis for diagnosis and understanding of the pathogenesis of myoclonus-dystonia.

Inheritance and recombination of mitochondrial genomes in plants, fungi and animals

It is generally assumed that mitochondrial genomes are uniparentally transmitted, homoplasmic and nonrecombining. However, these assumptions draw largely from early studies on animal mitochondrial DNA (mtDNA). In this review, we show that plants, animals and fungi are all characterized by episodes of biparental inheritance, recombination among genetically distinct partners, and selfish elements within the mitochondrial genome, but that the extent of these phenomena may vary substantially across taxa. We argue that occasional biparental mitochondrial transmission may allow organisms to achieve the best of both worlds by facilitating mutational clearance but continuing to restrict the spread of selfish genetic elements. We also show that methodological biases and disproportionately allocated study effort are likely to have influenced current estimates of the extent of biparental inheritance, heteroplasmy and recombination in mitochondrial genomes from different taxa. Despite these complications, there do seem to be discernible similarities and differences in transmission dynamics and likelihood of recombination of mtDNA in plant, animal and fungal taxa that should provide an excellent opportunity for comparative investigation of the evolution of mitochondrial genome dynamics.

Application of ES cells for generation of respiration-deficient mice carrying mtDNA with a large-scale deletion

In a previous study, we used mouse zygotes as recipients of mtDNA with a large-scale deletion mutation (DeltamtDNA) and generated respiration-deficient mice (mito-mice) carrying DeltamtDNA. In this study, we used mouse ES cells as recipients of DeltamtDNA, and generated mito-mice with DeltamtDNA only when the ES cells carried 17% DeltamtDNA. No chimera mice or their F(1) progenies were obtained from ES cells carrying more than 61% DeltamtDNA. These observations suggest that respiratory defects of ES cells inhibit their normal differentiation into chimera mice and mito-mice, and that ES cells are more effective than zygotes for generation of mito-mice carrying mtDNAs without significant pathogenic mutations.

MITOCHONDRIAL DNA AND HUMAN EVOLUTION

Several unique properties of human mitochondrial DNA (mtDNA), including its high copy number, maternal inheritance, lack of recombination, and high mutation rate, have made it the molecule of choice for studies of human population history and evolution. Here we review the current state of knowledge concerning these properties, how mtDNA variation is studied, what we have learned, and what the future likely holds. We conclude that increasingly, mtDNA studies are (and should be) supplemented with analyses of the Y-chromosome and other nuclear DNA variation. Some serious issues need to be addressed concerning nuclear inserts, database quality, and the possible influence of selection on mtDNA variation. Nonetheless, mtDNA studies will continue to play an important role in such areas as examining socio-cultural influences on human genetic variation, ancient DNA, certain forensic DNA applications, and in tracing personal genetic history.

Specific location of sperm mitochondria in mussel Mytilus galloprovincialis zygotes stained by MitoTracker

In Mytilidae, mitochondrial DNA (mtDNA) in the offspring is inherited from male and female parents. Sperm mitochondria are only incorporated into the testes. This phenomenon is called doubly uniparental inheritance (DUI). Sperm mitochondria should locate in the primordial germ cell during development to maintain DUI. However, the mechanism of sperm mitochondria localization is still unknown. To reveal the mechanism, we followed the location of sperm mitochondria in Mytilus galloprovincialis zygotes fertilized with sperm stained by MitoTracker. Just after fertilization, sperm mitochondria, which were found to enter eggs from various sites, remained at sperm entry point. Five sperm mitochondria located at the male pronucleus. After pronuclear expansion, sperm mitochondria migrated to the center of the egg together with the male pronucleus. At anaphase of cleavage-I, the distribution pattern of sperm mitochondria was divided into two patterns. In pattern A, sperm mitochondria located in the equatorial region of the eggs. In pattern B, sperm mitochondria migrated and divided into two groups with chromosomes. From observations of colchicine-treated eggs, we suggest that sperm mitochondria migration from fertilization to anaphase of cleavage-I depends on the microtubules. The difference between pattern A and pattern B may be caused by whether sperm mitochondria migrated or not by the microtubules at cleavage-I.

Rate of gene transfer from mitochondria to nucleus: effects of cytoplasmic inheritance system and intensity of intracellular competition

Endosymbiotic theory states that mitochondria originated as bacterial intracellular symbionts, the size of the mitochondrial genome gradually reducing over a long period owing to, among other things, gene transfer from the mitochondria to the nucleus. Such gene transfer was observed in more genes in animals than in plants, implying a higher transfer rate of animals. The evolution of gene transfer may have been affected by an intensity of intracellular competition among organelle strains and the organelle inheritance system of the organism concerned. This article reveals a relationship between those factors and the gene transfer rate from organelle to nuclear genomes, using a mathematical model. Mutant mitochondria that lose a certain gene by deletion are considered to replicate more rapidly than normal ones, resulting in an advantage in intracellular competition. If the competition is intense, heteroplasmic individuals possessing both types of mitochondria change to homoplasmic individuals including mutant mitochondria only, with high probability. According to the mathematical model, it was revealed that the rate of gene transfer from mitochondria to the nucleus can be affected by three factors, the intensity of intracellular competition, the probability of paternal organelle transmission, and the effective population size. The gene transfer rate tends to increase with decreasing intracellular competition, increasing paternal organelle transmission, and decreasing effective population size. Intense intracellular competition tends to suppress gene transfer because it is likely to exclude mutant mitochondria that lose the essential gene due to the production of lethal individuals.

Potential Cytoplasmic Inheritance in Wisteria sinensis and Robinia pseudoacacia (Leguminosae)

We examined pollen cells of Wisteria sinensis and Robinia pseudoacacia (Leguminosae) to determine a possible mode for cytoplasmic inheritance in these species. Epifluorescence microscopy revealed distinct mature generative cells. Mature generative cells of W. sinensis were associated with large numbers of punctuated fluorescent signals corresponding to cytoplasmic DNA aggregates, but no fluorescent signals were observed in the generative cells of R. pseudoacacia. Closer examination showed that the punctate fluorescent signals corresponded to plastid but not mitochondrial DNA. These results suggest a strong potential for paternal transmission of the plastid genome in W. sinensis. Electron microscopy confirmed the presence of plastids in the generative cells of W. sinensis and the absence of plastids in R. pseudoacacia cells due to an unequal distribution of plastids during the first pollen mitosis. Mitochondria were present and intact in the mature generative cells of both species. The lack of fluoresced mitochondrial DNA suggests a very low level of mitochondrial DNA in the cells. Immunoelectron microscopy demonstrated that the labeling of mitochondrial DNA in these cells was reduced by nearly 90% during pollen development. Such a dramatic reduction suggests an active degradation of paternal mitochondrial DNA, which may contribute greatly to the maternal inheritance of mitochondria. In short, we found that W. sinensis exhibits a strong potential for paternal transmission of plastids and that both W. sinensis and R. pseudoacacia appear to share the same mechanism for maternal mitochondrial inheritance.

Microinjection of Cytoplasm or Mitochondria Derived from Somatic Cells Affects Parthenogenetic Development of Murine Oocytes1

Cloned mammals are readily obtained by nuclear transfer using cultured somatic cells; however, the rate of generating live offspring from the reconstructed embryos remains low. In nuclear transfer procedures, varying quantities of donor cell mitochondria are transferred with nuclei into recipient oocytes, and mitochondrial heteroplasmy has been observed. A mouse model was used to examine whether transferred mitochondria affect the development of the reconstructed oocytes. Cytoplasm or purified mitochondria from somatic cells derived from the external ear, skeletal muscle, and testis of Mus spretus mice or cumulus cells of Mus musculus domesticus mice were transferred into M. m. domesticus (B6SJLF1 and B6D2F1) oocytes to observe parthenogenetic development through the morula stage. All B6D2F1 oocytes injected with somatic cytoplasm or mitochondria showed delayed development when compared to oocytes injected with buffer. The developmental rates were not different among injected cell sources, with the exception of testis-derived donor cells injected into B6SJLF1 oocytes (P < 0.01). The developmental rate of B6D2F1 oocytes injected with buffer alone (98.8% survival) was different from those injected with somatic cytoplasm (60.8% survival) or somatic mitochondria (56.5% survival) (P < 0.01). Conversely, injection of ooplasm into B6D2F1 oocytes did not affect parthenogenetic development (100% survival). Our results indicate that injection of somatic cytoplasm or mitochondria affected parthenogenetic development of murine oocytes. These results have further implications for in vitro fertilization protocols employing ooplasmic transfer where primary oocyte failure is not confirmed.

Germline bottlenecks, biparental inheritance and selection on mitochondrial variants: a two-level selection model

Selection on mitochondrial mutations potentially occurs at different levels: at the mitochondria, cell, and organism levels. Several factors affect the strength of selection at these different levels; in particular, mitochondrial bottlenecks during germline development and reduced paternal transmission decrease the genetic variance within cells, while they increase the variance between cells and between organisms, thus decreasing the strength of selection within cells and increasing the strength of selection between cells and organisms. However, bottlenecks and paternal transmission also affect the effective mitochondrial population size, thus affecting genetic drift. In this article, we use a simple model of a unicellular life cycle to investigate the effects of bottlenecks and paternal transmission on the probability of fixation of mitochondrial mutants and their frequency at mutation-selection equilibrium. We find that bottlenecks and reduced paternal transmission decrease the mean frequency of alleles with sm>sc (approximately), where sm and sc are the strengths of selection for an allele within and between cells, respectively, and increase the frequency of alleles with sm<sc. Effects on fixation probabilities are different; for example, bottlenecks reduce the fixation probability of mutants with sm>0 (unless sm is very small relative to sc) and increase the fixation probability of mutants with sm<0.

Giant spermatozoon coiled in small egg: fertilization mechanisms and their implications for evolutionary studies on Ostracoda (Crustacea)

Ostracods of the superfamily Cypridoidea have giant spermatozoa. However, little data exist on the sperm-egg interaction in this group: only two publications have so far given the most ambiguous indication that the entire sperm enters the egg on fertilization. These assumptions have not yet been tested with modern techniques, nor has their putative value for developmental and evolutionary investigations been realized. The present paper gives the first, clear, light- and scanning electron microscopical evidence of the entire giant ostracod spermatozoon being incorporated into the egg. Coiling of the sperm underneath the egg shell is shown in the early zygotes of the species Mytilocypris praenuncia and Pseudocandona marchica. Additionally, data on the morphology of female and male reproductive tracts are given for M. praenuncia. Hypotheses on the evolution of giant filiform sperm in the Animal Kingdom are reviewed, and their applicability to ostracods is discussed. The demonstrated ingression of the entire sperm implies the entry of the two giant paternal mitochondrial derivates into the zygote in Cypridoidea and potentially casts doubt upon the dogma of strict maternal inheritance of mitochondrial DNA. Evidence of paternal inheritance of mtDNA in several organisms has recently given rise to a controversial debate on this issue; the possible significance of this phenomenon for molecular studies on ostracod phylogeny and evolution is discussed.

Rare creation of recombinant mtDNA haplotypes in mammalian tissues

The problem of whether recombinant mtDNAs are created in mammalian cells has been controversial for many years. We show convincing evidence for the very rare creation of recombinant mtDNA haplotypes by isolating human somatic hybrid cells and by generating mice carrying two different mtDNA haplotypes. To avoid misinterpretation of PCR-jumping products as recombinants, we used purified mtDNAs for cloning and sequencing. The results showed that only three of 318 clones of mtDNA purified from mouse tissues corresponded to recombinant mtDNA haplotypes, whereas no recombinants were found in human somatic hybrid cells. Such an extremely low frequency of mtDNA recombination does not require any revision of important concepts on human evolution that are based on its absence. Considering the high concentration of reactive oxygen species around the mtDNA and its frequent strand breakage, recombinant clones would correspond to gene conversion products created by repair of nucleotide mismatches.

Paternal mitochondrial DNA transmission during nonhuman primate nuclear transfer

Offspring produced by nuclear transfer (NT) have identical nuclear DNA (nDNA). However, mitochondrial DNA (mtDNA) inheritance could vary considerably. In sheep, homoplasmy is maintained since mtDNA is transmitted from the oocyte (recipient) only. In contrast, cattle are heteroplasmic, harboring a predominance of recipient mtDNA along with varying levels of donor mtDNA. We show that the two nonhuman primate Macaca mulatta offspring born by NT have mtDNA from three sources: (1) maternal mtDNA from the recipient egg, (2) maternal mtDNA from the egg contributing to the donor blastomere, and (3) paternal mtDNA from the sperm that fertilized the egg from which the donor blastomere was isolated. The introduction of foreign mtDNA into reconstructed recipient eggs has also been demonstrated in mice through pronuclear injection and in humans through cytoplasmic transfer. The mitochondrial triplasmy following M. mulatta NT reported here forces concerns regarding the parental origins of mtDNA in clinically reconstructed eggs. In addition, mtDNA heteroplasmy might result in the embryonic stem cell lines generated for experimental and therapeutic purposes ("therapeutic cloning").

[Mitochondria and reproduction]

Mitochondria play a primary role in cellular energetic metabolism. They possess their own DNA, which is exclusively maternally transmitted. The relatively recent idea that mitochondria may be directly involved in human reproduction is arousing increasing interest in the scientific and medical community. It has been shown that the functional status of mitochondria contributes to the quality of oocytes and spermatozoa, and plays a part in the process of fertilisation and embryo development. Moreover, new techniques, such as ooplasm transfer, compromise the uniquely maternal inheritance of mitochondrial DNA, raising important ethical questions. This review discusses recent information about mitochondria in the field of human fertility and reproduction.

The impact of mitochondrial genetics on male infertility

Summary Human mitochondrial DNA (mtDNA) encodes 13 of the polypeptides associated with the process of oxidative phosphorylation (OXPHOS), the cells most important ATP generating pathway. Until recently, the effects of mtDNA rearrangements on male fertility have been largely ignored. However, it is becoming increasingly evident that both point mutations and large-scale deletions may have an impact on sperm motility and morphology. We discuss the implications of these rearrangements in the context of the clinical setting. We further discuss the possible consequences resulting from the transmission of sperm mtDNA deletions to the offspring. The role of nucleo-cytoplasmic interaction is investigated in the context of nuclear transcription and replication factors that regulate mtDNA transcription and replication.

Nuclear DNA but not mtDNA controls tumor phenotypes in mouse cells

Recent studies showed high frequencies of homoplasmic mtDNA mutations in various human tumor types, suggesting that the mutated mtDNA haplotypes somehow contribute to expression of tumor phenotypes. We directly addressed this issue by isolating mouse mtDNA-less (rho(0)) cells for complete mtDNA replacement between normal cells and their carcinogen-induced transformants, and examined the effect of the mtDNA replacement on expression of tumorigenicity, a phenotype forming tumors in nude mice. The results showed that genome chimera cells carrying nuclear DNA from tumor cells and mtDNA from normal cells expressed tumorigenicity, whereas those carrying nuclear DNA from normal cells and mtDNA from tumor cells did not. These observations provided direct evidence that nuclear DNA, but not mtDNA, is responsible for carcinogen-induced malignant transformation, although it remains possible that mtDNA mutations and resultant respiration defects may influence the degree of malignancy, such as invasive or metastatic properties.

In vivo interaction between mitochondria carrying mtDNAs from different mouse species

Mitochondrial disease model mice, mitomice, were created using zygotes of B6mtspr strain mice carrying mitochondrial DNA (mtDNA) from Mus spretus as recipients of exogenous mitochondria carrying wild-type and a deletion mutant mtDNA (DeltamtDNA) of M. musculus domesticus. In these experiments, mtDNAs from different mouse species were used for identification of exo- and endogenous wild-type mtDNAs in the mitomice. Results showed transmission of exogenous DeltamtDNA, but not exogenous wild-type mtDNA, of M. m. domesticus to following generations through the female germ line. Complete elimination of exogenous wild-type mtDNA would be due to stochastic segregation, whereas transmission of exogenous DeltamtDNA would be due to its smaller size leading to a propagational advantage. Tissues in mitomice of the F3 generation carrying exogenous DeltamtDNA showed protection from respiration defects until DeltamtDNA accumulated predominantly. This protection from expression of mitochondrial dysfunction was attained with the help of endogenous wild-type mtDNA of M. spretus, since mitomice did not possess exogenous wild-type mtDNA of M. m. domesticus. These observations provide unambiguous evidence for the presence of interaction between exogenous mitochondria carrying DeltamtDNA and endogenous mitochondria carrying M. spretus wild-type mtDNA.

Effect of cryopreservation on mitochondrial DNA of zebrafish (Danio rerio) blastomere cells

Cryopreservation has been extensively used in human reproductive medicine, aquaculture and conservation programmes for endangered species. However, despite the growing successes of cryopreservation, post-thaw recovery of reproductive and embryonic cells very often remains poor. Many studies have been devoted to the mechanisms of cryodamage. It is known that cryopreservation causes extensive damage to membranes; reduce the metabolic activity of cells; and disturbs the mitochondrial bioenergetical processes of cells. But few investigations on the genetic stability of cells during cryopreservation have been performed, and the role of any genetic impact cryopreservation needs to be determined. Some indirect data in the literature suggests that progress in this field might come from investigating freezing damage to mitochondrial DNA (mtDNA), nuclear DNA and other genome-related structures. In this study, zebrafish (Danio rerio) blastomeres were treated in three different ways: control suspension of blastomere cells in phosphate buffered saline; equilibration of blastomeres with 2M dimethyl sulfoxide (Me2SO) for 1h at room temperature and cryopreservation using Me2SO as a cryoprotectant. Mitochondrial DNA was analysed in fresh cells and after the different treatments. Two different loci of mtDNA were amplified with the help of PCR and sequenced. The sequences were analysed and nuclear base substitutions were counted for both control and treated samples. The results showed that cryopreservation significantly increased the frequency of mutations (0.78+/-0.27% in comparison to 0.16+/-0.25% of control), whilst 2M Me2SO treatment did not bring a significant increase in frequency of mutations (0.24+/-0.28%). The distributions of the mutation locations were analysed. More investigations are needed to determine whether optimisation of cryopreservation protocol is possible to reduce these adverse effects; whether such mutations interfere with overall function of the cells; whether similar changes also occur in the nuclear DNA and whether such mutations happen in other species. Meanwhile, it is important to be cautious in making judgements of the effect of cryopreservation technique in assisted reproduction. This is the first report on the effect of cryopreservation on mtDNA.

Nonsynonymous Site Heteroplasmy in Fish Mitochondrial DNA

Heteroplasmic nucleotide polymorphisms are rarely observed in wild animal mitochondrial DNA. The occurrence of such site heteroplasmy is expected to be extremely rare at nonsynonymous sites where the number of nucleotide substitutions per site is low due to functional constraints. This report deals with nonsynonymous mitochondrial heteroplasmy from two wild fish species, chum salmon and Japanese flounder. We detected an A/C nonsynonymous heteroplasmic site corresponding to putative amino acids, Ile or Met, in NADH dehydrogenase subunit-5 (ND5) region of chum salmon. The heteroplasmic site was at the 3rd position of 58th codon. As for Japanese flounder we detected a C/T nonsynonymous heteroplasmic site corresponding to putative amino acids, Leu or Pro, in ND4 region. The heteroplasmic site was at the 2nd position of 450th codon. We also verified heteroplasmy at these sites by sequencing cloned fragments.

Microinsemination and Nuclear Transfer Using Male Germ Cells

Microinsemination has been widely used in basic reproductive research and in human-assisted reproductive technology for treating infertility. Historically, microinsemination in mammals started with research on the golden hamster; since then, it has provided invaluable information on the mechanisms of mammalian fertilization. Thanks to advances in animal genetic engineering and germ-cell technologies, microinsemination techniques are now used extensively to identify the biological significance of genes of interest or to confirm the genetic normality of gametes produced by experimental manipulations in vitro. Fortunately, in mice, high rates of embryo development to offspring can be obtained so long as postmeiotic spermatogenic cells are used as male gametes-that is, round spermatids, elongated spermatids, and spermatozoa. For some other mammalian species, using immature spermatogenic cells significantly decreases the efficiency of microinsemination. Physically unstable chromatin and low oocyte-activating capacity are the major causes of fertilization failure. The youngest male germ cells, including primordial germ cells and gonocytes, can be used in the construction of diploid embryos by nuclear-transfer cloning. The cloned embryos obtained in this way provide invaluable information on the erasure and reestablishment of genomic imprinting in germ cells.

INDEPENDENT GENE PHYLOGENIES AND MORPHOLOGY DEMONSTRATE A MALAGASY ORIGIN FOR A WIDE-RANGING GROUP OF SWALLOWTAIL BUTTERFLIES

Madagascar is home to numerous endemic species and lineages, but the processes that have contributed to its endangered diversity are still poorly understood. Evidence is accumulating to demonstrate the importance of Tertiary dispersal across varying distances of oceanic barriers, supplementing vicariance relationships dating back to the Cretaceous, but these hypotheses remain tentative in the absence of well-supported phylogenies. In the Papilio demoleus group of swallowtail butterflies, three of the five recognized species are restricted to Madagascar, whereas the remaining two species range across the Afrotropical zone and southern Asia plus Australia. We reconstructed phylogenetic relationships for all species in the P. demoleus group, as well as 11 outgroup Papilio species, using 60 morphological characters and about 4 kb of nucleotide sequences from two mitochondrial (cytochrome oxidase I and II) and two nuclear (wg and EF-1alpha) genes. Of the three endemic Malagasy species, the two that are formally listed as endangered or at risk represented the most basal divergences in the group, while the more common third endemic was clearly related to African P. demodocus. The fifth species, P. demoleus, showed little differentiation across southern Asia, but showed divergence from its subspecies sthenelus in Australia. Dispersal-vicariance analysis using cladograms derived from morphology and three independent genes indicated a Malagasy diversification of lime swallowtails in the middle Miocene. Thus, diversification processes on the island of Madagascar may have contributed to the origin of common butterflies that now occur throughout much of the Old World tropical and subtemperate regions. An alternative hypothesis, that Madagascar is a refuge for ancient lineages resulting from successive colonizations from Africa, is less parsimonious and does not explain the relatively low continental diversity of the group.

Differential segregation patterns of sperm mitochondria in embryos of the blue mussel (Mytilus edulis)

In Mytilus, females carry predominantly maternal mitochondrial DNA (mtDNA) but males carry maternal mtDNA in their somatic tissues and paternal mtDNA in their gonads. This phenomenon, known as doubly uniparental inheritance (DUI) of mtDNA, presents a major departure from the uniparental transmission of organelle genomes. Eggs of Mytilus edulis from females that produce exclusively daughters and from females that produce mostly sons were fertilized with sperm stained with MitoTracker Green FM, allowing observation of sperm mitochondria in the embryo by epifluorescent and confocal microscopy. In embryos from females that produce only daughters, sperm mitochondria are randomly dispersed among blastomeres. In embryos from females that produce mostly sons, sperm mitochondria tend to aggregate and end up in one blastomere in the two- and four-cell stages. We postulate that the aggregate eventually ends up in the first germ cells, thus accounting for the presence of paternal mtDNA in the male gonad. This is the first evidence for different behaviors of sperm mitochondria in developing embryos that may explain the tight linkage between gender and inheritance of paternal mitochondrial DNA in species with DUI.

Proteasomal Interference Prevents Zona Pellucida Penetration and Fertilization in Mammals1

The ubiquitin-proteasome pathway has been implicated in the penetration of ascidian vitelline envelope by the fertilizing spermatozoon (Sawada et al., Proc Natl Acad Sci U S A 2002; 99:1223-1228). The present study provides experimental evidence demonstrating proteasome involvement in the penetration of mammalian zona pellucida (ZP). Using porcine in vitro fertilization as a model, penetration of ZP was completely inhibited by specific proteasomal inhibitors MG-132 and lactacystin. Three commercial rabbit sera recognizing 20S proteasomal core subunits beta-1i, beta-2i, alpha-6, and beta-5 completely blocked fertilization at a very low concentration (i.e., diluted 1/2000 to 1/8000 in fertilization medium). Neither proteasome inhibitors nor antibodies had any effects on sperm-ZP binding and acrosome exocytosis in zona-enclosed oocytes or on fertilization rates in zona-free oocytes, which were highly polyspermic. Consistent with a possible role of ubiquitin-proteasome pathway in ZP penetration, ubiquitin and various alpha and beta type proteasomal subunits were detected in boar sperm acrosome by specific antibodies, immunoprecipitated and microsequenced by MALDI-TOF from boar sperm extracts. Antiubiquitin-immunoreactive substrates were detected on the outer face of ZP by epifluorescence microscopy. This study therefore provides strong evidence implicating the ubiquitin-proteasome pathway in mammalian fertilization and zona penetration. This finding opens a new line of acrosome/ZP research because further studies of the sperm acrosomal proteasome can provide new tools for the management of polyspermia during in vitro fertilization and identify new targets for contraceptive development.

The incomplete natural history of mitochondria

Mitochondrial DNA (mtDNA) has been used to study molecular ecology and phylogeography for 25 years. Much important information has been gained in this way, but it is time to reflect on the biology of the mitochondrion itself and consider opportunities for evolutionary studies of the organelle itself and its ecology, biochemistry and physiology. This review has four sections. First, we review aspects of the natural history of mitochondria and their DNA to show that it is a unique molecule with specific characteristics that differ from nuclear DNA. We do not attempt to cover the plethora of differences between mitochondrial and nuclear DNA; rather we spotlight differences that can cause significant bias when inferring demographic properties of populations and/or the evolutionary history of species. We focus on recombination, effective population size and mutation rate. Second, we explore some of the difficulties in interpreting phylogeographical data from mtDNA data alone and suggest a broader use of multiple nuclear markers. We argue that mtDNA is not a sufficient marker for phylogeographical studies if the focus of the investigation is the species and not the organelle. We focus on the potential bias caused by introgression. Third, we show that it is not safe to assume a priori that mtDNA evolves as a strictly neutral marker because both direct and indirect selection influence mitochondria. We outline some of the statistical tests of neutrality that can, and should, be applied to mtDNA sequence data prior to making any global statements concerning the history of the organism. We conclude with a critical examination of the neglected biology of mitochondria and point out several surprising gaps in the state of our knowledge about this important organelle. Here we limelight mitochondrial ecology, sexually antagonistic selection, life-history evolution including ageing and disease, and the evolution of mitochondrial inheritance.

Accumulation of pathogenic ΔmtDNA induced deafness but not diabetic phenotypes in mito-mice

Mito-mice carrying various proportions of deletion mutant mtDNA (DeltamtDNA) were generated by introduction of the DeltamtDNA from cultured cells into fertilized eggs of C57BL/6J (B6) strain mice. Great advantages of mito-mice are that they share exactly the same nuclear-genome background, and that their genetic variations are restricted to proportions of pathogenic DeltamtDNA. Since accumulation of DeltamtDNA to more than 75% induced respiration defects, the disease phenotypes observed exclusively in mito-mice carrying more than 75% DeltamtDNA should be due to accumulated DeltamtDNA. In this study, we focused on the expressions of hearing loss and diabetic phenotypes, since these common age-associated abnormalities have sometimes been reported to be inherited maternally and to be associated with pathogenic mutant mtDNAs. The results showed that accumulation of exogenously introduced DeltamtDNA was responsible for hearing loss, but not for expression of diabetic phenotypes in mito-mice.

The evolutionary processes of mitochondrial and chloroplast genomes differ from those of nuclear genomes

This paper first introduces our present knowledge of the origin of mitochondria and chloroplasts, and the organization and inheritance patterns of their genomes, and then carries on to review the evolutionary processes influencing mitochondrial and chloroplast genomes. The differences in evolutionary phenomena between the nuclear and cytoplasmic genomes are highlighted. It is emphasized that varying inheritance patterns and copy numbers among different types of genomes, and the potential advantage achieved through the transfer of many cytoplasmic genes to the nucleus, have important implications for the evolution of nuclear, mitochondrial and chloroplast genomes. Cytoplasmic genes transferred to the nucleus have joined the more strictly controlled genetic system of the nuclear genome, including also sexual recombination, while genes retained within the cytoplasmic organelles can be involved in selection and drift processes both within and among individuals. Within-individual processes can be either intra- or intercellular. In the case of heteroplasmy, which is attributed to mutations or biparental inheritance, within-individual selection on cytoplasmic DNA may provide a mechanism by which the organism can adapt rapidly. The inheritance of cytoplasmic genomes is not universally maternal. The presence of a range of inheritance patterns indicates that different strategies have been adopted by different organisms. On the other hand, the variability occasionally observed in the inheritance mechanisms of cytoplasmic genomes reduces heritability and increases environmental components in phenotypic features and, consequently, decreases the potential for adaptive evolution.

The consequences of nuclear transfer for mammalian foetal development and offspring survival. A mitochondrial DNA perspective

The introduction of nuclear transfer (NT) and other technologies that involve embryo reconstruction require us to reinvestigate patterns of mitochondrial DNA (mtDNA) transmission, transcription and replication. MtDNA is a 16.6 kb genome located within each mitochondrion. The number of mitochondria and mtDNA copies per organelle is specific to each cell type. MtDNA is normally transmitted through the oocyte to the offspring. However, reconstructed oocytes often transmit both recipient oocyte mtDNA and mtDNA associated with the donor nucleus. We argue that the transmission of two populations of mtDNA may have implications for offspring survival as only one allele might be actively transcribed. This could result in the offspring phenotypically exhibiting mtDNA depletion-type syndromes. A similar occurrence could arise when nucleo-cytoplasmic interactions fail to regulate mtDNA transcription and replication, especially as the initiation of mtDNA replication post-implantation is a key developmental event. Furthermore, failure of the donor somatic nucleus to be reprogrammed could result in the early initiation of replication and the loss of cellular mtDNA specificity. We suggest investigations should be conducted to enhance our understanding of nucleo-cytoplasmic interactions in order to improve NT efficiency.

Divergent potentials for cytoplasmic inheritance within the genus Syringa. A new trait associated with speciogenesis

Epifluorescence microscopic detection of organelle DNA in the mature generative cell is a rapid method for determining the potential for the mode of cytoplasmic inheritance. We used this method to examine 19 of the known 22 to 27 species in the genus Syringa. Organelle DNA was undetectable in seven species, all in the subgenus Syringa, but was detected in the 12 species examined of the subgenera Syringa and Ligustrina. Therefore, species within the genus Syringa display differences in the potential cytoplasmic inheritance. Closer examination revealed that the mature generative cells of the species in which organelle DNA was detected contained both mitochondria and plastids, but cells of the species lacking detectable organelle DNA contained only mitochondria, and the epifluorescent organelle DNA signals from the mature generative cells corresponded to plastid DNA. In addition, semiquantitative analysis was used to demonstrate that, during pollen development, the amount of mitochondrial DNA decreased greatly in the generative cells of the species examined, but the amount of plastid DNA increased remarkably in the species containing plastids in the generative cell. The results suggest that all Syringa species exhibit potential maternal mitochondrial inheritance, and a number of the species exhibit potential biparental plastid inheritance. The difference between the modes of potential plastid inheritance among the species suggests different phylogenies for the species; it also supports recent conclusions of molecular, systematic studies of the Syringa. In addition, the results provide new evidence for the mechanisms of maternal mitochondrial inheritance in angiosperms.

The mitochondrial plasmid of the true slime mold Physarum polycephalum bypasses uniparental inheritance by promoting mitochondrial fusion

Mitochondrial DNA (mtDNA) is inherited maternally in most eukaryotes. Linear mitochondrial plasmids in higher plants and fungi are also transmitted from the maternal parent to the progeny. However, mF, which is a mitochondrial linear plasmid of Physarum polycephalum, evades uniparental mitochondrial inheritance. We examined 36 myxamoebal strains of Physarum and isolated three novel mF+ strains (JE8, TU111, NG111) that harbored free mF plasmids. These strains were mated with the mF- strain KM88. Of the three mF- x mF+ crosses, only KM88 x JE8 displayed complete uniparental inheritance. However, in KM88 x TU111 and KM88 x NG111, the mtDNA of KM88 and mF of TU111 and NG111 were inherited by the plasmodia and showed recombination. For example, although the mtDNA of TU111 was eliminated, the mF of TU111 persisted and became inserted into the mtDNA of KM88, such that recombinant mtDNA represented 80% of the total mtDNA. The parental mitochondria fused to yield giant mitochondria with two or more mitochondrial nucleoids. The mF appears to exchange mitochondria from the recipient (paternal) to the donor (maternal) by promoting mitochondrial fusion.

Differential Segregation Patterns of Sperm Mitochondria in Embryos of the Blue Mussel (Mytilus edulis)

In Mytilus, females carry predominantly maternal mitochondrial DNA (mtDNA) but males carry maternal mtDNA in their somatic tissues and paternal mtDNA in their gonads. This phenomenon, known as doubly uniparental inheritance (DUI) of mtDNA, presents a major departure from the uniparental transmission of organelle genomes. Eggs of Mytilus edulis from females that produce exclusively daughters and from females that produce mostly sons were fertilized with sperm stained with MitoTracker Green FM, allowing observation of sperm mitochondria in the embryo by epifluorescent and confocal microscopy. In embryos from females that produce only daughters, sperm mitochondria are randomly dispersed among blastomeres. In embryos from females that produce mostly sons, sperm mitochondria tend to aggregate and end up in one blastomere in the two- and four-cell stages. We postulate that the aggregate eventually ends up in the first germ cells, thus accounting for the presence of paternal mtDNA in the male gonad. This is the first evidence for different behaviors of sperm mitochondria in developing embryos that may explain the tight linkage between gender and inheritance of paternal mitochondrial DNA in species with DUI.

Cloning: questions answered and unsolved

Cloning by the transfer of adult somatic cell nuclei to oocytes has produced viable offspring in a variety of mammalian species. The technology is still in its initial stages of development. Studies to date have answered several basic questions related to such issues as genome potency, life expectancy of clones, mitochondrial fates, and feasibility of inter-species nuclear transfer. They have also raised new questions related to the control of nuclear reprogramming and function. These questions are reviewed here.

Presence of interaction but not complementation between human mtDNAs carrying different mutations within a tRNALeu(UUR) gene

This study provided answers to fundamental questions on mammalian mitochondrial genetics: Could respiratory function in mitochondria be maintained by their exchange of genetic contents even when mutations were created within the same genes in different mitochondrial DNA (mtDNA) molecules? Using cell fusion techniques, we created a chance to coexist two types of respiration-deficient syn(-) mitochondria carrying different mtDNA mutations within the same tRNA(Leu(UUR)) gene obtained from patients with mitochondrial diseases. The results showed that two syn(-) mitochondria exchanged their genetic contents, but did not restore respiration defects, suggesting that mitochondrial interaction could not complement the mutations created within the same gene in different mtDNA molecules.

The potential risks of abnormal transmission of mtDNA through assisted reproductive technologies

The recent introduction of more invasive assisted reproductive techniques offers the possibility to provide a wider treatment profile to patients. However, some of these technologies are of considerable concern as they are fraught with the possible transmission of genetic abnormalities to the offspring they create. To date, much analysis of these technologies has been conducted at the chromosomal DNA level. While some analysis has been conducted on the extranuclear, mitochondrial genome (mtDNA), this has been mainly descriptive. In the vast majority of cases, it appears that mtDNA is maternally inherited. The impact that leakage of sperm mtDNA transmission might have for the offspring is discussed in the light of the recent identification of sperm mtDNA presence in a patient with mtDNA disease. The implications of introducing donor mtDNA into a recipient oocyte through both cytoplasmic and nuclear transfer are also discussed. Again, the implications for offspring survival are discussed and suggestions made as to why the techniques might provide valuable insights into mtDNA transmission, replication and transcription. In order to be confident that patients and their offspring are being offered safe treatment, it is argued that potentially some of these treatments may be of considerable benefit in the future but significant scientific research is required before these treatments can be effectively employed in the clinic.