Maternal Inheritance Of Mitochondria Research Articles

Inheritance of Mitochondria in Pelargonium Section Ciconium (Sweet) Interspecific Crosses

We have studied the inheritance of mitochondria in Pelargonium section Ciconium using 36 interspecific crosses generated. We designed KASP markers targeting four mitochondrial loci, belonging to the mitomes of four main crossing parents, enabling tracking the transmission of each mitome in the crosses. These markers discriminate between an individual species versus the other section Ciconium species. We found that maternal inheritance of mitochondria is most frequent, with occasional occurrences of paternal inheritance, while biparental inheritance is rare. For a P. multibracteatum crossing series, we found ambiguous results. Our results confirm those of previous studies, namely, that paternal inheritance of mitochondria can occur in P. sect Ciconium but that the instance is rare and much less common than is the case for chloroplasts.

Mother's Curse effects on lifespan and aging.

The Mother's Curse hypothesis posits that mothers curse their sons with harmful mitochondria, because maternal mitochondrial inheritance makes selection blind to mitochondrial mutations that harm only males. As a result, mitochondrial function may be evolutionarily optimized for females. This is an attractive explanation for ubiquitous sex differences in lifespan and aging, given the prevalence of maternal mitochondrial inheritance and the established relationship between mitochondria and aging. This review outlines patterns expected under the hypothesis, and traits most likely to be affected, chiefly those that are sexually dimorphic and energy intensive. A survey of the literature shows that evidence for Mother's Curse is limited to a few taxonomic groups, with the strongest support coming from experimental crosses in Drosophila. Much of the evidence comes from studies of fertility, which is expected to be particularly vulnerable to male-harming mitochondrial mutations, but studies of lifespan and aging also show evidence of Mother's Curse effects. Despite some very compelling studies supporting the hypothesis, the evidence is quite patchy overall, with contradictory results even found for the same traits in the same taxa. Reasons for this scarcity of evidence are discussed, including nuclear compensation, factors opposing male-specific mutation load, effects of interspecific hybridization, context dependency and demographic effects. Mother's Curse effects may indeed contribute to sex differences, but the complexity of other contributing factors make Mother's Curse a poor general predictor of sex-specific lifespan and aging.

Maternal mitochondrial function affects paternal mitochondrial inheritance in Drosophila.

The maternal inheritance of mitochondria is a widely accepted paradigm, and mechanisms that prevent paternal mitochondria transmission to offspring during spermatogenesis and postfertilization have been described. Although certain species do retain paternal mitochondria, the factors affecting paternal mitochondria inheritance in these cases are unclear. More importantly, the evolutionary benefit of retaining paternal mitochondria and their ultimate fate are unknown. Here we show that transplanted exogenous paternal D. yakuba mitochondria can be transmitted to offspring when maternal mitochondria are dysfunctional in D. melanogaster. Furthermore, we show that the preserved paternal mitochondria are functional, and can be stably inherited, such that the proportion of paternal mitochondria increases gradually in subsequent generations. Our work has important implications that paternal mitochondria inheritance should not be overlooked as a genetic phenomenon in evolution, especially when paternal mitochondria are of significant differences from the maternal mitochondria or the maternal mitochondria are functionally abnormal. Our results improve the understanding of mitochondrial inheritance and provide a new model system for its study.

Assembly and Comparative Analysis of the Complete Mitochondrial Genome of Two Species of Calla Lilies (Zantedeschia, Araceae).

In this study, the mitochondrial genomes of two calla species, Zantedeschia aethiopica Spreng. and Zantedeschia odorata Perry., were assembled and compared for the first time. The Z. aethiopica mt genome was assembled into a single circular chromosome, measuring 675,575 bp in length with a 45.85% GC content. In contrast, the Z. odorata mt genome consisted of bicyclic chromosomes (chromosomes 1 and 2), measuring 719,764 bp and exhibiting a 45.79% GC content. Both mitogenomes harbored similar gene compositions, with 56 and 58 genes identified in Z. aethiopica and Z. odorata, respectively. Analyses of codon usage, sequence repeats, gene migration from chloroplast to mitochondrial, and RNA editing were conducted for both Z. aethiopica and Z. odorata mt genomes. Phylogenetic examination based on the mt genomes of these two species and 30 other taxa provided insights into their evolutionary relationships. Additionally, the core genes in the gynoecium, stamens, and mature pollen grains of the Z. aethiopica mt genome were investigated, which revealed maternal mitochondrial inheritance in this species. In summary, this study offers valuable genomic resources for future research on mitogenome evolution and the molecular breeding of calla lily.

Assessment of the Role of NuclearENDOGGene and mtDNA Variations on Paternal Mitochondrial Elimination (PME) in Infertile Men: An Experimental Study.

In humans and most animals, maternal inheritance of mitochondria and mitochondrial DNA (mtDNA) is considered as an universal assumption. Recently, several lines of evidence suggest that different species seem to employ distinct mechanisms to prevent the inheritance of paternal mtDNA. There are few studies in the literature on the molecular basis of sperm mtDNA elimination in mammals and paternal mtDNA transmission in humans. Endonuclease G (ENDOG) is a mitochondrial nuclease encoded by nuclear ENDOG gene. The critical importance of ENDOG gene on paternal mitochondrial elimination (PME) has been previously demonstrated in model organisms such as C. elegans and D. melanogaster. However, its mechanism in human is still unclear. Therefore, we aimed to evaluate whether nuclear ENDOG gene copy number could be a potential marker of paternal mtDNA transmission or not.Male factor infertility patients diagnosed with different infertility subgroups such as azoospermia, oligoteratozoospermia, astheno-teratozoospermia were included in this study: 13 infertile men and 25 healthy men as control group. Quantitative real-time polymerase chain reaction (qPCR) analysis and dual-color Fluorescence in situ hybridization (FISH) method were used to compare the groups. FISH method was applied to verify qPCR results and two signals were observed in nearly all patients. ENDOG gene copy number data were evaluated by comparing them with entire human mtDNA next-generation sequencing (NGS) analysis results obtained through bioinformatics and proteomics tools. Mitochondrial whole genome sequencing (WGS) data allowed determination of novel and reported variations such as single nucleotide polymorphisms (SNPs), multiple nucleotide polymorphism (MNP), insertion/deletion (INDEL). Missense variants causing amino acid substitution were filtered out from patients' mtDNA WGS data.Relative copy number of target ENDOG gene in male infertility patients [0.49 (0.31 - 0.77)] was lower than healthy controls [1.00 (0.66 - 1.51)], and statistical results showed significant differences between the groups (p < 0.01). A total of 38 missense variants were detected in the genes encoding the proteins involved in the respiratory chain complex. Moreover, we detected paternal mtDNA transmissions in the children of these patients who applied to assisted reproductive techniques.In conclusion, this study reveals that ENDOG gene may be an important factor for the PME mechanism in humans. To the best of our knowledge, this is the first study in humans about this topic and assessment of ENDOG gene sequencing and gene expression studies in a larger sample size including patients with male factor infertility would be our future project.

Sexually antagonistic evolution of mitochondrial and nuclear linkage.

Across eukaryotes, genes encoding bioenergetic machinery are located in both mitochondrial and nuclear DNA, and incompatibilities between the two genomes can be devastating. Mitochondria are often inherited maternally, and theory predicts sex-specific fitness effects of mitochondrial mutational diversity. Yet how evolution acts on linkage patterns between mitochondrial and nuclear genomes is poorly understood. Using novel mito-nuclear population-genetic models, we show that the interplay between nuclear and mitochondrial genes maintains mitochondrial haplotype diversity within populations, and selects both for sex-independent segregation of mitochondrion-interacting genes and for paternal leakage. These effects of genetic linkage evolution can eliminate male-harming fitness effects of mtDNA mutational diversity. With maternal mitochondrial inheritance, females maintain a tight mitochondrial-nuclear match, but males accumulate mismatch mutations because of the weak statistical associations between the two genomic components. Sex-independent segregation of mitochondria-interacting loci improves the mito-nuclear match. In a sexually antagonistic evolutionary process, male nuclear alleles evolve to increase the rate of recombination, whereas females evolve to suppress it. Paternal leakage of mitochondria can evolve as an alternative mechanism to improve the mito-nuclear linkage. Our modelling framework provides an evolutionary explanation for the observed paucity of mitochondrion-interacting genes on mammalian sex chromosomes and for paternal leakage in protists, plants, fungi and some animals.

Cytoplasmic inheritance of mitochondria and chloroplasts in the anisogamous brown alga Mutimo cylindricus (Phaeophyceae).

Based on the morphology of gametes, sexual reproduction in brown algae is usually classified into three types: isogamy, anisogamy, and oogamy. In isogamy, chloroplasts and chloroplast DNA (chlDNA) in the sporophyte cells are inherited biparentally, while mitochondria (or mitochondrial DNA, mtDNA) is inherited maternally. In oogamy, chloroplasts and mitochondria are inherited maternally. However, the patterns of mitochondrial and chloroplast inheritance in anisogamy have not been clarified. Here, we examined derivation of mtDNA and chlDNA in the zygotes through strain-specific PCR analysis using primers based on single nucleotide polymorphism in the anisogamous brown alga Mutimo cylindricus. In 20-day-old sporophytes after fertilization, mtDNA and chlDNA derived from female gametes were detected, thus confirming the maternal inheritance of both organelles. Additionally, the behavior of mitochondria and chloroplasts in the zygotes was analyzed by examining the consecutive serial sections using transmission electron microscopy. Male mitochondria were isolated or compartmentalized by a double-membrane and then completely digested into a multivesicular structure 2h after fertilization. Meanwhile, male chloroplasts with eyespots were observed even in 4-day-old, seven-celled sporophytes. The final fate of male chloroplasts could not be traced. Organelle DNA copy number was also examined in female and male gametes. The DNA copy number per chloroplast and mitochondria in male gametes was lower compared with female organelles. The degree of difference is bigger in mtDNA. Thus, changes in different morphology and DNA amount indicate that maternal inheritance of mitochondria and chloroplasts in this species may be based on different processes and timing after fertilization.

Sex-specific effects of mitochondrial haplotype on metabolic rate in Drosophila melanogaster support predictions of the Mother's Curse hypothesis.

Evolutionary theory proposes that maternal inheritance of mitochondria will facilitate the accumulation of mitochondrial DNA (mtDNA) mutations that are harmful to males but benign or beneficial to females. Furthermore, mtDNA haplotypes sampled from across a given species distribution are expected to differ in the number and identity of these 'male-harming' mutations they accumulate. Consequently, it is predicted that the genetic variation which delineates distinct mtDNA haplotypes of a given species should confer larger phenotypic effects on males than females (reflecting mtDNA mutations that are male-harming, but female-benign), or sexually antagonistic effects (reflecting mutations that are male-harming, but female-benefitting). These predictions have received support from recent work examining mitochondrial haplotypic effects on adult life-history traits in Drosophila melanogaster. Here, we explore whether similar signatures of male-bias or sexual antagonism extend to a key physiological trait-metabolic rate. We measured the effects of mitochondrial haplotypes on the amount of carbon dioxide produced by individual flies, controlling for mass and activity, across 13 strains of D. melanogaster that differed only in their mtDNA haplotype. The effects of mtDNA haplotype on metabolic rate were larger in males than females. Furthermore, we observed a negative intersexual correlation across the haplotypes for metabolic rate. Finally, we uncovered a male-specific negative correlation, across haplotypes, between metabolic rate and longevity. These results are consistent with the hypothesis that maternal mitochondrial inheritance has led to the accumulation of a sex-specific genetic load within the mitochondrial genome, which affects metabolic rate and that may have consequences for the evolution of sex differences in life history. This article is part of the theme issue 'Linking the mitochondrial genotype to phenotype: a complex endeavour'.

From the Literature

From the Literature

Cortical tethering of mitochondria by the anchor protein Mcp5 enables uniparental inheritance.

During sexual reproduction in eukaryotes, processes such as active degradation and dilution of paternal mitochondria ensure maternal mitochondrial inheritance. In the isogamous organism fission yeast, we employed high-resolution fluorescence microscopy to visualize mitochondrial inheritance during meiosis by differentially labeling mitochondria of the two parental cells. Remarkably, mitochondria, and thereby mitochondrial DNA from the parental cells, did not mix upon zygote formation but remained segregated at the poles by attaching to clusters of the anchor protein Mcp5 via its coiled-coil domain. We observed that this tethering of parental mitochondria to the poles results in uniparental inheritance of mitochondria, wherein two of the four spores formed subsequently contained mitochondria from one parent and the other spores contained mitochondria from the other parent. Further, the presence of dynein on an Mcp5 cluster precluded the attachment of mitochondria to the same cluster. Taken together, we reveal a distinct mechanism that achieves uniparental inheritance by segregation of parental mitochondria.

The domestic pig as a model for the study of mitochondrial inheritance.

Maternal mitochondrial inheritance is a fundamental paradigm within reproductive biology, yet the molecular mechanisms which underlie this process remain poorly understood. The ubiquitin proteasome system (UPS) and branches of the autophagic pathway have been implicated in taking part in the active degradation of sperm mitochondria post-fertilization. Despite this knowledge, there remains much unknown about this process, including the cofactors and substrates involved, as well as the implications of what occurs when these systems of degradation fail. Mitochondrial inheritance research has utilized a variety of animal models. However, one model that is of particular importance, especially when attempting to link mitochondrial inheritance research to humans, is the domestic pig. Pigs offer relatively easy collection of gametes which are similar to those of humans. Furthermore, pigs are physiologically and anatomically more similar to humans than the majority of other model systems available. Porcine in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), and novel cell-free systems are research tools which can be exploited to provide greater insight into the processes behind sperm mitochondrial degradation. In the future studies of mitochondrial inheritance, pigs will likely play a crucial role as an animal model system.

Abstract 122: Cardiac Hypertrophy and Vascular Function are Directly Proportional to Mitochondrial Variations in Intrinsic Exercise Capacity

Exercise is a primary lifestyle intervention to control cardiovascular diseases such as hypertension. Exercise capacity and its health benefits are inheritable. However, the mechanisms by which exercise capacity differences influence overall health state remain unclear. We first hypothesized that low and high intrinsic exercise capacity lead to distinct heart weight and vascular function. Animal model (male, 20-24-weeks old, n = 6): heterogeneous stock selectively bred for over 20 years to produce low-capacity running rats (LCR) and high-capacity running rats (HCR), and an intrinsic control, high response trainer rats (HRT, control). Statistics: t-test: *p&lt;0.05. Left ventricular (LV) weight was normalized by tibia length. We observed that HCR presented with LV hypertrophy compared to LCR and control (Control: 0.0155 ± 0.0006 vs. HCR: 0.0192 ± 0.0007* vs. LCR: 0.0164 ± 0.0004). Performing wire myography with 3 rd -order mesenteric resistance arteries (MRA), we observed that LCR MRA had an impaired maximum relaxation response (E max ) to acetylcholine (ACh) compared to HCR and control (Control: 94.99 ± 1.79 vs. HCR: 93.49 ± 12.17 vs. LCR: 72.95 ± 12.40*). Next, we questioned if these differences were the result of mitochondria. We used two reciprocal conplastic strains created by utilizing maternal mitochondrial inheritance (male, 30-36-weeks old, n = 4): LCR nuclear genome with HCR mitochondria (LCR.HCR mt ) and the reciprocal HCR.LCR mt . We observed that LCR.HCR mt presented with LV hypertrophy compared to LCR (0.0195 ± 0.0002* vs 0.0164 ± 0.0004) while HCR.LCR mt abolished HCR LV hypertrophy (0.0151 ± 0.0002* vs. 0.0192 ± 0.0007). Interestingly, in a separate group of animals (n = 4 - 5), LCR had higher mean arterial pressure (MAP) compared to HCR, and LCR.HCR mt had reduced MAP compared to LCR (LCR: 119 ± 2 vs. LCR.HCR mt : 111 ± 1*). No changes were observed in HCR.LCR mt compared to HCR. Further, LCR.HCR mt MRA presented with improved E max to ACh compared to LCR MRA (LCR: 72 ± 12 vs. LCR.HCR mt : 93 ± 2*) while no changes were observed in HCR.LCR mt MRA compared to HCR MRA. Overall, we conclude that intrinsic exercise capacity is directly proportional to cardiovascular effects. Similar to the nuclear genome, mitochondria are the key to this hereditary predisposition.

A maternal high-fat, high-sucrose diet induces transgenerational cardiac mitochondrial dysfunction independently of maternal mitochondrial inheritance.

Maternal obesity is correlated with cardiovascular disease in offspring, with a 1.3-fold increase in events observed in offspring of obese women. We have observed that obesity-exposed oocytes demonstrate impaired mitophagy and transmit damaged mitochondria to the offspring. Accordingly, we hypothesized that maternal obesity induces cardiac mitochondrial dysfunction in the offspring via transgenerational inheritance of abnormal oocyte mitochondria. We mated female mice fed a high-fat/high-sucrose (HFS) diet (or chow) with chow-fed males and assessed cardiac structure and function in their descendants that were chow fed in each generation. All F1 to F3 descendants bred via the female in each generation were nonobese and demonstrated cardiac mitochondrial abnormalities with crystal rarefaction and reduced oxygen consumption pointing to a transgenerational effect, while obese F0 dams' hearts were unaffected. Furthermore, male offspring from F1 to F3 generations and female F1 and F2 offspring developed increased left ventricular (LV) mass (vs. chow-fed controls). Increased LV mass was also observed in offspring generated by in vitro fertilization of obesity-exposed oocytes and gestation in nonobese surrogates, ruling out a gestational environment effect. Contrary to our hypothesis, male F1 also transmitted these effects to their offspring, ruling out maternal mitochondria as the primary mode of transmission. We conclude that transmission of obesity-induced effects in the oocyte nucleus rather than abnormal mitochondria underlie transgenerational inheritance of cardiac mitochondrial defects in descendants of obese females. These findings will spur exploration of epigenetic alterations in the oocyte genome as potential mechanisms whereby a family history of maternal obesity predisposes to cardiovascular disease in humans.

Maternal mitochondrial inheritance in two Fusarium pathogens of prickly ash (Zanthoxylum bungeanum) in northern China

ABSTRACTMitochondrial inheritance in Fusarium zanthoxyli and F. continuum, two canker-inducing pathogens of prickly ash (Zanthoxylum bungeanum) in northern China, was investigated by genotyping ascospore progeny obtained from laboratory crosses. Polymorphic regions of the mitochondrial genomes (mitogenomes) that contained indels and single-nucleotide polymorphisms (SNPs) were identified via comparative analyses of the complete mitogenomes of the parents used in the intraspecific crosses. A reciprocal genetic cross of F. zanthoxyli NRRL 66714 × NRRL 66285, and a separate cross of F. continuum ♀ NRRL 66286 × ♂ NRRL 66218, revealed that mitochondria were only inherited from the maternal parent. In addition, the reciprocal cross demonstrated that mitochondrial inheritance is not linked to mating type. Gene order in the circular mitogenomes of the prickly ash pathogens was identical to that previously reported for other fusaria and members of the Hypocreales, except that the TRNL tRNAs were duplicated in F. zanthoxyli NRRL 66714. The genomes contained 14 polypeptide-encoding genes involved in oxidative respiration, one intron-encoded ribosomal protein (rps3) gene, two ribosomal RNA (rRNA) genes, and 26–28 tRNA genes. The F. zanthoxyli mitogenomes were 80.9 and 98.7 kb in length, whereas those of F. continuum were considerably shorter and nearly identical in length at 63.4 kb. The significant differences in mitogenome length were primarily due to variable numbers of introns and open reading frames (ORFs) encoding hypothetical proteins.

Mitochondrial genetic effects on reproductive success: signatures of positive intrasexual, but negative intersexual pleiotropy.

Theory predicts that maternal inheritance of mitochondria will facilitate the accumulation of mtDNA mutations that are male biased, or even sexually antagonistic, in effect. While there are many reported cases of mtDNA mutations conferring cytoplasmic male sterility in plants, historically it was assumed such mutations would not persist in the streamlined mitochondrial genomes of bilaterian metazoans. Intriguingly, recent cases of mitochondrial variants exerting male biases in effect have come to light in bilaterians. These cases aside, it remains unknown whether the mitochondrial genetic variation affecting phenotypic expression, and in particular reproductive performance, in bilaterians is routinely composed of sex-biased or sex-specific variation. If selection consistently favours mtDNA variants that augment female fitness, but at cost to males, this could shape patterns of pleiotropy and lead to negative intersexual correlations across mtDNA haplotypes. Here, we show that genetic variation across naturally occurring mitochondrial haplotypes affects components of reproductive success in both sexes, in the fruit fly Drosophila melanogaster We find that intrasexual correlations across mitochondrial haplotypes, for components of reproductive success, are generally positive, while intersexual correlations are negative. These results accord with theoretical predictions, suggesting that maternal inheritance has led to the fixation of numerous mutations of sexually antagonistic effect.

Sex-specific influences of mtDNA mitotype and diet on mitochondrial functions and physiological traits in Drosophila melanogaster.

Here we determine the sex-specific influence of mtDNA type (mitotype) and diet on mitochondrial functions and physiology in two Drosophila melanogaster lines. In many species, males and females differ in aspects of their energy production. These sex-specific influences may be caused by differences in evolutionary history and physiological functions. We predicted the influence of mtDNA mutations should be stronger in males than females as a result of the organelle’s maternal mode of inheritance in the majority of metazoans. In contrast, we predicted the influence of diet would be greater in females due to higher metabolic flexibility. We included four diets that differed in their protein: carbohydrate (P:C) ratios as they are the two-major energy-yielding macronutrients in the fly diet. We assayed four mitochondrial function traits (Complex I oxidative phosphorylation, reactive oxygen species production, superoxide dismutase activity, and mtDNA copy number) and four physiological traits (fecundity, longevity, lipid content, and starvation resistance). Traits were assayed at 11 d and 25 d of age. Consistent with predictions we observe that the mitotype influenced males more than females supporting the hypothesis of a sex-specific selective sieve in the mitochondrial genome caused by the maternal inheritance of mitochondria. Also, consistent with predictions, we found that the diet influenced females more than males.

Autophagy and ubiquitin–proteasome system contribute to sperm mitophagy after mammalian fertilization

Maternal inheritance of mitochondria and mtDNA is a universal principle in human and animal development, guided by selective ubiquitin-dependent degradation of the sperm-borne mitochondria after fertilization. However, it is not clear how the 26S proteasome, the ubiquitin-dependent protease that is only capable of degrading one protein molecule at a time, can dispose of a whole sperm mitochondrial sheath. We hypothesized that the canonical ubiquitin-like autophagy receptors [sequestosome 1 (SQSTM1), microtubule-associated protein 1 light chain 3 (LC3), gamma-aminobutyric acid receptor-associated protein (GABARAP)] and the nontraditional mitophagy pathways involving ubiquitin-proteasome system and the ubiquitin-binding protein dislocase, valosin-containing protein (VCP), may act in concert during mammalian sperm mitophagy. We found that the SQSTM1, but not GABARAP or LC3, associated with sperm mitochondria after fertilization in pig and rhesus monkey zygotes. Three sperm mitochondrial proteins copurified with the recombinant, ubiquitin-associated domain of SQSTM1. The accumulation of GABARAP-containing protein aggregates was observed in the vicinity of sperm mitochondrial sheaths in the zygotes and increased in the embryos treated with proteasomal inhibitor MG132, in which intact sperm mitochondrial sheaths were observed. Pharmacological inhibition of VCP significantly delayed the process of sperm mitophagy and completely prevented it when combined with microinjection of autophagy-targeting antibodies specific to SQSTM1 and/or GABARAP. Sperm mitophagy in higher mammals thus relies on a combined action of SQSTM1-dependent autophagy and VCP-mediated dislocation and presentation of ubiquitinated sperm mitochondrial proteins to the 26S proteasome, explaining how the whole sperm mitochondria are degraded inside the fertilized mammalian oocytes by a protein recycling system involved in degradation of single protein molecules.

Stem Cell Aging and Sex: Are We Missing Something?

Longevity differs between sexes, with females being longer-lived in most mammals, including humans. One hallmark of aging is the functional decline of stem cells. Thus, a key question is whether the aging of stem cells differs between males and females and whether this has consequences for disease and lifespan.

Apocrine secretion of the egg hull in oogenesis and exclusion of sperm organelles at fertilization make reproduction in Chitonida (Mollusca) unique

Gametogenesis in Chitonida is highly unusual, giving rise to eggs with elaborate, often spiny hulls and sperm with needle-like nuclear extensions and tiny acrosomes. My studies of oogenesis in a variety of chitonid families have revealed unique mechanisms of egg hull formation. Three-dimensional reconstruction of serial sections of oogenesis in the chitonid Acanthochitona garnoti (Blainville) confirms that the egg hull is formed largely by microapocrine secretions, rather than by exocytosis of Golgi vesicles. This novel process, detailed for the first time in Mollusca, bypasses the endomembrane system and releases compounds formed in the cytoplasm directly from the cell surface via budding of ‘microaposomes’. Fertilization is no less unusual. In virtually all Metazoa, except chitonids, the entire sperm enters the egg, and sperm mitochondria are digested. Typically, the centrosome that derives from the sperm centriole acts as a microtubule organizing centre, moving the two pronuclei together to form the zygote nucleus and later initiating cleavage of the embryo. However, in chitonids the sperm digest a minute pore in the egg hull that allows injection of DNA but blocks entry of sperm organelles, including the nuclear membrane. Serial sections of fertilized eggs of Mopalia muscosa (Gould) confirm that a membrane bag, abandoned on the egg surface, contains the sperm organelles. Thus, not only is there maternal inheritance of mitochondria and centrioles in Chitonida but also other sperm proteins normally associated with fertilization may be blocked from entering the egg.

The quest for Doubly Uniparental Inheritance in heterodont bivalves and its detection inMeretrix lamarckii(Veneridae: Meretricinae)

Doubly Uniparental Inheritance (DUI) is possibly the most striking exception to the well-known maternal inheritance of mitochondria. It poses several stimulating questions concerning among others the function of these organelles, sex determination, embryonic development, and evolutionary consequences. At present, DUI has been found in few species of bivalve molluscs, but more research is necessary to obtain a clearer picture of its distribution within the group, a picture that is mandatory to make any reasonable inference about its origin and evolutionary meaning. The debate about a single evolutionary origin of DUI versus multiple origins is still open. In this manuscript, we investigated seven species of heterodont bivalves and provide evidence for the presence of DUI in the venerid Meretrix lamarckii.