Rapid, selective digestion of mitochondrial DNA in accordance with the matA hierarchy of multiallelic mating types in the mitochondrial inheritance of Physarum polycephalum

Diet and mitonuclear haplotype interactions affect growth rate in a slime mould

Trait expression in metazoans is strongly influenced by the balance of macronutrients (i.e. protein, carbohydrate and fat) in the diet. At the same time, an individual's genetic background seems to regulate the magnitude of phenotypic response to a particular diet. It needs to be better understood whether interactions between diet, genetic background and trait expression are found in unicellular eukaryotes. A protist-the slime mould, Physarum polycephalum can choose diets based on protein-to-carbohydrate (P:C) content to support optimal growth rate. Yet, the role of genetic background (variation in the mitochondrial and nuclear DNAs) in mediating growth rate response to dietary P:C ratios in the slime mould is unknown. Here, we studied the effects of interactions between mitochondrial and nuclear DNA haplotypes and diet (i.e. G × G × E interactions) on the growth rate of P. polycephalum. A genetic panel of six distinct strains of P. polycephalum that differ in their mitochondrial and nuclear DNA haplotypes was used to measure growth rate across five diets that varied in their P:C ratio and total calories. We first determined the strains' growth rate (total biomass and surface area) when grown on a set menu with access to a particular diet. We then assessed whether the growth rate of strains increased on a buffet menu with access to all diets. Our findings show that the growth rate of P. polycephalum is generally higher on diets containing more carbohydrates than protein and that total calories negatively affect the growth rate. Three-way interactions between mitochondrial, nuclear haplotypes and dietary P:C ratios affected the strains' surface area of growth but not biomass. Intriguingly, strains did not increase their surface area and biomass when they had access to all diets on the buffet menu. Our findings have broad implications for our understanding of the effect of mitonuclear interactions on trait expression across diverse eukaryotic lineages.

G × G × E effect on phenotype expression in a non-conventional model organism, the unicellular slime mould Physarum polycephalum

In metazoans, the expression of key phenotypic traits is sensitive to two- and three-way interactions between variation in mitochondrial DNA, nuclear DNA and the external environment. Whether gene-by-environment interactions affect phenotypes in single-celled eukaryotes is poorly studied, except in a few species of yeast and fungi. We developed a genetic panel of the unicellular slime mould, Physarum polycephalum containing strains differing in mitochondrial and nuclear DNA haplotypes. The panel also included two strains harbouring a selfishly replicating mitochondrial-fusion (mF) plasmid that could affect phenotype expression. We assayed movement and growth rate differences among the strains across two temperature regimes: 24° and 28°C. We found that the slime mould's growth rate, but not movement, is affected by G × G × E interactions. Predictably, mtDNA × nDNA interactions significantly affected both traits. The inter-trait correlation across the strains in each temperature regime was positive. Surprisingly, the mF plasmid had no negative effects on our chosen traits. Our study is the first to demonstrate genetic regulation of phenotype expression in a unicellular slime mould. The genetic effect on phenotypes manifests via epistatic interactions with the thermal environment, thus shedding new light on the role of G × G × E interactions in trait evolution in protists.

Semi-in vitro detection of Mg2+-dependent DNase that specifically digest mitochondrial nucleoids in the zygote of Physarum polycephalum

The maternal/uniparental inheritance of mitochondria is controlled by the selective elimination of paternal/uniparental mitochondria and digestion of their mitochondrial DNA (mtDNA). In isogamy, the selective digestion of mtDNA in uniparental mitochondria is initiated after mating and is completed prior to the elimination of mitochondria, but the molecular mechanism of the digestion of uniparental mtDNA remains unknown. In this study, we developed a semi-in vitro assay for DNase, wherein the digestion of mitochondrial nucleoids (mt-nucleoids) was microscopically observed using isolated mitochondria from Physarum polycephalum and the DNase involved in uniparental inheritance was characterized. When myxamoebae of AI35 and DP246 are crossed, mtDNA and mt-nucleoid from only the DP246 parent are digested. The digestion of mt-nucleoids was observed in zygotes 3 h after plating for mating. During the digestion of mt-nucleoids, mitochondrial membrane integrity was maintained. In the semi-in vitro assay, the digestion of mt-nucleoids was only observed in the presence of Mg²⁺ at pH 7.5-9.0. Moreover, such Mg²⁺-dependent DNase activity was specifically detected in mitochondria isolated from zygotes 3 h after plating for mating. Therefore, Mg²⁺-dependent DNase is potentially involved in uniparental inheritance. Our findings provide insights into the DNase involved in uniparental inheritance and its regulatory mechanism.

Mitochondrial genome and diverse inheritance patterns in Pleurotus pulmonarius

Pleurotus pulmonarius, a member of the Pleurotaceae family in Basidiomycota, is an edible, economically important mushroom in most Asian countries. In this study, the complete mitochondrial genomes (mtDNA) of three P. pulmonarius strains - two monokaryotic commercial (J1-13 and ZA3) and one wild (X1-15) - were sequenced and analyzed. In ZA3 and X1-15, the mtDNA molecule was found to be a single circle of 68,305 bp and 73,435 bp, respectively. Both strains contain 14 core protein-coding genes and two ribosomal RNA (rRNA) subunit genes. The ZA3 strain has 22 transfer RNA (tRNA) genes and nine introns: eight in cytochrome c oxidase subunit 1 (coxl), and one in the rRNA large subunit (rnl). Monokaryotic J1-13 and ZA3 mtDNAs were found to be similar in their structure. However, the wild strain X1-15 contains 25 tRNA genes and only seven introns in coxl. Open reading frames (ORFs) of ZA3/J1-13 and X1-15 encode LAGLIDADG, ribosomal protein S3, and DNA polymerase II. In addition, mtDNA inheritance in J1-13, ZA3, and X1-15 was also studied. Results showed that the mtDNA inheritance pattern was uniparental and closely related to dikaryotic hyphal location with respect to the parent. Results also show that mtDNA inheritance is influenced by both the parental nuclear genome and mitogenome in the zone of contact between two compatible parents. In summary, this analysis provides valuable information and a basis for further studies to improve our understanding of the inheritance of fungal mtDNA.

TALE homeobox heterodimer GSM1/GSP1 is a molecular switch that prevents unwarranted genetic recombination in Chlamydomonas

Eukaryotic sexual life cycles alternate between haploid and diploid stages, the transitions between which are delineated by cell fusion and meiotic division. Transcription factors in the TALE-class homeobox family, GSM1 and GSP1, predominantly control gene expression for the haploid-to-diploid transition during sexual reproduction in the unicellular green alga, Chlamydomonas reinhardtii. To understand the roles that GSM1 and GSP1 play in zygote development, we used gsm1 and gsp1 mutants and examined fused gametes that normally undergo the multiple organellar fusions required for the genetic unity of the zygotes. In gsm1 and gsp1 zygotes, no fusion was observed for the nucleus and chloroplast. Surprisingly, mitochondria and endoplasmic reticulum, which undergo dynamic autologous fusion/fission, did not undergo heterologous fusions in gsm1 or gsp1 zygotes. Furthermore, the mutants failed to resorb their flagella, an event that normally renders the zygotes immotile. When gsm1 and gsp1 zygotes resumed the mitotic cycle, their two nuclei fused prior to mitosis, but neither chloroplastic nor mitochondrial fusion took place, suggesting that these fusions are specifically turned on by GSM1/GSP1. Taken together, this study shows that organellar restructuring during zygotic diploidization does not occur by default but is triggered by a combinatorial switch, the GSM1/GSP1 dyad. This switch may represent an ancient mechanism that evolved to restrict genetic recombination during sexual development.

Selfish Mitonuclear Conflict

Mitochondria, a nearly ubiquitous feature of eukaryotes, are derived from an ancient symbiosis. Despite billions of years of cooperative coevolution - in what is arguably the most important mutualism in the history of life - the persistence of mitochondrial genomes also creates conditions for genetic conflict with the nucleus. Because mitochondrial genomes are present in numerous copies per cell, they are subject to both within- and among-organism levels of selection. Accordingly, 'selfish' genotypes that increase their own proliferation can rise to high frequencies even if they decrease organismal fitness. It has been argued that uniparental (often maternal) inheritance of cytoplasmic genomes evolved to curtail such selfish replication by minimizing within-individual variation and, hence, within-individual selection. However, uniparental inheritance creates conditions for cytonuclear conflict over sex determination and sex ratio, as well as conditions for sexual antagonism when mitochondrial variants increase transmission by enhancing maternal fitness but have the side-effect of being harmful to males (i.e., 'mother's curse'). Here, we review recent advances in understanding selfish replication and sexual antagonism in the evolution of mitochondrial genomes and the mechanisms that suppress selfish interactions, drawing parallels and contrasts with other organelles (plastids) and bacterial endosymbionts that arose more recently. Although cytonuclear conflict is widespread across eukaryotes, it can be cryptic due to nuclear suppression, highly variable, and lineage-specific, reflecting the diverse biology of eukaryotes and the varying architectures of their cytoplasmic genomes.

Malaria parasites of long-tailed macaques in Sarawak, Malaysian Borneo: a novel species and demographic and evolutionary histories

Background

Non-human primates have long been identified to harbour different species of Plasmodium. Long-tailed macaques (Macaca fascicularis), in particular, are reservoirs for P. knowlesi, P. inui, P. cynomolgi, P. coatneyi and P. fieldi. A previous study conducted in Sarawak, Malaysian Borneo, however revealed that long-tailed macaques could potentially harbour novel species of Plasmodium based on sequences of small subunit ribosomal RNA and circumsporozoite genes. To further validate this finding, the mitochondrial genome and the apicoplast caseinolytic protease M genes of Plasmodium spp. were sequenced from 43 long-tailed macaque blood samples.

Results

Apart from several named species of malaria parasites, long-tailed macaques were found to be potentially infected with novel species of Plasmodium, namely one we refer to as “P. inui-like.” This group of parasites bifurcated into two monophyletic clades indicating the presence of two distinct sub-populations. Further analyses, which relied on the assumption of strict co-phylogeny between hosts and parasites, estimated a population expansion event of between 150,000 to 250,000 years before present of one of these sub-populations that preceded that of the expansion of P. knowlesi. Furthermore, both sub-populations were found to have diverged from a common ancestor of P. inui approximately 1.5 million years ago. In addition, the phylogenetic analyses also demonstrated that long-tailed macaques are new hosts for P. simiovale.

Conclusions

Malaria infections of long-tailed macaques of Sarawak, Malaysian Borneo are complex and include a novel species of Plasmodium that is phylogenetically distinct from P. inui. These macaques are new natural hosts of P. simiovale, a species previously described only in toque monkeys (Macaca sinica) in Sri Lanka. The results suggest that ecological factors could affect the evolution of malaria parasites.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1170-9) contains supplementary material, which is available to authorized users.

Uniparental Inheritance Promotes Adaptive Evolution in Cytoplasmic Genomes

Eukaryotes carry numerous asexual cytoplasmic genomes (mitochondria and plastids). Lacking recombination, asexual genomes should theoretically suffer from impaired adaptive evolution. Yet, empirical evidence indicates that cytoplasmic genomes experience higher levels of adaptive evolution than predicted by theory. In this study, we use a computational model to show that the unique biology of cytoplasmic genomes-specifically their organization into host cells and their uniparental (maternal) inheritance-enable them to undergo effective adaptive evolution. Uniparental inheritance of cytoplasmic genomes decreases competition between different beneficial substitutions (clonal interference), promoting the accumulation of beneficial substitutions. Uniparental inheritance also facilitates selection against deleterious cytoplasmic substitutions, slowing Muller's ratchet. In addition, uniparental inheritance generally reduces genetic hitchhiking of deleterious substitutions during selective sweeps. Overall, uniparental inheritance promotes adaptive evolution by increasing the level of beneficial substitutions relative to deleterious substitutions. When we assume that cytoplasmic genome inheritance is biparental, decreasing the number of genomes transmitted during gametogenesis (bottleneck) aids adaptive evolution. Nevertheless, adaptive evolution is always more efficient when inheritance is uniparental. Our findings explain empirical observations that cytoplasmic genomes-despite their asexual mode of reproduction-can readily undergo adaptive evolution.

Gamete Dialogs in Green Lineages

Gamete fusion is a core process of sexual reproduction and, in both plants and animals, different sex gametes fuse within species. Although most of the molecular factors involved in gamete interaction are still unknown in various sex-possessing eukaryotes, reports of such factors in algae and land plants have been increasing in the past decade. In particular, knowledge of gamete interaction in flowering plants and green algae has increased since the identification of the conserved gamete fusion factor generative cell specific 1/hapless 2 (GCS1/HAP2). GCS1 was first identified as a pollen generative cell-specific transmembrane protein in the lily (Lilium longiflorum), and was then shown to function not only in flowering plant gamete fusion but also in various eukaryotes, including unicellular protists and metazoans. In addition, although initially restricted to Chlamydomonas, knowledge of gamete attachment in flowering plants was also acquired. This review focuses on recent progress in the study of gamete interaction in volvocine green algae and flowering plants and discusses conserved mechanisms of gamete recognition, attachment, and fusion leading to zygote formation.

Selection against heteroplasmy explains the evolution of uniparental inheritance of mitochondria

Why are mitochondria almost always inherited from one parent during sexual reproduction? Current explanations for this evolutionary mystery include conflict avoidance between the nuclear and mitochondrial genomes, clearing of deleterious mutations, and optimization of mitochondrial-nuclear coadaptation. Mathematical models, however, fail to show that uniparental inheritance can replace biparental inheritance under any existing hypothesis. Recent empirical evidence indicates that mixing two different but normal mitochondrial haplotypes within a cell (heteroplasmy) can cause cell and organism dysfunction. Using a mathematical model, we test if selection against heteroplasmy can lead to the evolution of uniparental inheritance. When we assume selection against heteroplasmy and mutations are neither advantageous nor deleterious (neutral mutations), uniparental inheritance replaces biparental inheritance for all tested parameter values. When heteroplasmy involves mutations that are advantageous or deleterious (non-neutral mutations), uniparental inheritance can still replace biparental inheritance. We show that uniparental inheritance can evolve with or without pre-existing mating types. Finally, we show that selection against heteroplasmy can explain why some organisms deviate from strict uniparental inheritance. Thus, we suggest that selection against heteroplasmy explains the evolution of uniparental inheritance.

Mitochondria contain genes that encode the machinery needed to power cells. Unlike the nuclear genome, the mitochondrial genome is typically inherited from one parent only (uniparental inheritance). The most common explanation for uniparental inheritance is the genomic conflict theory, which states that uniparental inheritance evolved to prevent the spread of ‘selfish’ mitochondria that replicate quickly but produce energy inefficiently. Current explanations have a major problem: when using realistic parameters, mathematical models cannot show that uniparental inheritance can replace biparental inheritance. Clearly, we need a new explanation that fits with standard population-genetic theory. Recent evidence suggests cells may incur a cost when they carry multiple types of mitochondria. Here we show mathematically that uniparental inheritance could have evolved to avoid the costs of maintaining multiple mitochondrial lineages within a cell. Our results explain the long-standing evolutionary mystery of uniparental inheritance and provide insight into the evolution of mating types and binary sexes. Selection against heteroplasmy also has implications for the evolution of the mitochondrial genome because new mitochondrial haplotypes always lead to heteroplasmy before becoming fixed in the population. Thus, selection against heteroplasmy may explain why mtDNA coding-genes have slower substitution rates than analogous genes within the nucleus.

Maternal inheritance of mitochondrial DNA by diverse mechanisms to eliminate paternal mitochondrial DNA

The mitochondrion is an organelle that has its own DNA (mtDNA). Mitochondria play essential roles in energy production and in various cellular processes such as metabolism and signal transduction. In most animals, including humans, although the sperm-derived paternal mitochondria enter the oocyte cytoplasm after fertilization, their mtDNA is never transmitted to the offspring. This pattern of mtDNA inheritance is well known as "maternal inheritance." However, how the paternal mitochondria and mtDNA are eliminated from the cytoplasm of gametes or zygotes remains an enigma. Recently, a variety of mechanisms, including specific nuclease-dependent systems, ubiquitin-proteasome system, and autophagy have been shown to degrade the paternal mtDNA or the paternal mitochondria themselves in order to prevent paternal mtDNA transmission. In this review, we will address the current state of knowledge of the molecular mechanisms underlying the elimination of paternal mtDNA or mitochondrial structures for ensuring the maternal transmission of mtDNA.

Inheritance of mitochondrial DNA in the Pennate diatom Haslea ostrearia (Naviculaceae) during auxosporulation suggests a uniparental transmission

We present the first study examining mtDNA transmission in diatoms, using sexual progeny of the pennate species Haslea ostrearia (Naviculaceae). A fragment of the cytochrome oxidase subunit I gene (cox1) with 7 nucleic substitutions between parental clones was used as a parental tracer in 16 F1 clones obtained from two pairs of mating crosses. Each cross involved a parental clone isolated from France (Bay of Bourgneuf) and Sweden (Kattegat Bay). We determined that all progeny possessed only one cox1 parental haplotype. These results suggest that the mitochondrial DNA transmission in H. ostrearia is uniparental. Implications and new topics of investigation are discussed.

Mitochondrial inheritance in fungi

Faithful inheritance of mitochondria is essential for growth and development. Uniparental inheritance of mitochondria is a common phenomenon in sexual eukaryotes and has been reported for numerous fungal species. Uniparental inheritance is a genetically regulated process, aimed to gain a homoplasmic state within cells, and this is often associated with selective elimination of one parental mitochondria population. This review will focus on recent developments in our understanding of common and specified regulatory circuits of selective mitochondrial inheritance during sexual development. It further refers to the influence of mitochondrial fusion on generation of recombinant mitochondrial DNA molecules. The latter aspect appears rather exciting in the context of intron homing and could bring a new twist to the debate on the significance of uniparental inheritance. The emergence of genome-wide studies offers new perspectives to address potential relationships between uniparental inheritance, vegetative inheritance and last but not least cellular scavenging systems to dispose of disintegrated organelles.

Having sex, yes, but with whom? Inferences from fungi on the evolution of anisogamy and mating types

The advantage of sex has been among the most debated issues in biology. Surprisingly, the question of why sexual reproduction generally requires the combination of distinct gamete classes, such as small and large gametes, or gametes with different mating types, has been much less investigated. Why do systems with alternative gamete classes (i.e. systems with either anisogamy or mating types or both) appear even though they restrict the probability of finding a compatible mating partner? Why does the number of gamete classes vary from zero to thousands, with most often only two classes? We review here the hypotheses proposed to explain the origin, maintenance, number, and loss of gamete classes. We argue that fungi represent highly suitable models to help resolve issues related to the evolution of distinct gamete classes, because the number of mating types vary from zero to thousands across taxa, anisogamy is present or not, and because there are frequent transitions between these conditions. We review the nature and number of gamete classes in fungi, and we attempt to draw inferences from these data on the evolutionary forces responsible for their appearance, loss or maintenance, and number.

Uniparental inheritance of cpDNA and the genetic control of sexual differentiation in Chlamydomonas reinhardtii

An intriguing feature of most eukaryotes is that chloroplast (cp) and mitochondrial (mt) genomes are inherited almost exclusively from one parent. Uniparental inheritance of cp/mt genomes was long thought to be a passive outcome, based on the fact that eggs contain multiple numbers of organelles, while male gametes contribute,at best, only a few cp/mtDNA. However, the process is likely to be more dynamic because uniparental inheritance occurs in organisms that produce gametes of identical sizes (isogamous). In Chlamydomonas reinhardtii,the uniparental inheritance of cp/mt genomes is achieved by a series of mating type-controlled events that actively eliminate the mating type minus (mt-) cpDNA.The method by which Chlamydomonas selectively degrades mt- cpDNA has long fascinated researchers, and is the subject of this review.

Mitochondrial fusion and inheritance of the mitochondrial genome

Although maternal or uniparental inheritance of mitochondrial genomes is a general rule, biparental inheritance is sometimes observed in protists and fungi,including yeasts. In yeast, recombination occurs between the mitochondrial genomes inherited from both parents.Mitochondrial fusion observed in yeast zygotes is thought to set up a space for DNA recombination. In the last decade,a universal mitochondrial fusion mechanism has been uncovered, using yeast as a model. On the other hand, an alternative mitochondrial fusion mechanism has been identified in the true slime mold Physarum polycephalum.A specific mitochondrial plasmid, mF, has been detected as the genetic material that causes mitochondrial fusion in P. polycephalum. Without mF, fusion of the mitochondria is not observed throughout the life cycle, suggesting that Physarum has no constitutive mitochondrial fusion mechanism.Conversely, mitochondria fuse in zygotes and during sporulation with mF. The complete mF sequence suggests that one gene, ORF640, encodes a fusogen for Physarum mitochondria. Although in general, mitochondria are inherited uniparentally, biparental inheritance occurs with specific sexual crossing in P. polycephalum.An analysis of the transmission of mitochondrial genomes has shown that recombinations between two parental mitochondrial genomes require mitochondrial fusion,mediated by mF. Physarum is a unique organism for studying mitochondrial fusion.

Review of cytological studies on cellular and molecular mechanisms of uniparental (maternal or paternal) inheritance of plastid and mitochondrial genomes induced by active digestion of organelle nuclei (nucleoids)

In most sexual organisms, including isogamous, anisogamous and oogamous organisms, uniparental transmission is a striking and universal characteristic of the transmission of organelle (plastid and mitochondrial) genomes (DNA). Using genetic, biochemical and molecular biological techniques, mechanisms of uniparental (maternal and parental) and biparental transmission of organelle genomes have been studied and reviewed. Although to date there has been no cytological review of the transmission of organelle genomes, cytology offers advantages in terms of direct evidence and can enhance global studies of the transmission of organelle genomes. In this review, I focus on the cytological mechanism of uniparental inheritance by "active digestion of male or female organelle nuclei (nucleoids, DNA)" which is universal among isogamous, anisogamous, and oogamous organisms. The global existence of uniparental transmission since the evolution of sexual eukaryotes may imply that the cell nuclear genome continues to inhibit quantitative evolution of organelles by organelle recombination.

Maternal inheritance of mitochondria: multipolarity, multiallelism and hierarchical transmission of mitochondrial DNA in the true slime mold Physarum polycephalum

Direct evidence of digestion of paternal mitochondrial DNA (mtDNA) has been found in the true slime mold Physarum polycephalum. This is the first report on the selective digestion of mtDNA inside the zygote, and is striking evidence for the mechanism of maternal inheritance of mitochondria. Moreover, two mitochondrial nuclease activities were detected in this organism as-candidates for the nucleases responsible for selective digestion of mtDNA. In the true slime mold, there is an additional-feature of the uniparental inheritance of mitochondria.Although mitochondria are believed to be inherited from the maternal lineage in nearly all eukaryotes, the mating types of the true slime mold P. polycephalum is not restricted to two: there are three mating loci--matA, matB,and matC--and these loci have 16, 15, and 3 alleles,-respectively. Interestingly, the transmission patterns of mtDNA are determined by the matA locus, in a hierarchical-fashion (matA hierarchy) as follows: matA7[matA2[matA11[matA12[matA15/matA16[matA1[matA6.The strain possessing the higher status of matA would be the mtDNA donor in crosses. Furthermore, we have found that some crosses showed biparental inheritance of mitochondria.This review describes the phenomenon of hierarchical transmission of mtDNA in true slime molds, and discusses the presumed molecular mechanism of maternal and biparental inheritance.

Cytoplasmic inheritance in green algae: patterns, mechanisms and relation to sex type

Cytological and genetic investigations of two major groups of green algae, chlorophyte and streptophyte green algae, show a predominance of uniparental inheritance of the plastid and mitochondrial genomes in most species. However, in some crosses of isogamous species of Ulva compressa, these genomes are transmitted from mt+, mt(-), and both parents. In species with uniparental organelle inheritance, various mechanisms can eliminate organelles and their DNA during male gametogenesis or after fertilization. Concerning plastid inheritance, two major mechanisms are widespread in green algae: (1) digestion of plastid DNA during male gametogenesis, during fertilization, or after fertilization; and (2) disintegration or fusion of the plastid in the zygote. The first mechanism also eliminates the mitochondrial DNA in anisogamous and oogamous species. These mechanisms would ensure the predominantly uniparental inheritance of organelle genomes in green algae. To trace the evolutionary history of cytoplasmic inheritance in green algae, the relations between uniparental inheritance and sex type were considered in isogamous, anisogamous, and oogamous species using sex-specific features that might be nearly universal among Chlorophyta.

Mechanisms of organelle division and inheritance and their implications regarding the origin of eukaryotic cells

Mitochondria and plastids have their own DNAs and are regarded as descendants of endosymbiotic prokaryotes. Organellar DNAs are not naked in vivo but are associated with basic proteins to form DNA-protein complexes (called organelle nuclei). The concept of organelle nuclei provides a new approach to explain the origin, division, and inheritance of organelles. Organelles divide using organelle division rings (machineries) after organelle-nuclear division. Organelle division machineries are a chimera of the FtsZ (filamentous temperature sensitive Z) ring of bacterial origin and the eukaryotic mechanochemical dynamin ring. Thus, organelle division machineries contain a key to solve the origin of organelles (eukaryotes). The maternal inheritance of organelles developed during sexual reproduction and it is also probably intimately related to the origin of organelles. The aims of this review are to describe the strategies used to reveal the dynamics of organelle division machineries, and the significance of the division machineries and maternal inheritance in the origin and evolution of eukaryotes.

The a2 mating-type locus genes lga2 and rga2 direct uniparental mitochondrial DNA (mtDNA) inheritance and constrain mtDNA recombination during sexual development of Ustilago maydis

Uniparental inheritance of mitochondria dominates among sexual eukaryotes. However, little is known about the mechanisms and genetic determinants. We have investigated the role of the plant pathogen Ustilago maydis genes lga2 and rga2 in uniparental mitochondrial DNA (mtDNA) inheritance during sexual development. The lga2 and rga2 genes are specific to the a2 mating-type locus and encode small mitochondrial proteins. On the basis of identified sequence polymorphisms due to variable intron numbers in mitochondrial genotypes, we could demonstrate that lga2 and rga2 decisively influence mtDNA inheritance in matings between a1 and a2 strains. Deletion of lga2 favored biparental inheritance and generation of recombinant mtDNA molecules in combinations in which inheritance of mtDNA of the a2 partner dominated. Conversely, deletion of rga2 resulted in predominant loss of a2-specific mtDNA and favored inheritance of the a1 mtDNA. Furthermore, expression of rga2 in the a1 partner protected the associated mtDNA from elimination. Our results indicate that Lga2 in conjunction with Rga2 directs uniparental mtDNA inheritance by mediating loss of the a1-associated mtDNA. This study shows for the first time an interplay of mitochondrial proteins in regulating uniparental mtDNA inheritance.

The fate of paternal mitochondria in marmoset pre-implantation embryos

BACKGROUND

Sperm-derived mitochondria are integrated into the oocyte at fertilization but seem to vanish during the early cleavage phase. The developmental potential of pre-implantation embryos seems to be closely related to their ability to induce degeneration of these mitochondria, but the mechanisms underlying their loss of function are not yet understood. This study focuses on the fate of paternal mitochondria in pre-implantation embryos.

METHODS

Stimulation, collection and in vitro culture of oocytes from Callithrix jacchus, allows the study of the destiny of paternal mitochondria by utilizing immunostaining of pre-implantation embryos, fluorescence and laserscanning microscopy. Live pre-implantation embryos were stained with a fluorescence indicator reflecting mitochondrial membrane potential.

RESULTS

Evidence indicating the loss of mitochondrial function was not found nor that apoptosis pathways were involved in the disappearance of paternally derived mitochondria.

CONCLUSIONS

These findings may have implications for mitochondrially inherited diseases and could lead to new strategies for improving assisted reproduction.

The mating type-specific homeodomain genes SXI1 alpha and SXI2a coordinately control uniparental mitochondrial inheritance in Cryptococcus neoformans

In the great majority of sexual eukaryotes, mitochondrial genomes are inherited almost exclusively from a single parent. While many hypotheses have been proposed to explain this phenomenon, very little is known about the genetic elements controlling uniparental mitochondria inheritance. In the bipolar, isogamous basidiomycete yeast Cryptococcus neoformans, progeny from crosses between strains of mating type a (MATa) and mating type alpha (MATalpha) typically inherit mitochondrial DNA (mtDNA) from the MATa parent. We recently demonstrated that a mating type alpha (MATalpha)-specific gene SXI1a, controls mitochondrial inheritance in C. neoformans. Here, we show that another homeodomain gene SXI2a in the alternative mating type MATa is also required for uniparental mtDNA inheritance in this fungus. Disruption of SXI2a resulted in biparental mtDNA inheritance in the zygote population with significant numbers of progeny inheriting mtDNA from the MATa parent, the MATalpha parent, and both the MATa and the MATalpha parents. In addition, progeny from same-sex mating between MATalpha strains showed a biparental mitochondrial inheritance pattern. Our results suggest that SXI1alpha and SXI2a coordinately control uniparental mitochondrial inheritance in C. neoformans.

The inheritance of organelle genes and genomes: patterns and mechanisms

Unlike nuclear genes and genomes, the inheritance of organelle genes and genomes does not follow Mendel's laws. In this mini-review, I summarize recent research progress on the patterns and mechanisms of the inheritance of organelle genes and genomes. While most sexual eukaryotes show uniparental inheritance of organelle genes and genomes in some progeny at least part of the time, increasing evidence indicates that strictly uniparental inheritance is rare and that organelle inheritance patterns are very diverse and complex. In contrast with the predominance of uniparental inheritance in multicellular organisms, organelle genes in eukaryotic microorganisms, such as protists, algae, and fungi, typically show a greater diversity of inheritance patterns, with sex-determining loci playing significant roles. The diverse patterns of inheritance are matched by the rich variety of potential mechanisms. Indeed, many factors, both deterministic and stochastic, can influence observed patterns of organelle inheritance. Interestingly, in multicellular organisms, progeny from interspecific crosses seem to exhibit more frequent paternal leakage and biparental organelle genome inheritance than those from intraspecific crosses. The recent observation of a sex-determining gene in the basidiomycete yeast Cryptococcus neoformans, which controls mitochondrial DNA inheritance, has opened up potentially exciting research opportunities for identifying specific molecular genetic pathways that control organelle inheritance, as well as for testing evolutionary hypotheses regarding the prevalence of uniparental inheritance of organelle genes and genomes.

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.

GENERATIVE CELL SPECIFIC 1 is essential for angiosperm fertilization

The double fertilization process in angiosperms is based on the delivery of a pair of sperm cells by the pollen tube (the male gametophyte), which elongates towards an embryo sac (the female gametophyte) enclosing an egg and a central cell. Several studies have described the mechanisms of gametophyte interaction, and also the fertilization process - from pollination to pollen tube acceptance. However, the mechanisms of gamete interaction are not fully understood. Cytological studies have shown that male gametes possess distinct cell-surface structures and genes specific to male gametes have been detected in cDNA libraries. Thus, studies of isolated gametes may offer clues to understanding the sperm-egg interaction. In this study, we identified a novel protein, designated GCS1 (GENERATIVE CELL SPECIFIC 1), using generative cells isolated from Lilium longiflorum pollen. GCS1 possesses a carboxy-terminal transmembrane domain, and homologues are present in various species, including non-angiosperms. Immunological assays indicate that GCS1 is accumulated during late gametogenesis and is localized on the plasma membrane of generative cells. In addition, Arabidopsis thaliana GCS1 mutant gametes fail to fuse, resulting in male sterility and suggesting that GCS1 is a critical fertilization factor in angiosperms.

The organization and inheritance of the mitochondrial genome

Mitochondrial DNA (mtDNA) encodes essential components of the cellular energy-producing apparatus, and lesions in mtDNA and mitochondrial dysfunction contribute to numerous human diseases. Understanding mtDNA organization and inheritance is therefore an important goal. Recent studies have revealed that mitochondria use diverse metabolic enzymes to organize and protect mtDNA, drive the segregation of the organellar genome, and couple the inheritance of mtDNA with cellular metabolism. In addition, components of a membrane-associated mtDNA segregation apparatus that might link mtDNA transmission to mitochondrial movements are beginning to be identified. These findings provide new insights into the mechanisms of mtDNA maintenance and inheritance.

Early zygote-specific nuclease in mitochondria of the true slime mold Physarum polycephalum

The active, selective digestion of mtDNA from one parent is a possible molecular mechanism for the uniparental inheritance of mtDNA. In Physarum polycephalum, mtDNA is packed by DNA-binding protein Glom, which packs mtDNA into rod-shaped mt-nucleoids. After the mating, mtDNA from one parent is selectively digested, and the Glom began to disperse. Dispersed Glom was retained for at least 6 h after mtDNA digestion, but disappeared completely by about 12 h after mixing two strains. We identified two novel nucleases using DNA zymography with native-PAGE and SDS-PAGE. One is a Ca2+-dependent, high-molecular-weight nuclease complex (about 670 kDa), and the other is a Mn2+-dependent, high-molecular-weight nuclease complex (440-670 kDa); the activity of the latter was detected as a Mn2+-dependent, 13-kDa DNase band on SDS-PAGE. All mitochondria isolated from myxamoebae had mt-nucleoids, whereas half of the mitochondria isolated from the zygotes at 12 h after mixing had lost the mt-nucleoids. The activity of the Mn2+-dependent nuclease in the isolated mitochondria was detected at least 8 h after mixing of two strains. The timing and localization of the Mn2+-dependent DNase activity matched the selective digestion of mtDNA.

Rearrangements in the Physarum polycephalum mitochondrial genome associated with a transition from linear mF-mtDNA recombinants to circular molecules

Although mitochondrial DNA (mtDNA) is transmitted to progeny from one parent only in Physarum polycephalum, the mtDNAs of progeny of mF+ plasmodia vary in structure. To clarify the mechanisms associated with the mitochondrial plasmid mF that generate mtDNA polymorphisms, 91 progeny of four strains (KM88 x JE8, KM88 x TU111, KM88 x NG111, Je90) were investigated using RFLP analysis, PCR, and pulse-field gel electrophoresis (PFGE). Nine mtDNA rearrangement types were found, with rearrangements occurring exclusively in the mF regions. PFGE revealed that, in the groups containing rearranged mtDNA, the linear mF-mtDNA recombinants had recircularized. Sequencing the rearranged region of one of the progeny suggested that the mF plasmid and the mtDNA recombine primarily at the ID sequences, linearizing the circular mtDNA. Recombination between the terminal region of the mF plasmid and a region about 1 kbp upstream of the mitochondrial/plasmid ID sequence results in a rearranged circular mtDNA, with variations caused by differences in the secondary recombination region.

Mitochondria, Sex, and Mortality

It has been proposed that prior to the evolution of sex, the endosymbiotic relationship between mitochondria and nuclear genomes would have selected mechanisms that maintained the optimum interaction between the two genomes. Once sex evolved, mating would introduce different, competitive, mtDNA and/or nDNA gene products that could well upset the balance. Mechanisms, such as the specific degradation of one mitochondrial genome that is known to occur, could have been selected to prevent part of such competition. Unlike most protein complexes in the cell, the proteins of the multienzyme complexes of the ox-phos system are derived from both nuclear-genome-coded genes and mitochondrial-genome-coded genes. Minor mutations in either mtDNA or nDNA coding for these proteins are known to lead to major and catastrophic diseases of humans, suggesting that very tight and precise interactions are required. To maintain the evolutionarily established balance after mating, monoallelic expression of the nuclear-coded genes would be advantageous and prevent subtly different competitive proteins from interacting with the resident mitochondria. This would require regulation of the expression of those specific nuclear genes, possibly under the control of the resident mitochondria. It is possible that aging cells could lose the requisite tight regulation and allow expression of proteins derived from the formerly repressed nuclear alleles that would compete for mitochondrial complex sites. With age, random failure of this control could lead to increasingly inefficient mitochondria in different tissues and organs and eventually to senescence and death.

Degradation of paternal mitochondria after fertilization: implications for heteroplasmy, assisted reproductive technologies and mtDNA inheritance

Maternal inheritance of mitochondrial DNA has long been regarded as a major paradox in developmental biology. While some confusion may still persist in popular science, research data clearly document that the paternal sperm-borne mitochondria of most mammalian species enter the ooplasm at fertilization and are specifically targeted for degradation by the resident ubiquitin system. Ubiquitin is a proteolytic chaperone that forms covalently linked polyubiquitin chains on the targeted proteinaceous substrates. The polyubiquitin tag redirects the substrate proteins to a 26-S proteasome, a multi-subunit proteolytic organelle. Thus, specific proteasomal inhibitors reversibly block sperm mitochondrial degradation in ooplasm. Lysosomal degradation and the activity of membrane-lipoperoxidating enzyme 15-lipoxygenase (15-LOX) may also contribute to sperm mitochondrial degradation in the ooplasm, but probably is not crucial. Prohibitin, the major protein of the inner mitochondrial membrane, appears to be ubiquitinated in the sperm mitochondria. Occasional occurrence of paternal inheritance of mtDNA has been suggested in mammals including humans. While most such evidence has been widely disputed, it warrants further examination. Of particular concern is the documented heteroplasmy, i.e. mixed mtDNA inheritance after ooplasmic transplantation. Intracytoplasmic sperm injection (ICSI) has inherent potential for delaying the degradation of sperm mitochondria. However, paternal mtDNA inheritance after ICSI has not been documented so far.