Rapid disappearance of one parental mitochondrial genotype after isogamous mating in the myxomycete Physarum polycephalum

The complete mitochondrial genome of a new species of the genus Schizothorax from Sichuan, China (Cypriniformes: Cyprinidae)

Schizothorax gulinensis sp. nov. is a new species of the genus Schizothorax from Sichuan, China (Cypriniformes: Cyprinidae). In this study, we have first reported the complete mitochondrial genome of S. gulinensis with Illumina sequencing. There were 16,587 nucleotide pairs in the mitochondrial genome (mitogenome) of S. gulinensis, including 13 protein-coding genes (PCGs), two ribosomal RNAs (rRNAs), and 22 transfer RNAs (tRNAs), as well as one non-coding control region (CR). The proportion of nucleotides in mitochondrial genome was 29.67% (A), 25.45% (T), 17.84% (G), 27.05% (C), and A + T content was 55.12%. All PCGs have the same start codon of the standard ATG, excepting for that of NADH dehydrogenase subunit 1 (nad1) which was the ATC, NADH dehydrogenase subunit 5 (nad5) which was the ATT and cytochrome c oxidase 1 (cox1) which was the ATC. Phylogenetic analysis results supported that S. gulinensis was closely related to Schizothorax grahami. The complete mitochondrial sequence of S. gulinensis will contribute to mitochondrial genome database and provide useful resources for population genetics and evolution analyses.

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.

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.

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.

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.

EVOLUTION OF HIERARCHICAL CYTOPLASMIC INHERITANCE IN THE PLASMODIAL SLIME MOLD PHYSARUM POLYCEPHALUM

A striking linear dominance relationship for uniparental mitochondrial transmission is known between many mating types of plasmodial slime mold Physarum polycephalum. We herein examine how such hierarchical cytoplasmic inheritance evolves in isogamous organisms with many self-incompatible mating types. We assume that a nuclear locus determines the mating type of gametes and that another nuclear locus controls the digestion of mitochondria DNAs (mtDNAs) of the recipient gamete after fusion. We then examine the coupled genetic dynamics for the evolution of self-incompatible mating types and biased mitochondrial transmission between them. In Physarum, a multiallelic nuclear locus matA controls both the mating type of the gametes and the selective elimination of the mtDNA in the zygotes. We theoretically examine two potential mechanisms that might be responsible for the preferential digestion of mitochondria in the zygote. In the first model, the preferential digestion of mitochondria is assumed to be the outcome of differential expression levels of a suppressor gene carried by each gamete (suppression-power model). In the second model (site-specific nuclease model), the digestion of mtDNAs is assumed to be due to their cleavage by a site-specific nuclease that cuts the mtDNA at unmethylated recognition sites. Also assumed is that the mtDNAs are methylated at the same recognition site prior to the fusion, thereby being protected against the nuclease of the same gamete, and that the suppressor alleles convey information for the recognition sequences of nuclease and methylase. In both models, we found that a linear dominance hierarchy evolves as a consequence of the buildup of a strong linkage disequilibrium between the mating-type locus and the suppressor locus, though it fails to evolve if the recombination rate between the two loci is larger than a threshold. This threshold recombination rate depends on the number of mating types and the degree of fitness reduction in the heteroplasmic zygotes. If the recombination rate is above the threshold, suppressor alleles are equally distributed in each mating type at evolutionary equilibrium. Based on the theoretical results of the site-specific nuclease model, we propose that a nested subsequence structure in the recognition sequence should underlie the linear dominance hierarchy of mitochondrial transmission.

A linear dominance hierarchy among clones in chimeras of the social amoeba Dictyostelium discoideum

Amoebae from different clones of Dictyostelium discoideum aggregate into a common slug, which migrates towards light for dispersal, then forms a fruiting body consisting of a somatic, dead stalk, holding up a head of living spores. Contributions of two clones in a chimera to spore and stalk are often unequal, with one clone taking advantage of the other's stalk contribution. To determine whether there was a hierarchy of exploitation among clones, we competed all possible pairs among seven clones and measured their relative representation in the prespore and prestalk stages and in the final spore stage. We found a clear linear hierarchy at the final spore stage, but not at earlier stages. These results suggest that there is either a single principal mechanism or additive effects for differential contribution to the spore, and that it involves more than spore/stalk competition.

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

Although mitochondria are inherited uniparentally in nearly all eukaryotes, the mechanism for this is unclear. When zygotes of the isogamous protist Physarum polycephalum were stained with DAPI, the fluorescence of mtDNA in half of the mitochondria decreased simultaneously to give small spots and then disappeared completely approximately 1.5 hr after nuclear fusion, while the other mitochondrial nucleoids and all of the mitochondrial sheaths remained unchanged. PCR analysis of single zygote cells confirmed that the loss was limited to mtDNA from one parent. The vacant mitochondrial sheaths were gradually eliminated by 60 hr after mating. Using six mating types, the transmission patterns of mtDNA were examined in all possible crosses. In 39 of 60 crosses, strict uniparental inheritance was confirmed in accordance with a hierarchy of relative sexuality. In the other crosses, however, mtDNA from both parents was transmitted to plasmodia. The ratio of parental mtDNA was estimated to be from 1:1 to 1:10(-4). Nevertheless, the matA hierarchy was followed. In these crosses, the mtDNA was incompletely digested, and mtDNA replicated during subsequent plasmodial development. We conclude that the rapid, selective digestion of mtDNA promotes the uniparental inheritance of mitochondria; when this fails, biparental inheritance occurs.

Cellular and molecular mechanisms of sexual incompatibility in plants and fungi

Plants and fungi show an astonishing diversity of mechanisms to promote outbreeding, the most widespread of which is sexual incompatibility. Sexual incompatibility involves molecular recognition between mating partners. In fungi and algae, highly polymorphic mating-type loci mediate mating through complementary interactions between molecules encoded or regulated by different mating-type haplotypes, whereas in flowering plants polymorphic self-incompatibility loci regulate mate recognition through oppositional interactions between molecules encoded by the same self-incompatibility haplotypes. This subtle mechanistic difference is a consequence of the different life cycles of fungi, algae, and flowering plants. Recent molecular and biochemical studies have provided fascinating insights into the mechanisms of mate recognition and are beginning to shed light on evolution and population genetics of these extraordinarily polymorphic genetic systems of incompatibility.

Mitochondrial inheritance in Aspergillus nidulans

Mitochondrial chloramphenicol and oligomycin resistance mutations were used to investigate mitochondrial inheritance in A. nidulans. Mitochondrial RFLPs could not be used to distinguish between paternal and maternal mitochondria because none were detected in the 54 isolates investigated. Several thousand ascospores from each of 111 hybrid cleistothecia from 21 different crosses between 7 heterokaryon incompatible isolates were tested for biparental inheritance. All mitochondrial inheritance was strictly uniparental. Not one instance of paternal inheritance of mitochondria was observed. The implications of our results for the theory that uniparental inheritance evolved to avoid cytoplasmic conflict are discussed. Possible explanations for the maintenance of strict uniparental inheritance of mitochondria in an inbreeding homothallic organism are suggested. The chloramphenicol resistance marker was inherited preferentially to the oligomycin resistance marker probably due to the inhibited energy production of mitochondria with the oligomycin resistance mutation. The maternal parent was determined for 93 hybrid cleistothecia from 17 crosses between 7 different strains. Contrary to previous reports A. nidulans strains functioned as both maternal and paternal parent in most crosses.

Rearrangements of mitochondrial DNA and the mitochondrial fusion-promoting plasmid (mF) are associated with defective mitochondrial fusion in Physarum polycephalum

A specific linear mitochondrial plasmid (mF) is genetically associated with the fusion of mitochondria in the true slime mould, Physarum polycephalum. In matings between mF+ and mF- strains, which respectively carry and do not carry the mF plasmid, mitochondrial fusion occurs in the zygote. Mitochondrial fusion induces recombination between specific sites in the mitochondrial DNA (mtDNA) and in the mF plasmid. To detect a region which is associated with the mitochondrial fusion in the mF plasmid, we isolated, by fluorescence microscopy, strains which showed defective mitochondrial fusion (delta mif-) from those which showed normal mitochondrial fusion (mif+). Analysis of the mitochondrial genomes of delta mif- strains showed only mtDNA which recombined with the mF plasmid in mitochondria. Comparison of this recombinant mtDNA in one delta mif- strain (NG 15) with that of a mif+ strain showed that a 2.2-kbp region, which included the integration site of the mF plasmid, was deleted in the delta mif- strain by recombination between the main mtDNA and the mF plasmid. In other strains, in addition to this deletion, a 6-kbp region which included both termini was deleted by recombination at six repeats of AAT sequences in the mF plasmid. Moreover, transcripts of the mF plasmid were not detected in NG15 by slot hybridization.

Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution

In nearly all eukaryotes, at least some individuals inherit mitochondrial and chloroplast genes from only one parent. There is no single mechanism of uniparental inheritance: organelle gene inheritance is blocked by a variety of mechanisms and at different stages of reproduction in different species. Frequent changes in the pattern of organelle gene inheritance during evolution suggest that it is subject to varying selective pressures. Organelle genes often fail to recombine even when inherited biparentally; consequently, their inheritance is asexual. Sexual reproduction is apparently less important for genes in organelles than for nuclear genes, probably because there are fewer of them. As a result organelle sex can be lost because of selection for special reproductive features such as oogamy or because uniparental inheritance reduces the spread of cytoplasmic parasites and selfish organelle DNA.

A mating type-linked mutation that disrupts the uniparental inheritance of chloroplast DNA also disrupts cell-size control in Chlamydomonas

An intriguing feature of early zygote development in Chlamydomonas reinhardtii is the active elimination of chloroplast DNA from the mating-type minus parent due presumably to the action of a zygote-specific nuclease. Meiotic progeny thus inherit chloroplast DNA almost exclusively from the mating-type plus parent. The plus-linked nuclear mutation mat3 prevents this selective destruction of minus chloroplast DNA and generates progeny that display a biparental inheritance pattern. Here we show that the mat3 mutation creates additional phenotypes not previously described: the cells are much smaller than wild type and they possess substantially reduced amounts of both mitochondrial and chloroplast DNA. We propose that the primary defect of the mat3 mutation is a disruption of cell-size control and that the inhibition of the uniparental transmission of chloroplast genomes is a secondary consequence of the reduced amount of chloroplast DNA in the mat3 parent.

Selfish genetic elements and their role in evolution: the evolution of sex and some of what that entails

An individual is often considered (sometimes implicitly) to be the product of a well functioning mutualism between its constituent genes. This however need not be so. One consequence of sexual reproduction is that costly competition within an individual between genes that are effectively allelic can provide the conditions for the spread of suppressors of such competition. The spread of both these ultracompetitive alleles (alias selfish genetic elements) and their suppressors is evidence of a 'conflict of interests' within the genome. That this conflict is a potentially important force in the evolution of genetic systems is illustrated by consideration of the problem of the evolution of sexes (alias mating types). One hypothesis holds that sexes are the result of selection on nuclear genes to coordinate the inheritance of cytoplasmic genomes (usually this means the enforcement of uniparental inheritance) so as to prevent competition between unrelated cytoplasmic genomes. This hypothesis is tested against five comparative predictions and shown to receive considerable empirical support.

Sexuality of mitochondria: fusion, recombination, and plasmids

Mitochondrial fusion, recombination, and mobile genetic elements, which are essential for mitochondrial sexuality, are well established in various organisms. The recombination of mitochondrial DNA (mtDNA) depends upon fusion between parental mitochondria, and between their mtDNA-containing areas (mt-nuclei), to allow pairing between the parental mtDNAs. Such mitochondrial fusion followed by recombination may be called "mitochondrial sex." We have identified a novel mitochondrial plasmid named mF. This plasmid is apparently responsible for promoting mitochondrial fusion and crosses over with mtDNA in successive sexual crosses with mF- strains. Only in mF+ strains carrying the mF plasmid did small spherical mitochondria fuse which subsequently underwent fusion between the mt-nuclei that contained the mtDNA derived from individual mitochondria. Several successive mitochondrial divisions followed, accompanied by mt-nuclear divisions. The resulting mitochondria contained recombinant mtDNA with the mF plasmid. Such features remind us also of the bacterial conjugative plasmids such as F plasmid. Therefore, in the final part of this chapter, we discuss the origin of sex and its relationship to the sexuality of mitochondria.

Uniparental Mitochondrial Transmission in the Cultivated Button Mushroom, Agaricus bisporus

A uniparental mitochondrial (mt) transmission pattern has been previously observed in laboratory matings of the cultivated mushroom Agaricus bisporus on petri dishes. In this study, four sets of specific matings were further examined by taking mycelial plugs from the confluent zone of mated homokaryons and inoculating these plugs into rye grain for laboratory fruiting and for fruiting under industrial conditions. Examination of the mt genotype of each individual fruit body for mt-specific restriction fragment length polymorphisms further confirmed that the mt genome was inherited uniparentally. The vegetative radial growth and the fruiting activity of two pairs of intraspecific heterokaryons, each pair carrying the same combination of nuclear genomes but different mt genotypes, were compared. Our results suggested that the mt genotype did not appreciably affect radial growth or fruiting activity. The failure to recover both heterokaryons, each carrying either parental mt genotype in any given cross, therefore clearly indicated that in matings of A. bisporus , the mt genome from one of the parental homokaryons is either selectively excluded in the newly formed heterokaryon or selectively eliminated in the immediate heterokaryotic mitotic progeny of the newly formed heterokaryon.

Molecular and cellular mechanisms of mitochondrial nuclear division and mitochondriokinesis

Our present understanding of mitochondrial division can be summarized as follows: Mitochondria contain a specific genome, synthesize their own DNA, and multiply semi-autonomously. Strands of mitochondrial DNA (mt-DNA) in the in vivo organelles of all eukaryotes are organized to form mitochondrial nuclei (nucleoids) (mt-nuclei) with specific proteins including a histone-like protein and transcription factors at the central region of the mitochondrion. We can easily observe the mt-nucleus in vivo mitochondria in various organisms such as fungi, algae, plants, and animals by using high-resolution epifluorescence microscopy. Therefore, the process of mitochondrial division can be clearly separated into two main events: division of the mt-nuclei and mitochondriokinesis analogous to cytokinesis. Mitochondria undergo binary division which is accompanied by the division of the mt-nucleus. A remarkable characteristic of mitochondrial multiplication during the mitochondrial life cycle is that mitochondria can multiply the mt-chromosome by endoduplication until 50-100 copies are present. Mitochondria can then divide without mitochondrial DNA synthesis to eventually contain 1-5 copies of the mt-chromosome. This characteristic phenomenon can be observed during cell differentiation, such as during the formation of plasmodia and sclerotia of Physarum polycephalum and during embryogenesis and the formation of meristematic tissues in plants. The mitochondrial chromosome has a mitochondrial "kinetochore (centromere)" which is A-T rich and contains specific sequences such as topoisomerase binding sites, tandem repeats, and inverted repeats. A bridge of proteins may exist between the kinetochore DNA and membrane systems. Mitochondrial chromosomes can divide according to the growth of a membrane system between the kinetochores. Mitochondriokinesis progresses steadily along with mitochondrial nuclear division. As the membrane at the equatorial region of a mitochondrion contracts, the neck of the cleavage furrow narrows, and eventually the daughter mitochondria are separated. An actin-like protein may power mitochondriokinesis by separating the daughter mitochondria. In general, mitochondriokinesis occurs by contraction rather than by partition of the inner membrane.

Transfer of paternal mitochondrial DNA during fertilization of honeybee (Apis mellifera L.) eggs

Strict maternal inheritance of mitochondrial (mt) DNA is believed to be the rule in most eukaryotic organisms because of exclusion of paternal mitochondria from the egg cytoplasm during fertilization. In honeybees, polyspermic fertilization occurs, and many spermatozoa, including their mitochondria-rich flagellum, can completely penetrate the egg, thus allowing for a possibly high paternal leakage. In order to identify paternal mtDNA in honeybee eggs, restriction fragment length polymorphisms (RFLP) of different subspecies were used. Total DNA extracts of different developmental stages of an Apis mellifera carnica x Apis mellifera capensis hybrid brood were tested with a radioactively-labelled diagnostic mtDNA probe. Densitograms of autoradiographs indicated that the male contribution represents up to 27% of the total mitochondrial DNA in the fertilized eggs 12 h after oviposition. In subsequent developmental stages the portion of paternal mtDNA slowly decreased until hatching of the larvae when only traces were found. Although rapid disintegration of paternal mtDNA does not occur, the initially high paternal mitochondrial contribution is not maintained in the adult animal.

Constitutive homologous recombination between mitochondrial DNA and a linear mitochondrial plasmid in Physarum polycephalum

In one particular myxamoebal strain (NG7; mF+) of Physarum polycephalum, a linear mitochondrial plasmid (mF plasmid) which promotes mitochondrial fusion has been identified. A mating between mF- strains, that do not carry the mF plasmid, resulted in uniparental inheritance of the mtDNA. In matings between mF+ and mF- strains a recombination occurred between the mtDNA and the mF plasmid, and recombinant mtDNA was generated with the end of the mF plasmid as its ends. The DNA sequences of the recombination site in the mtDNA and the mF plasmid, and of the recombinant mtDNA, revealed that the mF plasmid had a 473-bp sequence that was identical to, but slightly shorter than, a 477-bp sequence of the mtDNA. This so-called identical sequence was found at the junction between unique sequences of the mF plasmid and the mtDNA in the recombinant mtDNA. Thus, the recombination between the mtDNA and the mF plasmid was due to reciprocal crossing-over at the identical sequence.

Population genetic aspects of deleterious cytoplasmic genomes and their effect on the evolution of sexual reproduction

A conflict of interest may arise between intra-cellular genomes and their host cell. The example explicitly investigated is that of a 'selfish' mitochondrion which increases its own rate of replication at the cost of reduced metabolic activity which is deleterious to the host cell. The results apply to deleterious cytoplasmic agents in general, such as intracellular parasites. Numerical simulation suggests that selfish mitochondria are able to invade an isogamous sexual population and are capable of reducing its fitness to below 5% of that prior to their invasion. Their spread is enhanced by decreasing the number of mitotic divisions between meioses, and this may constitute a significant constraint on the evolution of lifecycles. The presence of such deleterious cytoplasmic agents favours a nuclear mutation whose expression prevents cytoplasm from the other gamete entering the zygote at fertilization, resulting in uniparental inheritance of cytoplasm. Such a mutation appears physiologically plausible and can increase in frequency despite its deleterious effect in halving the amount of cytoplasm in the zygote. It is suggested that these were the conditions under which anisogamy evolved. These results have implications for the evolution of sexual reproduction. Standard theory suggests there is no immediate cost of sex, a twofold cost being incurred later as anisogamy evolves. The analysis described here predicts a large, rapid reduction in fitness associated with isogamous sexual reproduction, due to the spread of deleterious cytoplasmic agents with fitness only subsequently rising to a maximum twofold cost as uniparental inheritance of cytoplasm and anisogamy evolve.

The fate of mitochondrial DNAs of mt+ and mt- origin in gametes and zygotes of Chlamydomonas

In order to study the mechanism responsible for the uniparental transmission of the mitochondrial genome in crosses between Chlamydomonas reinhardtii and C. smithii, we have analyzed the fate of mitochondrial DNA during gametogenesis, zygospore differentiation and sporulation by hybridization experiments. Both mt+ and mt- gametes contain the same amount of mitochondrial DNA and the two parental genomes persist for several days in the zygotes. The DNA of mt+ origin is slowly eliminated during the period of zygote maturation. Light is required for total elimination of mt+ mitochondrial DNA in the zygospores. Using appropriate restriction enzymes, we have been unable to detect methylation of the mitochondrial DNA during gametogenesis or zygospore formation. The possibility that the mt+ mitochondria themselves are specifically eliminated in the course of zygote maturation is discussed.