Why are most organelle genomes transmitted maternally?

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Why the DNA-containing organelles, chloroplasts, and mitochondria, are inherited maternally is a long standing and unsolved question. However, recent years have seen a paradigm shift, in that the absoluteness of uniparental inheritance is increasingly questioned.

Here, we review the field and propose a unifying model for organelle inheritance. We odna that the predominance of the maternal mode is a result of higher mutational load in the paternal gamete.

Uniparental inheritance evolved from relaxed organelle inheritance patterns because it avoids odna spread of selfish cytoplasmic elements. However, on evolutionary timescales, uniparentally inherited organelles are susceptible to mutational meltdown Muller's ratchet. To prevent this, fall-back to relaxed inheritance patterns occurs, allowing low levels of sexual organelle recombination. Since sexual organelle recombination is insufficient to mitigate the effects of selfish cytoplasmic elements, various mechanisms for uniparental inheritance then evolve again independently.

Organelle inheritance must therefore be seen as sex evolutionary unstable trait, with a strong general bias to the uniparental, maternal, mode.

The eukaryotic genome is distributed sex different genetic compartments that follow contrasting modes of inheritance 1. Nuclear genes usually display Mendelian segregation. Odna contrast, non-Mendelian inheritance patterns are characteristic of the DNA-containing cell organelles: plastids chloroplasts and mitochondria. The non-Mendelian inheritance of organelles is predominantly uniparental, usually maternal.

Thus, organelle inheritance can be recognized as reciprocal difference in sexual crosses Fig. Other features of organelle inheritance include somatic segregation sorting-out of genetically distinct organelles Box 1; Fig. Due to the different evolutionary origins and inheritance modes of the genomes of the eukaryotic cell, severe evolutionary consequences arise:. Paternal leakage, biparental chloroplast inheritance, sorting-out, plastome-genome incompatibility, and gamete controlled paternal exclusion.

A: Paternal leakage of plastids in tobacco seedlings detected by antibiotic selection. Green areas correspond odna cells harboring spectinomycin-resistant paternal chloroplasts, whereas white sectors contain only cells with antibiotic-sensitive maternal plastids Diffuse areas of green tissue indicate incomplete sorting-out of maternal and paternal plastids Box 1.

B: Biparental chloroplast inheritance in evening primroses, as evidenced by variegated progeny from the inter-specific cross Oenothera villaricae x Oe. The two species are diploid structural heterozygotes that, due to the genetic phenomenon of permanent translocation heterozygosity, inherit their haploid genomes as complete units.

It is heteroplasmic for the plastids of Oe. The chloroplast genome of Oe. Odna that sorting-out in this particular individual is likely completed, as odna by sex sharp borders between green and chlorotic tissue sectors. Scale bars: 0. In contrast to the nuclear genome, organelle genomes occur at high copy numbers and are usually distributed among multiple organelles per cell. Polyploidy and free vegetative segregation of organelles and their genomes are hallmarks of cytoplasmic inheritance.

Since the distribution of organelles and their DNA to daughter cells is, in principle, a stochastic process, mixed cells usually disappear after a certain number odna cell divisions, and homoplasmic cell lineages arise. Speed and sorting mechanisms are variable between organisms and organelles. For example, sorting-out of plastids in seed plants is a rapid process that is typically completed before flower formation Fig. In contrast, at least in some animal systems, heteroplasmy in the germ line can persist for several generations.

Sorting-out results in intra-organismic genetic drift. The process does not change allele frequencies of neutral alleles within a population, but it does so within an organism. It further provides an opportunity for selection on particular oDNA genotypes, if a mutation is harmful or the two genome types differ in their replication speed. The copy number of organelle genomes in the germline is often drastically reduced compared to the vast amount of organelle genome copies present in sex tissues, thus resulting sex rapid segregation to homoplasmy at high probability 112 However, uniparental organelle inheritance alone does not seem to represent a sufficiently strong driving force for the evolution of anisogamy and of two sexes 56 ; Box 2.

In both plant and animal systems, an increased female fitness associated with the organellar genotype cytotype has been observed 78. The best studied case is cytoplasmic male sterility CMS in plants.

This typically mitochondrially encoded trait mediates sex determination in gynodioecious populations and induces a counter-selection for nuclear fertility restorer genes 810 Although the organellar sex of related species are often very similar and typically have identical coding capacities, organelles are not freely exchangeable between species.

Enforced by uniparental inheritance and lack of sexual recombination, co-evolution, and co-adaptation of the genetic compartments lead to tight genetic interdependence of the nucleus and the organelles 712 Combination of a nuclear genome with an alien mitochondrial or plastid genome thus can result in inter-specific hybrids that display so-called cytoplasmic incompatibilities Fig.

Such incompatibilities can create hybridization barriers and contribute to speciation 813 Despite being of enormous importance, the causes of the predominantly maternal inheritance mode of organelles are not fully understood e. Uniparental inheritance excludes organelles from sexual recombination.

However, recombination is believed to be necessary to allow genomes to escape mutational meltdown, a process known as Muller's ratchet.

Uniparental maternal organelle transmission should therefore be an evolutionary dead end. However, accumulating evidence for at least occasional biparental transmission paternal leakage provides opportunities for sporadic sexual recombination events between organellar genomes. Those could significantly slow down Muller's ratchet 16 — The past few years have seen a paradigm shift in that the absoluteness of maternal organelle transmission is increasingly challenged 151618 — Nevertheless, there must be a selection pressure toward the evolution of uniparental transmission, for example to avoid the spreading of selfish cytoplasmic elements.

Such elements can be mutant organellar genomes that replicate faster than the wild-type genome, but are maladaptive to the organism. However, whether these elements indeed represent the driving force sex to uniparental inheritance and predominance of the maternal mode has remained enigmatic.

Further, the validity of the assumption that rare biparental transmission and sporadic sexual recombination of organelle DNA oDNA can stop the ratchet remains to be assessed. This article describes recent progress in our understanding of organelle inheritance. It discusses the current views on the driving forces and evolutionary consequences of maternal inheritance in plants, animals, odna, and fungi and highlights important unresolved problems.

We suggest a unifying model of organelle inheritance, and sex that the dominance of uniparentally maternal transmission is an evolutionary unstable trait. Mutational meltdown of organelle genomes is overcome by episodes of recombination between organelle genomes.

The driving force for the fall-back to strict uniparental sex comes from a certain type of selfish cytoplasmic elements i. Importantly, such elements cannot be disarmed by recombination. Finally, we propose experimental strategies to test the assumptions underlying our model.

One of the most commonly suggested models for the existence of two sexes is based on uniparental organelle inheritance. Is it assumed that two mating types exist to avoid costs of cytonuclear conflicts, for example, by competing and maladaptive cytotypes.

Uniparental inheritance has first evolved in isogamous organisms and was then enforced by anisogamy to regulated uniparental inheritance via only one gamete. In this way, the organelles define the sex 2324 ; see sex text. However, besides the fact that various other models for the evolution of anisogamy and two sexes exist, the cytonuclear conflict model can be questioned.

First, there are some fungi where organelle inheritance is regulated independently of gamete size or mating type. Second, it is difficult to judge if organelle inheritance is just a by-product of anisogamy. Organelle inheritance could be coupled secondarily to an already sex mating type. Also, it may typically associate with the larger gamete in a quantitative manner reviewed, e. If one assumes a higher mutational load of the smaller paternal gamete as driving force for the maternal predominance of organelle transmission as we propose herethis would be a very reasonable scenario.

Most eukaryotes are unicellular and many of them are isogamous. In many isogamous odna, oDNA inheritance appears to be linked to a mating type. Hence it seems reasonable to assume that the organelles indeed define the mating type. Importantly, one of the arguments standing against this view can be questioned based on our present theory of oDNA inheritance.

If oDNA inheritance is phylogenetically unstable Box 3; see main textfungi odna regulate oDNA inheritance independently of gamete size or mating type can be interpreted as derived forms. Another important point to clarify is whether uniparental organelle inheritance represents a by-product of the evolution of mating types or its cause?

This question is difficult to address in organisms that carry only one organelle type mitochondria. Disregarding biparental transmission, mitochondrial inheritance is almost always associated with the larger gamete, and so far, studies in isogamous organisms have not provided a clear answer either. However, if uniparental oDNA inheritance was a prerequisite for anisogamy, one would expect a clear linkage between mating type and organelle inheritance in those isogamous species that possess two types of organelles.

This can be tested in algae that contain both plastids and mitochondria. This example can be interpreted as evidence for uniparentally maternal oDNA inheritance indeed being a by-product of anisogamy. However, since oDNA inheritance is phylogenetically unstable see main texta much larger dataset on organelle transmission in algae should be analyzed. Unfortunately, mostly due to technical constraints, organelle transmission in isogamous algae is largely understudied.

While data for red algae are essentially lacking, the few odna reported so far for green and brown algae argue against regular co-transmission of plastid and mitochondria in isogamous algal species 84 The inheritance mode is abbreviated with U, uniparental; M, maternal; P, paternal; B, biparental; PL, paternal leakage; M, maternal predominance; P, paternal predominance; UU, doubly uniparental; BMP, biparental, maternal and paternal progeny.

Reference list: 1. Boynton et al. Aoyama et al. Kuroiwa et al. Adams et al. KuroiwaJ Plant Res ; 6. Diers a, Planta ; 7. Owens and MorrisAm J Bot ; 9. Owens and MorrisAm J Bot ; Mogensen and RuscheProtoplasma 1; Sodmergen et al.

Miyamura et al.


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The past few years have seen a paradigm shift in that the absoluteness of maternal organelle transmission is increasingly challenged 15 , 16 , 18 — Nevertheless, there must be a selection pressure toward the evolution of uniparental transmission, for example to avoid the spreading of selfish cytoplasmic elements.

Such elements can be mutant organellar genomes that replicate faster than the wild-type genome, but are maladaptive to the organism. However, whether these elements indeed represent the driving force leading to uniparental inheritance and predominance of the maternal mode has remained enigmatic.

Further, the validity of the assumption that rare biparental transmission and sporadic sexual recombination of organelle DNA oDNA can stop the ratchet remains to be assessed.

This article describes recent progress in our understanding of organelle inheritance. It discusses the current views on the driving forces and evolutionary consequences of maternal inheritance in plants, animals, algae, and fungi and highlights important unresolved problems. We suggest a unifying model of organelle inheritance, and argue that the dominance of uniparentally maternal transmission is an evolutionary unstable trait. Mutational meltdown of organelle genomes is overcome by episodes of recombination between organelle genomes.

The driving force for the fall-back to strict uniparental inheritance comes from a certain type of selfish cytoplasmic elements i. Importantly, such elements cannot be disarmed by recombination. Finally, we propose experimental strategies to test the assumptions underlying our model. One of the most commonly suggested models for the existence of two sexes is based on uniparental organelle inheritance. Is it assumed that two mating types exist to avoid costs of cytonuclear conflicts, for example, by competing and maladaptive cytotypes.

Uniparental inheritance has first evolved in isogamous organisms and was then enforced by anisogamy to regulated uniparental inheritance via only one gamete. In this way, the organelles define the sex 23 , 24 ; see main text. However, besides the fact that various other models for the evolution of anisogamy and two sexes exist, the cytonuclear conflict model can be questioned.

First, there are some fungi where organelle inheritance is regulated independently of gamete size or mating type. Second, it is difficult to judge if organelle inheritance is just a by-product of anisogamy.

Organelle inheritance could be coupled secondarily to an already pre-existing mating type. Also, it may typically associate with the larger gamete in a quantitative manner reviewed, e. If one assumes a higher mutational load of the smaller paternal gamete as driving force for the maternal predominance of organelle transmission as we propose here , this would be a very reasonable scenario. Most eukaryotes are unicellular and many of them are isogamous. In many isogamous species, oDNA inheritance appears to be linked to a mating type.

Hence it seems reasonable to assume that the organelles indeed define the mating type. Importantly, one of the arguments standing against this view can be questioned based on our present theory of oDNA inheritance.

If oDNA inheritance is phylogenetically unstable Box 3; see main text , fungi that regulate oDNA inheritance independently of gamete size or mating type can be interpreted as derived forms.

Another important point to clarify is whether uniparental organelle inheritance represents a by-product of the evolution of mating types or its cause? This question is difficult to address in organisms that carry only one organelle type mitochondria. Disregarding biparental transmission, mitochondrial inheritance is almost always associated with the larger gamete, and so far, studies in isogamous organisms have not provided a clear answer either.

However, if uniparental oDNA inheritance was a prerequisite for anisogamy, one would expect a clear linkage between mating type and organelle inheritance in those isogamous species that possess two types of organelles. This can be tested in algae that contain both plastids and mitochondria.

This example can be interpreted as evidence for uniparentally maternal oDNA inheritance indeed being a by-product of anisogamy. However, since oDNA inheritance is phylogenetically unstable see main text , a much larger dataset on organelle transmission in algae should be analyzed. Unfortunately, mostly due to technical constraints, organelle transmission in isogamous algae is largely understudied.

While data for red algae are essentially lacking, the few examples reported so far for green and brown algae argue against regular co-transmission of plastid and mitochondria in isogamous algal species 84 , The inheritance mode is abbreviated with U, uniparental; M, maternal; P, paternal; B, biparental; PL, paternal leakage; M, maternal predominance; P, paternal predominance; UU, doubly uniparental; BMP, biparental, maternal and paternal progeny.

Reference list: 1. Boynton et al. Aoyama et al. Kuroiwa et al. Adams et al. Kuroiwa , J Plant Res ; 6. Diers a, Planta ; 7. Owens and Morris , Am J Bot ; 9. Owens and Morris , Am J Bot ; Mogensen and Rusche , Protoplasma 1; Sodmergen et al.

Miyamura et al. Yu et al. Yu and Russell a, Sex Plant Reprod 7: ; Yu and Russell b, Planta ; Medgyesy et al. Diers b, Mol Gen Genet 56; Corriveau et al. Nagata et al. Forsthoefel et al. Mogensen , Am J Bot ; Corriveau and Coleman , Am J Bot ; Matsuura , Rep Cucurbit Genet Coop 31; Havey , J Hered ; Brennicke and Schwemmle , Z Naturforsch 39c: ; Chiu and Sears , Curr Genet ; Metzlaff et al.

Weihe et al. Moriyama and Kawano , Genetics ; Mannella et al. Birky et al. Birky , Annu Rev Genet ; Solieri , Trends Microbiol ; Ursprung and Schabtach , J Exp Zool ; Schabtach and Ursprung , J Exp Zool ; Breton et al. Zouros et al. Reilly and Thomas , Plasmid 3: ; Politi et al. Kondo et al. Meusel and Moritz , Curr Genet ; Kaneda et al. Cummins et al. Giles et al. Larsson et al. Cummins , Rev Reprod 3: Especially in vascular plants, where chloroplast transmission has been extensively studied, a large dataset supports repeated and independent evolution of biparental plastid transmission 16 , 46 , In many branches of the phylogenetic tree, plastid transmission modes vary from maternal, maternal with paternal leakage, biparental unbiased or with maternal or paternal dominance to paternal.

Interestingly, about one third of the plant species analyzed so far display the potential for biparental plastid transmission 49 , Relaxed uniparental maternal inheritance is also observed in ferns and algae. Moreover, mitochondria and plastids can be inherited independently of each other by different sexes. However, paternal or biparental mitochondrial inheritance is frequently found in fungi Biparental transmission or at least strong paternal leakage has been reported for bees.

Thus, in addition to maternal inheritance, various other types of organelle inheritance are observed. If inheritance is uniparental, it is often not strict. More and more evidence is accumulating that heteroplasmy and paternal leakage are quite common in natural populations of plants, animals, and fungi 15 , 18 — 20 , 88 — This seems particularly frequent in inter-species crosses, where exclusion mechanisms of different species may not function properly upon hybridization For a few plant species, paternal leakage frequencies of plastids could be determined experimentally.

The observed leakage frequencies are rather high, and thus blur the boundary between uniparental and biparental transmission 16 , 79 , The mechanisms of how uniparental organelle transmission is achieved are also very diverse 46 — 49 , 53 , Many different organelle exclusion mechanisms exist, and they can act either before, during or after fertilization Fig. Birky 16 lists 12 different cellular mechanisms for organelle exclusion.

Often, the mechanism is not even conserved between closely related taxa. For example, in mammals such as mouse, cow or rhesus monkey, the paternal mtDNA undergoes a reduction in the sperm but is fully degraded only later during early embryogenesis i. By contrast, in the Chinese hamster, mtDNA seems to be excluded during fertilization reviewed in In tomato, the male generative cell does not contain plastids. By contrast, in potato, the paternal plastids are eliminated at a later stage of gametogenesis.

Remarkably, tomato and potato belong to the same genus Fig. These two examples indicate that uniparental maternal inheritance evolved repeatedly even between closely related taxa. Different cytological mechanisms can result in maternal inheritance of plastids in angiosperms Species belonging to the Lycopersicon type tomato type , exclude plastids in pollen mitosis I. As the result of an unequal cell division, the resulting large vegetative cell receives all plastids, whereas the generative cell is devoid of plastids.

Species of the Solanum type potato type exclude plastids after pollen mitosis I. Their generative cell contains a few plastids which, however, are selectively degraded by an unknown mechanism prior to division of the generative cell into the two sperm cells in pollen mitosis II. Both mechanisms must be under genetic control of the paternal gamete.

Species of the Triticum type wheat type produce sperm cells that still contain plastids. However, the plastids are stripped off upon fertilization and thus do not enter the cytoplasm of the egg cell. Alternative mechanisms are possible in which the paternal plastids enter the egg cell, but do not contribute to the embryo.

The close phylogenetic relatedness of tomato and potato, which belong to the same family Solanaceae; nightshade family and, according to the most recent taxonomy, even to the same genus tomato, formerly called Lycopersicon esculentum , was renamed Solanum lycopersicum , suggests significant evolutionary flexibility and repeated independent evolution of the mechanisms leading to paternally controlled maternal plastid inheritance. The nuclear genetics of organelle exclusion also appears to be rather heterogeneous.

In many plant species, the mode of chloroplast inheritance depends on the crossing direction, and varies between crosses involving different ecotypes. Similar data are available for mitochondrial inheritance 15 , 20 , This indicates that organelle exclusion is, in many cases, haplotype dependent. Taken together, modes, mechanisms, phylogenetic distribution, and genetic architecture of organelle inheritance are very diverse among eukaryotes.

This strongly suggests repeated and independent evolution of diverse patterns of organelle transmission. Although not universal, maternal inheritance is the predominant mode of organelle transmission in all eukaryotic kingdoms. This raises the question as to which evolutionary forces favor its prevalence. Below, we briefly discuss the main models in the light of existing experimental evidence.

We point out unsettled questions and assumptions that remain to be scrutinized. It should be emphasized that these models are not necessarily mutually exclusive. As originally proposed by Grun 26 and based on genetic observations in evening primroses genus Oenothera , the most frequently expressed explanation for the evolution of uniparental organelle inheritance is the avoidance of cytonuclear conflicts 21 , From a modeling perspective, the two schemes cannot be fully discerned from each other 27 — Mutations in one of the genomes, for example in a locus determining the replication speed of the organelle, would allow one of the lineages to outgrow the other.

Also, mutations in oDNA might arise that mar the competing organelles by their attempts to gain a competitive advantage 24 , Considering organelles alone, negative interaction could be caused by loci in the two organellar genomes that are not co-adapted to each other, but combined by sexual recombination.

Alternatively, there is the possibility that different organelles harbor different alleles of one locus, and their heteroplasmic combination is maladaptive to the cell Obviously, a strict uniparental inheritance of organelles largely avoids these problems. Indeed, modeling of such scenarios frequently leads to the fixation of a nuclear inheritance modifier that causes switching from an ancestral biparental to a derived uniparental mode of inheritance.

Another starting point toward explaining uniparental inheritance is the assumption that sexual recombination of oDNA is not the only force that counteracts Muller's ratchet see below.

Hence, strict uniparental organelle transmission may be less harmful than widely assumed. Most relevant in this context is that organelles pass a genetic bottleneck when entering the germline Box 1. By this mechanism, organelle mutations can become purified by intra-cellular genetic drift in that genome segregation to homoplasmy occurs 38 , Subsequently, deleterious mutations can be eliminated effectively by selection 12 , Modeling work showed that paternal leakage or biparental transmission would interfere with this process Another model that deserves consideration was postulated recently The establishment of DNA-containing organelles by endosymbiosis was followed by massive gene transfer from the genome of the endosymbiont to the nuclear genome of the host cell Since many of the encoded gene products are re-imported into the organelle, organellar genomes and nuclear genomes rely on tight co-evolution and co-adaptation.

Mathematical modeling shows that co-adaptation is enhanced by both uniparental inheritance and the genetic bottleneck, suggesting that selection for co-adaptation was a driving force for uniparental inheritance and the evolution of two sexes. Like the other models, the co-adaptation model assumes lack of sexual oDNA recombination. In particular, the different types of genomic conflict hypotheses have been modeled extensively.

From this work, several theoretical problems arose. A general argument against these hypotheses is that a mutation leading to uniparental transmission can only be advantageous if a selfish cytoplasmic element is present, but not yet fixed in the population 6 , 16 , According to Hutson and Law 45 , fixation of an inheritance modifier inducing the switch from ancestral biparental inheritance to uniparental inheritance requires a heterozygous advantage at this locus and the tight linkage to a self-incompatibility allele.

Uniparental inheritance can, therefore, only evolve within rather strict boundary conditions. It seems that these problems can be solved by a recently proposed model 5. It makes the assumption that the gametes control organelle inheritance. It further takes the dynamics of the fitness costs of biparental inheritance into account in that cells do not suffer from a fixed cost of biparental inheritance, but the actual costs depend on the number of selfish or maladaptive mutations.

Consequently, the model predicts that the relative advantage of uniparental inheritance declines in a mutation frequency-dependent manner within a population. This appears to be the case under very broad parameters.

It can also account for genomic conflicts, mutation pressure, and nuclear-organelle co-adaptation as potential driving forces for uniparental inheritance. However, in agreement with previous modeling, it was found that an inheritance modifier that kills its own organelles cannot spread.

Paternal exclusion should, therefore, be evolutionarily unstable This is mainly due to the mechanistic problem that such an allele cannot be genetically linked with the fittest cytotype 5 , 6 , It is, however, obviously associated with fitness costs. However, in contrast to mammalian mitochondrial DNA, the nucleotide substitution frequencies in plastid and plant mitochondrial genomes are very low 51 , Developing the idea further, the occurrence of hermaphrodites with uniparental organelle transmission as is the case for many self-pollinating plant species is difficult to explain.

In these organisms, maternal transmission implies a costly mechanism for the organism to eliminate its own paternal cytoplasm. The second argument that can be raised against all models for uniparental inheritance is the implicit assumption that the cytotype transmitted into the hybrid typically the maternal cytotype is generally fitter than the excluded paternal cytotype e.

Hence, the current theoretical problem connected with organelle inheritance is not its sex linkage per se, but rather the dominance of the maternal over the paternal mode and in many cases its control by the paternal gamete. Arguing that gamete size simply determines organelle inheritance in a largely quantitative manner in that female gametes are larger and, therefore, harbor more organelles , is not satisfactory either. In view of the problems outlined above, some authors assume that the current models do not provide a fully satisfactory explanation for the prevalence of uniparental transmission of plastids and mitochondria in the entire eukaryotic domain 16 , 42 , 44 , Organelle genomes of plants and animals as well as those of unicellular and multicellular eukaryotes differ greatly in genome organization, coding capacity, copy number per cell and mutation rate, as do cell and gamete sizes and ecological niches.

In theory, modes of organelle transmission could even be explained as an evolutionary by-product of selection forces shaping organellar genomes in a lineage-specific manner On the other hand, the predominance of maternal organelle transmission, along with the virtual absence of sexual recombination between organelles in most lineages of eukaryotic evolution, is striking.

It thus appears likely that there is a general explanation for the observed pattern but also see The exclusion of organelles from the germline is an active process and should be costly 46 — Also, it has likely evolved repeatedly 16 , 46 , 49 ; Fig. Hence, there must be a strong, general selection pressure maintaining this trait.

By arguing from a physiological point of view, a possible explanation was offered by Allen It posits that only the maternal organelle DNA is maintained because it is protected from oxidative damage as caused by the electron transfer reactions in photosynthesis and respiration. Since the sessile egg cell has a lower energy demand than the mobile sperm, the paternal oDNA may suffer from higher oxidative damage and, therefore, is excluded from inheritance.

By contrast, the maternal germline cells are protected in specialized tissues, where organelles would display low metabolic rates. This assumption seems to be true for a wide range of animal systems 57 , and likely also for proplastids in plant meristems.

However, since the meristem confers plant growth and cellular differentiation, it has a high energy demand. Therefore, its mitochondria should not be protected from reactive oxygen species. Also, the hypothesis cannot apply to unicellular organisms. Thus, like most of the genetic models described above, the theory falls short of explaining organelle inheritance patterns for all eukaryotes.

Taking a number of theoretical considerations into account, we propose here a unifying model for organelle inheritance Fig. However, uniparental inheritance is evolutionarily unstable, because organelles are subject to Muller's ratchet. This drives a relaxation of strict maternal inheritance by paternal leakage or regular biparental transmission.

Biparental inheritance is again susceptible to the evolution of selfish genomes and, therefore, is repeatedly lost and restored over evolutionary timeframes.

In other words, the mutational meltdown by Muller's ratchet is escaped from by episodes or longer periods of sexual recombination between organelle genomes.

Importantly, sexual oDNA recombination is not sufficient to stop the spread of selfish cytoplasmic elements. What is the actual evidence for these assumptions?

Repeated origin and loss of uniparental organelle inheritance in evolution and selection pressures for uniparental and biparental organelle transmission. A: Biparental organelle inheritance likely represented the ancestral stage.

It is selected against to avoid the spread of selfish cytoplasmic elements left panel. This drives evolution for uniparental inheritance. It is typically maternal and, due to its lineage-dependent evolution, realized by various cellular mechanisms indicated by different colors.

Strict uniparental inheritance leads to organelle genome susceptibility to mutational meltdown middle panel. This, in turn, provides a driving force for a fall-back to relaxed organelle inheritance patterns to allow low levels of sexual oDNA recombination. Repeated evolution of uniparental inheritance is necessary, since biparental transmission allows the spread of selfish cytoplasmic elements, even if organelle genomes undergo sexual recombination right panel.

B: Selection pressure for uniparental organelle inheritance as caused by an aggressive and maladaptive cytoplasm. Identical offspring viability is achieved if both organelles are inherited biparentally and have identical multiplication speeds i.

This situation would provide a strong selection pressure for the evolution of uniparental inheritance lower panel. C: Spread of maladaptive and aggressive cytoplasmic genotypes cannot be prevented by sexual oDNA recombination. Box 4 that confer compatibility with the hybrid nucleus inc and normal replication speed fast. The latter is incompatible with the host nuclear genome, but substantially overrepresented in the hybrid population, thus conferring a strong selective disadvantage.

If uniparental inheritance is evolutionarily unstable, three major patterns in organelle inheritance should be observable. First, biparental transmission should evolve repeatedly and independently. Second, paternal leakage should be relatively frequent. Third, the switch back to sex-specific organelle exclusion uniparental inheritance should occur by diverse mechanisms that can differ between closely related species or even between haplotypes.

Strikingly, these patterns are indeed observed, throughout the eukaryotic domain Box 3; Fig. In addition, paternal gamete-controlled organelle exclusion certainly plays an important role in organelle exclusion. In all sexually reproducing eukaryotes, the zygote develops through fusion of an egg cell with a usually motile sperm cell.

It subsequently undergoes rapid divisions that incur a high energy demand. This should be the case for both mtDNA and ptDNA, because in many seed plant taxa, embryos are green at least in the early stages of seed development the embryo is exposed to light and perform photosynthesis If selection in the zygote is the driving force for paternal exclusion, one must, however, assume higher mutation rates for paternally inherited organelle genotypes. This can be tested in paternally inherited cytotypes as found in gymnosperms.

Strikingly, oDNA mutation rates are indeed higher in these taxa, suggesting that, compared to the egg cell, organelles in the pollen carry a higher mutational load 52 , 59 , However, since oxidative damage fails to explain relaxed maternal organelle inheritance patterns see above , it cannot be the sole and universal driving force for the observed patterns of oDNA inheritance. However, paternal oDNA copy numbers in the sperm cell are typically substantially smaller than maternal copy numbers in the larger egg cell.

Hence, genetic drift of oDNAs due to stronger genetic bottlenecking at the level of the gamete might represent an additional relevant factor.

This view is in line with theoretical considerations, arguing that the higher mutational load of organelle genomes in general is not due to asexuality per se, but is the result of the small effective population size of organellar genomes Taken together, maternal dominance in organelle inheritance could be due to a lower mutational load, since in most organisms, more oDNA copies are inherited by the mother.

However, in organisms where bottlenecking is less severe for the male gamete, paternal oDNA inheritance can evolve, thus potentially explaining why contrasting modes of organelle inheritance exist. This especially applies to isogamous organisms, carrying two organelles such as green algae Box 2. Finally, uniparental maternal inheritance must be seen as a consequence of, rather than the underlying reason for, anisogamy Box 2. Maternal dominance of uniparental inheritance could be explained by a higher mutational load of the paternal gamete.

However, why does uniparental inheritance exist at all, and what are the selection forces, leading to uniparental maternal inheritance? A commonly suggested putative selection force for uniparental inheritance is deleterious epistatic interaction between co-existing organelle genomes.

In the case of mitochondria, a possible mechanistic scenario could, for example, involve the unscheduled onset of apoptosis. That can be triggered by production of reactive oxygen species if improperly co-adapted subunits of the mitochondrial respiratory chain are combined with each other However, to what extent deleterious epistatic interactions between co-existing organelles occur in nature, is currently unclear. At least for plastids of seed plants, such interactions are difficult to image, since plastids usually do not undergo fusion 16 , 46 , There is no molecular or cell biological evidence for negative interactions between co-existing plastids, even though some classic genetic evidence could be interpreted in this direction pages — of 26 , Negative interactions between mitochondria in plants and animals seem to be possible, and were reported in some cell fusions 65 , Recently, it was demonstrated that heteroplasmic mice display reduced respiratory activity and behavioral phenotypes, whereas mice homoplasmic for either of the two mitochondrial genotypes had no phenotype Taken together, inter-organellar epistasis seems to exist, although its mechanisms are largely enigmatic.

However, analyses on sexual oDNA recombination in yeast and Chlamydomonas somewhat argue against the widespread occurrence of deleterious epistasis between oDNA alleles, since the expected segregation distortion is not normally observed Box 4. Given the few documented examples, the general significance of deleterious epistatic interactions between organelles is currently questionable.

Recombination between biparentally inherited organelle genomes has been reported for some taxa, but is controversial in others 15 , 16 , 18 , 20 for references.

It is important to note that, upon strict uniparental inheritance and lack of sexual recombination , selective sweeps should be frequent in organellar genomes, but are barely observed Detailed genetic linkage analyses based on sexual oDNA recombination were conducted in yeast e.

Linkage analyses in yeast and Chlamydomonas uncovered remarkable differences of oDNA recombination compared to recombination mapping in the nuclear genome. However, in contrast to a phage cross where titers and double infection rates can be easily determined, models for oDNA recombination usually assume that oDNA contribution from both parents is equal and that there is no intra-cellular selection for or against particular recombinants.

Furthermore, random pairing of oDNA molecules, multiple rounds of paring and recombination, and random segregation of oDNA copies is assumed In spite of these uncertainties, genetic distances obtained from segregation analyses usually correlate well with the physical distances of the genetic markers , , Also, recombination hotspots can exist in oDNAs Another important finding from genetic analyses was that oDNA recombination events are mostly non-reciprocal at the level of the individual, but reciprocal at the level of the population , This is likely due to gene conversion, but other factors may be involved as well The general features of sexual oDNA recombination as worked out for unicellular organisms may be transferable to many multicellular eukaryotes.

However, there are some limitations concerning the frequency of organelle mixing and fusion. For example, plastids of seed plants do not seem to regularly undergo recombination in crosses, not even in organisms with biparental plastid inheritance However, especially in cell fusion experiments ptDNA recombination was occasionally seen e. It appears likely that recombination between plastomes in sexual crosses of seed plants is largely prevented by the absence of plastid fusion in the zygote 46 , 47 , This is in contrast to Chlamydomonas , where plastid fusion occurs after syngamy.

Organelle recombination of plant mitochondrial genomes was repeatedly demonstrated in protoplast fusion 3 , 65 , and preliminary evidence for recombination in sexual crosses has also been obtained If biparental transmission occurs, sexual recombination of plant mtDNA is expected, because plant mitochondria regularly undergo fusion and fission , and homologous recombination events seem to occur frequently in mitochondrial genomes , In contrast to plants and fungi reviewed in 20 , occurrence and evolutionary relevance of mitochondrial genome recombination in animals are still controversial.

Mixed evidence is available in that recombination was detected in some animal species, but not in others 2 , 15 , , The general presence of sexual oDNA recombination has gained some support from investigations of natural populations. Circumstantial phylogenetic evidence points to sexual recombination in both plant and animal systems, but the currently available data are still sparse and a bit controversial 1 , 3 , 15 , 18 , While genetic studies in natural populations of campion genus Silene suggest presence of recombination 19 , somewhat contradicting evidence has been obtained for fruit flies and fungi 91 , More rigorous and systematic investigations of oDNA recombination in natural populations and hybrid zones are needed that, for example, also take into account the possibility of selection against recombinant genotypes.

In summary, it seems possible that sexual recombination of oDNA is widespread and perhaps even a general phenomenon. As paternal leakage of plastids occurs at least occasionally in many, if not all, species, sexual recombination of plastids in seed plants may be limited by the rarity of plastid fusion events.

In contrast, the limiting factor in sexual recombination of mtDNA may be paternal leakage and reduced recombination ability, at least in some animal taxa, most notably in mammals cf.

It is noteworthy in this respect that mammalian mitochondrial genomes have considerably higher nucleotide substitution rates than plastid genomes and plant mitochondrial genomes. Interestingly, plant mitochondria, which are likely subject to paternal leakage and regularly undergo fusion and mtDNA recombination, display one of the lowest nucleotide substitution rates known in nature 51 , However, whether or not oDNA recombination frequencies in all organisms, and especially in mammalian mitochondria and seed plant plastids, are high enough to overcome Muller's ratchet, remains to be determined see main text.

Another common posit is that uniparental inheritance has evolved to avoid the spread of selfish cytoplasmic elements. Some solid datasets are available for competition between organelle genomes in both plant and animal systems. Examples have come from cell fusion events, oDNA mutants and sexual crosses 6 , 12 , 18 , 67 , For example, in evening primroses, plastids display different multiplication speeds in sexual crosses depending on the plastid genotype If the avoidance of competition between organelles was the major driving force for the evolution of uniparental inheritance, a replication race between oDNAs must be harmful to the nucleus.

Although some human diseases are associated with altered mtDNA copy numbers 69 , in most eukaryotic systems studied so far, the amounts of oDNA versus nuclear DNA remain constant within a rather narrow range and are likely under nuclear control 1 , 12 , 69 — Hence, it appears unlikely that solely differences in oDNA replication speeds provide sufficient driving force for the evolution of uniparental inheritance.

A likely much stronger selection force for uniparental inheritance will arise if an organelle with a higher replication speed carries a genotype that is incompatible with or maladaptive to the host nucleus.

Prime examples are some of the petite mutants of yeast, which lack the capability for respiration due to large deletions in the mitochondrial genome.

Although yeast can grow anaerobically, growth rates achieved by fermentation are substantially lower. In evening primroses, the competitive advantage of specific plastid genotypes is largely independent of the nuclear background and is also observed when the more competitive plastid genotype is deleterious 73 , 74 , exemplifying a naturally occurring aggressive and maladaptive cytotype Strikingly, cytotypes that are maladaptive to the nucleus are well known in plants, fungi, and animals.

They lead to cytoplasmic incompatibilities, which are the result of diverging evolution between the organellar and nuclear genomes involved 7 , 8 , 13 , 14 ; Fig. Together with potentially ubiquitously present differences in organelle replication speeds, this can lead to a hitchhiking of cytoplasmic incompatibilities. Potentially, this provides a strong selection force for uniparental organelle inheritance 26 , 30 ; Fig.

As summarized by Birky 16 , the assumption that sexual recombination in oDNA is required to counteract Muller's ratchet has been challenged. Hence, the virtual absence of recombination may be less harmful than widely assumed. A major argument is that organelles generally undergo a genetic bottleneck when entering the germline.

Thus, organelle genomes become purified by intra-organismic genetic drift, rapidly segregate to homoplasmy, and therefore malfunctioning genotypes can be eliminated effectively by selection 12 , 38 , 39 ; Box 1. Also, organelles may have very efficient DNA repair mechanisms that might have evolved to cope with the constant exposure to high levels of reactive oxygen species that are generated as unavoidable by-products of respiratory and photosynthetic electron transfer reactions.

High genome copy numbers, together with active gene conversion, seem to be effective mechanisms for slowing down the ratchet, at least in plant oDNAs Nonetheless, the ratchet should still be clicking, raising the question how much recombination is needed to stop it. According to Charlesworth et al. If applied to a sexual oDNA recombination frequency of 3.

Although this estimate may be an over-simplification 78 , the calculated frequency comes close to the value expected to suffice. This could explain why uniparental inheritance of plastids is evolutionarily particularly unstable, and biparental transmission is more frequently observed for plastids than for mitochondria. As mentioned above, a major problem with the current theoretical modeling of the occurrence of uniparental inheritance lies in the frequent occurrence of paternal gamete-controlled exclusion of organelles.

However, the present models might be too simple to reflect the true pattern of organelle inheritance. For example, the probably best theoretical approximation to the naturally observed organelle inheritance patterns 5 assumes a unicellular organism, a simple single-locus genetics of nuclear control of organelle inheritance, and the absence of sexual recombination of oDNA.

Furthermore, no sex-specific mutational load is assumed. However, organelle inheritance can be controlled by multiple nuclear loci 80 , 81 , the mutational load in paternal oDNA may be elevated, and sexual recombination of oDNA is known to occur in many systems Box 4. In addition, improved models should take into account more complex patterns of sorting-out, as they occur in multicellular eukaryotes, and the underlying population genetics.

In the presence of occasional sexual oDNA recombination, the fittest alleles of the paternal cytotype might be able to escape a uniparental inheritance modifier. The key question then will be whether the theoretical values that can be deduced from refined modeling approaches are in agreement with observed paternal leakage frequencies, oDNA recombination rates, and the strength of the selection pressures for uniparental inheritance.

As suggested above, the avoidance of spreading of an incompatible but aggressive cytoplasm with a faster replicating genotype might be a major driving force for uniparental organelle inheritance.

However, for a full understanding of oDNA inheritance patterns, one needs to assess the fitness effects of all potential driving forces of uniparental inheritance. It further will be necessary to identify the nuclear factors responsible for organelle exclusion as well as the organellar loci controlling replication speed and the loci conferring deleterious epistatic interactions between co-existing organelles or between organelles and the nucleus.

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