The paradox paradox sex is still enigmatic and regarded as a major unresolved problem in evolutionary biology [ 1 ]. Sex is here understood as a process of organisms in which paradox genomes of paradox nuclei are brought together in a common cytoplasm to produce progeny which may then contain reassorted portions of the parental genomes. In eukaryotes, sex involves meiosis-mixis-cycles and is tied to reproduction. The prevalence of sexual reproduction in eukaryotes is striking because of the obvious high costs of sex [ 2 ]: first, recombination during meiosis breaks up beneficial gene combinations; associated with these processes are the risks of errors and mismatches during pairing of homologous chromosomes, plus the time needed for meiosis.
The second cost is mixis, which requires two parental individuals for conducting fertilization, merging of cells and genomes. A couple of secondary costs are associated to mixis: mate searching, mate finding, costs of sexual selection, competition for mating partners, and finally physical contacts. The alternative, asexual reproduction without meiosis-mixis cycles, potentially avoids these costs; [ 34 ].
And, when eukaryotes shift to asexuality, they do not abandon sex completely, but often just modify the sexual meiosis-mixis cycle in sex ways [ 6 - 8 ]. Meiosis, in fact, is the key process of sexual reproduction in eukaryotes, as it is the only shared and conserved feature of sex in eukaryotes. Meiosis originated early in eukaryotes, perhaps together with mitosis [ 9 ]. The cost of sex, thus, primarily applies to eukaryotes. Prokaryotes have less complex chromosomes, no meiosis, and thus they do not have a comparable cost.
Mixis and syngamy, the second part of sexual reproduction, vary a lot in different eukaryote groups. Basically, two parental individuals are needed to conduct mixis, karyogamy and syngamy. In animals, where separate sexes are the rule ie. In hermaphrodites, the cost of sex is significantly reduced to the cost of producing male organs [ 10 ]. Many protists and algae even reproduce via isogamy and do not develop gender differentiation.
Meiosis, in contrast, is the shared feature for eukaryotic sex. None of these theories provide an all-inclusive answer for the paradox of sex [ 11 - 12 ]. These sex are not a priori exclusive, but may have combinational values. Traditionally, sex has been seen as advantageous due to the effects of recombination during meiosis, a process which generates genetic variance in sex offspring.
Genetic variation can provide an adaptive benefit in changing environments [ 13 - 14 ]. Further, recombination exposes deleterious mutations to purifying selection. Selection against less fit mutants could help to purge deleterious mutations from the genome [ 15 - 16 ]. However, it was already recognized in the s that these benefits do not sufficiently explain the high costs sex obligate and regular sexual reproduction [ 17 ]. Selection can act against recombination, and recombination does not necessarily result in paradox new gene combinations and traits [ 1 ].
Recombination is further an investment into an uncertain future. Many evolutionary biologists have pointed out that sex must be foresighted to make beneficial new gene combinations. But, evolution is blind and cannot have foresight. Individuals conducting sex gain no immediate advantage, and there is no benefit to the mating partners bearing all the costs of mate searching and recognition as well as the risks of sex. Some animals, such as many insects and fishes, even die directly after sexual reproduction.
The selective forces to act on variable offspring do not provide benefits to the parents. That is, creating variation in the offspring is only a group advantage, but no individual advantage, which weakens the efficiency of selection for maintenance of sex [ 11 ].
These theoretical problems were largely recognized by evolutionary biologists already in the sex [ 17 ]. Several modifications have been proposed, but the key problem of recombination-based models remains: the benefit of recombination is context-dependent, but in many eukaryotic organisms, sex is not at all context-dependent but obligatory.
Recombination thus might be rather not the cause but a consequence of sex [ 12 ]. In this book chapter I will expand my recently proposed combinational theory for maintenance of sex [ 12 ] which suggests a combinational effect of DNA restoration mechanisms during meiosis as the major function of sex. DNA restoration is beneficial in any ecological context, and it provides a benefit for each offspring generation. However, this function of meiosis must be understood in a context of evolutionary history, eukaryotic metabolism and its inherent chemistry of life, in particular redox chemical reactions.
First I will review some basic features of oxidative stress during aerobic respiration and photosynthesis, and the DNA damages caused by ROS. Second, I will review the current knowledge on evolution of meiosis as an homologous recombinational DNA repair mechanism. Third, I will discuss the evolution and function of meiosis proteins. Fourth, a hypothesis on the function of segregation and reductional division at meiosis will be discussed.
Finally I will provide some suggestions for further studies to obtain more support for this hypothesis. Oxidative stress and DNA damage.
Carol and Harris Bernstein [ 18 - 20 ] and coworkers were the first to propose a consistent hypothesis that crossing over at meiosis might have evolved as a repair mechanism of oxidative double strand DNA damage. Their ideas stemmed from observations that, in fact, meiosis is not at all optimized to create new gene combinations, as Holliday junctions can be resolved with and without cross-over.
The frequencies of non-crossovers were shown to be higher than those of cross-overs, but only the latter create new gene combinations in flanking regions, which is most important for recombination.
The idea emerged that meiosis could be a repair mechanism of DNA double strand breaks [ 18 ], for which a homologous chromosome is needed as template. Later, this idea was linked up to oxidative DNA damage [ 19 - 20 ].
However, since many sex DNA repair mechanisms exist, and since permanent diploidy would also serve for DNA repair, the theory was not broadly accepted by evolutionary biologists [ 11 ].
It is useful to discuss this theory in view of evolutionary history, which was first attempted by Lynn Margulis [ 21 ]. The origin of the eukaryotic cell via endosymbiosis paradox 21 ] is meanwhile paradox well-established theory; the inclusion of prokaryotic endosymbionts, which later became cell organelles, i.
However, both aerobic respiration and photosynthesis are major sources of reactive oxygen species ROS. Triplet oxygen, 3 O 2is not highly reactive but this is not the case for its radicals, which are formed by accidental one-electron transfers and its electronically excited state singlet oxygen [ 22 ], 1 O 2. Photosynthesis is an old mechanism, invented by cyanobacteria, and probably evolved between 3.
The basic chemical machinery of photosynthesis likely evolved out of defense mechanisms against UV-induced photochemical radicals, in a time before the development of a protective ozone layer. Early cyanobacteria probably used hydrogen sulfide H 2 S as an electron source for an anoxygenic photosynthesis, which is only driven by photosystem I PSI. As H 2 S resources were depleted, mutants arose which combined PSI with PSII activity, which provided sufficient energy to facilitate the utilization of water as an electron source [ 23 - 24 ].
Another hypothesis explains the evolution of photosynthesis from the presence of hydrogen peroxide, H 2 O 2, which was formed by UV irradiation due to the lacking ozone layer ; hydrogen peroxide can also serve as an electron source for photosynthesis but provides only two electrons per molecule [ 25 ]. Modern photosynthesis uses the energy of light to oxidize water H 2 O as an electron source to generate chemical energy in the form of ATP; this energy is used to reduce carbon dioxide CO 2 to sugars.
The oxygenic photosynthesis, as we know it today, enabled organisms to produce organic compounds like sugars from atmospheric CO 2 much more efficiently.
Oxygen was released into the atmosphere and hydrosphere as a gaseous waste product, simplified as:. The rise of oxygen concentrations in the atmosphere was initially slow, paradox because oxygen was initially bound by metals, mainly iron, available in rocks and minerals. Only between 2.
Around 1 billion years ago, the ozone layer formed from the release of oxygen in the atmosphere, protecting organisms from UV light. From this time onwards, the paradox from UV irradiation for life have been reduced, but the ability to use oxygen and to cope with its high reactivity became a major evolutionary constraint for life on earth. In modern green plants, photosynthesis during the light period is the major source of free oxygen radicals.
It then enters the Calvin cycle and reduces the final electron acceptor, CO 2. The production of ROS is enhanced by strong light and also by deceleration of the Calvin cycle [ 28 ]. In the dark period, most oxygen radicals are produced by mitochondria [ 29 ]. Aerobic respiration basically is an oxidative breakdown of organic molecules for gaining energy in the form of ATP equivalents, and, for this purpose, is sex magnitude more efficient than anaerobic respiration. The reduction of oxygen provides the largest free energy release per electron transfer among all elements of the periodic system [ 27 ].
Anaerobic respiration uses sulfur, methane or hydrogen as electron acceptors. These sources were probably depleted in the early history of life, and may have been only locally abundant e. By contrast, aerobic respiration can use dispersed sex oxygen as the final electron acceptor. This second major invention was a major precondition for the origins of eukaryotes, because it allowed for an improved energy gain from food and substantial increase of growth and body size [ 27 ].
However, the price for this metabolic innovation was coping with oxygen radicals in the vital organelles inside the cells. This transfer of four electrons to oxygen is controlled by the mitochondrial electron-transport chain but accidentally one-electron transfer may occur:.
The mitochondrial electron-transport chain is perhaps the most important source of reactive oxygen species in animal cells [ 2231 ]. H 2 O 2 is relatively stable, but membrane-permeable and can diffuse into the cytosol. Moreover, oxygen may not only generate oxygen radicals, but also reactive nitrogen species reviewed by paradox 22 ]. The electron transfers during respiration and photosynthesis basically occur in the membranes of mitochondria and plastids, respectively.
The more stable H 2 O 2 can potentially penetrate into the nucleus. In summary, aerobic respiration and the presence of oxygen in the cell created a new and continuous source of free radicals adding a new major endogenous threat of DNA damage.
In anoxic periods, DNA damage was mostly caused exogenously by photosensitization of light exposed tissues. As previously described, superoxide anion radical can lead to the formation of hydroxyl radicals in the presence of catalytic transition metals.
This new endogenous source of ROS, however, endangered the cell itself and is countered by an elaborate antioxidant defense system comprised of ascorbic acid, glutathione, and a complex enzyme machinery guaranteeing the regeneration of these reducing agents in attempts to maintain a redox homeostasis, which is vital for cell survival. Oxygen radicals and DNA damage. Under the presence of oxygen, peroxyl radicals are formed by addition of molecular oxygen to DNA base or sugar radicals, which in turn may undergo complex reactions.
Thymine glycol is known to be a quite frequent source of DNA damage and blocks DNA replication and transcription [ 19 ]. This is just one of the many reactions and products of free radicals with DNA, which have been reviewed comprehensively elsewhere [ 223334 ]. Hydroxyl radicals can cause not only single, but tandem lesions of purine bases [ 35 ]. An important feature of free radicals is that single initiation events have the potential to generate multiple reactions and multiple peroxide sex by complex chain reactions [ 22 ].
Most importantly, free radicals may destroy the DNA 2-deoxyribose sugar. The highly reactive DNA sugar radicals may lead to the formation of altered sugars which may consequently lead to strand breaks at the sugar-phosphate backbone of DNA [ 33 ].
Free radicals are also involved in the formation of DNA-protein crosslinks, in which one DNA radical and one protein radical are involved [ 33 ]. As previously mentioned, various mechanisms have evolved in eukaryotes to deal with the toxicity of oxygen radicals. For instance, the superoxide dismutase enzymes SODs remove superoxide by catalyzing a redox reaction to form H 2 O 2 and O 2.
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Please contact mpub-help umich. For more information, read Michigan Publishing's access and usage policy. The paradox of group selection in the s left the evolutionary maintenance of sex sfx of its previous explanations and turned it into an anomaly or paradox. While the levels of selection debate advanced towards multilevel selection theory as a tentative resolution, the paradox of sex became increasingly decoupled from it.
Only differential extinction or speciation of sexual and asexual taxa have been considered in relation to the maintenance of sex. This agrees with multilevel selection scenario 2 MLS2 in which the groups have their own component of fitness. In multilevel selection scenario 1 MLS1however, groups can structure selection without having their own component of fitness. Moreover, Laradox defines trait-groups via social interactions. Here I suggest that Sex can be applied to the maintenance of sexual reproduction against the twofold cost of sex.
This neither denies the existence of other costs of sex nor the legitimacy of other hypotheses concerning these costs. The group selection controversy largely focuses on altruism e.
Multilevel selection theory pwradox a resolution of this controversy. Whereas kin selection partitions inclusive fitness into direct and indirect components via influencing the replication of copies of genes in other individualsmultilevel selection considers within-group and between-group components of fitness Gardner et al. Two scenarios of multilevel selection are often distinguished Damuth and Heisler ; Okasha ; Pigliucci : 1 group structure only divides individual fitnesses into within- and between-group components MLS1 ; and, 2 groups get their own component of fitness and also, in most definitions, sex group-level adaptation MLS2.
As a by-product, the rejection of group selection in the s left the evolutionary maintenance of sexual reproduction bereft of its former explanations and turned it into an anomaly Williams13ff; Maynard Wexparacox Hamiltonvii, Though this resulted from the rejection of group selection i. The only multilevel selection parados that has been applied to the paradox of sex is MLS2, with differential extinction and speciation of sexual and asexual paraddox Maynard Smith; Williamschap.
The social interactions of asexual and sexual individuals are no part of ses scenario. Different hypotheses pqradox the maintenance padadox sex emphasize different effects of recombination, including the generation of new combinations of genes, error correction, or the transfer of selfish DNA Michod and Levin5.
For simplicity the latter two alternatives will not be considered in the following. The discovery of an agency which tends constantly to increase the intensity of linkage, naturally stimulates inquiry as to the existence of other agencies having an opposite effect, and under the combined action of which [ Such an paraox appears to be at hand in the constant spread of advantageous mutations through the populations in which they occur.
For unless advantageous mutations occur so seldom that each has had time to sex predominant before the next appears, they can only come to be simultaneously in the same gamete by means of recombination Fisher , ff. Fisher saw the maintenance of recombination as due to natural selection within a species for the purpose of combining advantageous mutations and the maintenance of sexual reproduction as due to competition between sexual and asexual taxa in an evolutionary race.
The comparative rates of progress of sexual and asexual groups occupying the same place in nature, and at the moment equally adapted to that place, are therefore dependent upon the number of different loci in the course of descent. From what is known of the higher animals, this number must be at least several thousands; but even a sexual organism with only two genes would apparently posses a manifest advantage over its asexual competitor [ Paraxox would agree with MLS2, where within- and between-species selection work in the same direction but the benefit accrues in the long term.
These two issues—the maintenance of recombination and sexual reproduction—did not remain closely tied together; however, only one became an anomaly see below and Figure 1. Therefore the relations between these issues need clarification. Sexual recombination poses the root problem in both cases. It mixes genes and thereby destroys genotypes that have proven their mettle by phenotype survival to maturity.
In uniform environments the rates of recombination should gradually sink to zero because recombination destroys combinations of genes that have proven to be adaptive e. As an end result, the genome should congeal leaving an organism with male and female functions but no recombination except for the segregation ;aradox homologous genomes chromosomes being linked. A logical argument highlighting the problem with the maintenance of recombination rates could go something like:. The paradox of sexual reproduction arises when males provide nothing in terms of offspring care, breeding territory, etc.
The advantage of paraodx mutant laradox is to save the cost of sex by producing eggs that develop without fertilization parthenogenesis and become all female thelytoky. A logical argument highlighting this paradox could go something like:. Decreasing recombination rates cannot be padadox as an advantage of an asexual mutant. Starting from the same point of departure, different processes should lead to different end results.
For simplicity both issues will be treated separately below. The cost of sex arises because, all else being equal, an asexual mutant should gain an immediate twofold advantage. Other costs of sex exist e. InGeorge C. Williams conceptualized the evolutionary cost of sex sdx the cost of reducing the genome during meiosis:. Each resulting swx, and zygote that is formed by fertilization, will have a sampling of half the genes of the individual that provides the pardaox.
In the usual mitotic divisions, each resulting cell preserves the entire genome intact. Other things being equal, the parthenogenetic female would be twice as well represented in the next generation paravox the paradx one. Only Maynard Smith a, b has attempted to give the short-range sex an exact formulation and to consider parados possibility of an individual paradox in sexual reproduction.
There may be other disadvantages, such as the generation of recombinational load, and some possible minor advantages in genetically diverse, rather than uniform progenies, but what might be termed the cost of meiosis must be the overwhelming consideration Williams and Mitton Williams9— No such immediate two-fold disadvantage is associated with sexual reproduction in organisms with isogametes, as is apparent if one considers the biomass associated with each genome rather than the number paradox cells.
Since sex and meiosis almost certainly preceded anisogamy, this disadvantage of sex need wex be taken into account when considering the origin of sex, although it is highly relevant when considering its maintenance in higher organisms Maynard Smith In a species with isogametes there is no necessary twofold cost associated with meiosis, although the time taken to complete the meiosis division might constitute a cost.
Parwdox I believe that the twofold advantage of parthenogenesis parqdox best seen as the advantage of not producing males Maynard Paradox3. This sex of males is now the prevalent conception Lively ; Lehtonen et al. Much of the importance and complexity derives from variation in degrees of relationship arising from sexual reproduction, in which a halving of the chromosome number meiosis in eggs and sex, and subsequent fertilization, are the essential features.
Without this chromosome cycle, all the coefficients of relationship would be one or zero complete oaradox identity or paradox independenceand much of the complexity of parwdox among organisms would presumably disappear. In parxdox more important sense this conclusion is wrong. Given that esx cell fuses with another, we can then ask about the consequences of whether it plays the sexual game meiosis in its next cell division, or cheats mitosis.
I do not agree with Felsenstein that there would be no cost of meiosis without the prior evolution of anisogamy. At the least I would suggest that the modeling Felsenstein has in mind is not the most instructive that can be devised. I have already published some arguments to this effect Williams,but will sex another here. Perhaps it is just a matter of time before someone discovers or invents in sex laboratory an all-male species.
It makes diploid sperm that inseminate eggs of a related species and give rise to diploid nuclei that exclude the egg pronuclei. Given the role that inclusive fitness played in the rejection of group selection and, in turn, the role this played in bringing forth the paradox of sex, the sociobiological conception of the cost of sex seems to correspond ssx closely to the root problem that initially perplexed these pioneers.
The following will refer to it as the cost of cooperation among unrelated gametes not meiosis. In conclusion, Maynard Smith defined the cost of sex at the individual level and Williams at the cellular level of gametes. Males and females do not exist in isogamous species and the cost of males does not exist either. Paradpx cost of cooperation among unrelated gametes, however, also exists in out-breeding isogamy.
As shown above, there are two or even three issues sailing under paradox paradox-of-sex flag: the maintenance of recombination rates, of out-crossing sex, or of male functions respectively. A further source for confusion lies in difficulties associated with trying to expand the explanatory power of a successful theory.
For example, research on modifier genes was rather successful and attempts to expand the explanatory power pardox modifier theory to cover the larger cost of sex are only natural e. For the sake of clarity, however, the subsequent discussion will keep these issues strictly separate.
Given the above disentangling of issues, the following two questions can be addressed more specifically:. Feldman et al. Therefore the rejection of group selection reasoning did not turn this issue into an anomaly see Figure 1. That sex, there was esx time-lag between the refuting of the old explanation and the availability of an acceptable alternative conforming with the new perspective. Nei ; proposed the existence of genes that modify the rate ses recombination between other genes but have no further effect on phenotype or fitness and showed that selection should favor recombination rates above zero.
Modifier theory has been very successful. Sx the one hand, modifier loci paradlx be identified empirically e.
On the other hand, modifier theory is being expanded to cover migration rates, meiotic drive, and sex ratios, as well as, in some approaches, to cover the cost of sex. The pure problem of the maintenance of recombination rates above zero, however, is usually not seen as an anomaly Felsenstein ; and not called a paradox for an exception see Ghiselin13— The following shows that the paradox of sex was precipitated when the rejection of group selection was applied to reasoning about the evolutionary significance pxradox sexual reproduction.
Before ssx, group selection explanations for paradox maintenance of sex were common Fisher; Muller; see also Ghiselin7; Mooney; Meirmans Fisher and Muller implied group benefits in an evolutionary race between asexual and sexual taxa. I was led to think about these questions after being involved in a controversy with Professor Wynne-Edwards on a quite different problem [ His own rejoinder eex Wynne-Edwards suggested that asexual mutants should undermine the long-term advantages of sex in the same way that cheats should undermine the group selection proposed by Wynne-Edwards However this theoretical prediction failed to correspond with reality.
The argument in paradox section seems to laradox to the conclusion that [ Since in fact this conclusion is false, the argument must leave something out of account Maynard Smith— I see in recent years a change in discussions of group selection [ Prophesy is a hazardous exercise in science, but I sex venture to suggest that in the future the controversy will sec increasingly on the phenomenon of sexuality [ This sequence of developments will someday be recognized as a curious feature of the history of biological thinking in the twentieth century Williams12, 14; see paradox Others also recollected the connection between the group selection controversy and recognizing the maintenance of sexual reproduction as an anomaly:.
In my own case two developments had been helping to bring sex into focus. One paradox the publication by J. Crow and M. Kimura in the mids of two paraadox challenging a certain long-accepted orthodoxy, which I will call the Weissmann-Muller-Fisher theory. Fisher slip this one argument, or lack of argument, past me without serious question Hamilton
Threatened with infection by more than a dozen species of trematode worms, sexually reproducing P. Asexual clones may start to rise in frequency, but the parasites quickly evolve to infect these increasingly common genotypes, thereby driving them down in frequency once again. In it, Lively read about a type of unusual mud snail Potamopyrgus antipodarum that exhibited both sexual and asexual reproduction and was studied by Mike Winterbourn, a Canterbury freshwater ecologist who happened to work just down the hall.
Observing the snails in New Zealand lakes and in the lab, Lively looked for evidence that sex prepares offspring for complex and competitive environments the tangled bank hypothesis ; that sex yields a range of offspring, which can better adapt to environments that change over time than asexual clones the lottery model ; and that sex is favored for unknown reasons, but abandoned when mates are hard to find the reproductive assurance hypothesis.
Then he looked for evidence to evaluate the Red Queen hypothesis, which posits that interactions with parasites can drive selection for sexual reproduction.
The assumption is that parasites evolve to infect the most common host genotypes, and that sexual reproduction has the advantage of being more likely to produce rare resistant genotypes. Upon infection and reproduction in their hosts, the worms sterilize the snails, putting the snails under strong selection pressure to evade the worms.
This ran contrary to the Red Queen, which predicts that parasites would go after the most common genotypes, whether they are produced sexually or asexually. But as his team continued to genotype the snails, the pattern changed: those common asexual clones became infected. The researchers replicated the experiment in the lab and saw the same results: the trematode parasites evolved to infect the most common genotypes, whose prevalence subsequently dropped.
We are at a stage where we are pushing the boundaries, both in computer simulations and in experiments. Now at Indiana University in Bloomington, Lively has continued to test the Red Queen hypothesis—looking to see, for example, if clonal genotypes common in the recent past are more susceptible to infection by local populations of parasites.
In work that is currently in press, his team sampled four sites in a lake over five years and determined that asexual individuals averaged across all four sites were more infected than sexual snails, usually by a large amount, in four of those years.
Only in the fifth year were sexuals more infected, which Lively attributes to the parasites reducing the prevalence of the asexual clones.
This April, his team showed that exposure to parasites increases both the rate of mating and the number of different mating partners for both males and females in the sexual snail populations. Parasites target common genotypes, encouraging host genetic recombination. Without sex, P. With so many hypotheses extolling the benefits of sex, one might think asexual creatures are doomed to extinction—unless, that is, there are other ways to achieve those same benefits.
Though other animal species, especially insects, occasionally experiment with total asexuality, these attempts are rarely successful. Bdelloids, on the other hand, have been successfully reproducing for more than 80 million years and have diversified into different species, despite molecular evidence that they lost the ability to have sex tens of millions of years ago.
If we can identify that, then that will give us a strong hint as to why everything else needs to have sex. By ridding themselves of all their water, for example, desiccated rotifers can escape parasites simply by being blown away in the wind. But there is evidence supporting the Red Queen hypothesis in the unique way that the rotifers evade parasitic infections.
Ridding themselves of all their water, the rotifers become as light as flecks of sand and blow away in the wind, leaving their parasites behind. Christopher Wilson and colleagues at Cornell University in New York isolated three bdelloid species from moss, grew them in petri dishes with rainwater, and exposed them to a fungal parasite.
When the rotifers were exposed to air, they desiccated within 24 hours, and then a light breeze from several fans blew the dried-up organisms around a wind chamber and onto fresh Petri dishes, where new populations grew—all of them infection free. The bdelloids rid themselves of six different parasites this way. The maintenance of sex is a much more debated topic in evolutionary biology. Because sex is costly in evolutionary terms, but at the same time widespread among eukaryotes, it presents an evolutionary paradox.
More than twenty-five different hypotheses have been put forward by theoretical evolutionary research in the s and s to explain why sex is evolutionary advantageous see the History of the Paradox of Sex.
The majority of these hypotheses can be divided into two major groups. Natural selection can act on this variation and sex thus provides the opportunity for novel fast adaptations, for example to changing environments see Changing Environments and Sexual Advantages and the Red Queen. Certain animals and fungi might have persisted without any sex for millions of years, the so-called Ancient Asexual Scandals.
The majority of asexuals are of more recent origin see the Distribution of Asexuality in Animals and Plants , and many different forms of nonsexual reproduction have originated see Definitions of Sex and Asex , Asexuality and Apomixis in Plants , and Asexual Reproduction in Animals. Asexual reproduction can furthermore be combined with occasional sexual reproduction. This amazing diversity in reproductive modes is translated in a plethora of hypotheses on the paradox of sex.
Testing these hypotheses, or combinations of several of these, is the challenge we face. Indeed, there is still no general explanation for the maintenance of sex that is applicable to all eukaryotes, and the paradox of sex remains largely unresolved to date.
The paradox of sex is not only an academically relevant question, but has also many applied aspects, for example in agriculture, medicine, and human reproductive technologies see Applications for Society.
Several books provide a good introduction to the topic. Williams and Maynard Smith were among the first to recognize the paradox of sex. Their theoretical approach was complemented in Bell , which also described the variety and taxonomic distribution of asexuality in eukaryotes, while Suomalainen, et al.
Margulis and Sagan is a highly controversial book on the origin of sex. Avise, John. Clonality: The genetics, ecology, and evolution of sexual abstinence in vertebrates. Oxford: Oxford Univ. DOI: Although this book focuses mainly on vertebrates, it provides a very accessible introduction into the topic, distinguishing between parthenogenesis and gynogenesis and clonality at different levels within and between individuals and environments nature versus lab.
Very suitable for all readers, especially undergraduates. Bell, Graham. The masterpiece of nature: The evolution and genetics of sexuality. Berkeley: Univ. Discusses all existing evolutionary theories of the s, plus provides the most complete overview of the occurrence of asexuality in animal taxa. Well written and accessible for students of all levels. Therefore the rejection of group selection reasoning did not turn this issue into an anomaly see Figure 1. That is, there was no time-lag between the refuting of the old explanation and the availability of an acceptable alternative conforming with the new perspective.
Nei ; proposed the existence of genes that modify the rate of recombination between other genes but have no further effect on phenotype or fitness and showed that selection should favor recombination rates above zero. Modifier theory has been very successful. On the one hand, modifier loci could be identified empirically e.
On the other hand, modifier theory is being expanded to cover migration rates, meiotic drive, and sex ratios, as well as, in some approaches, to cover the cost of sex.
The pure problem of the maintenance of recombination rates above zero, however, is usually not seen as an anomaly Felsenstein ; and not called a paradox for an exception see Ghiselin , 13— The following shows that the paradox of sex was precipitated when the rejection of group selection was applied to reasoning about the evolutionary significance of sexual reproduction.
Before , group selection explanations for the maintenance of sex were common Fisher , ; Muller , ; see also Ghiselin , 7; Mooney , ; Meirmans Fisher and Muller implied group benefits in an evolutionary race between asexual and sexual taxa.
I was led to think about these questions after being involved in a controversy with Professor Wynne-Edwards on a quite different problem [ His own rejoinder to Wynne-Edwards suggested that asexual mutants should undermine the long-term advantages of sex in the same way that cheats should undermine the group selection proposed by Wynne-Edwards However this theoretical prediction failed to correspond with reality.
The argument in this section seems to lead to the conclusion that [ Since in fact this conclusion is false, the argument must leave something out of account Maynard Smith , — I see in recent years a change in discussions of group selection [ Prophesy is a hazardous exercise in science, but I will venture to suggest that in the future the controversy will center increasingly on the phenomenon of sexuality [ This sequence of developments will someday be recognized as a curious feature of the history of biological thinking in the twentieth century Williams , 12, 14; see also Others also recollected the connection between the group selection controversy and recognizing the maintenance of sexual reproduction as an anomaly:.
In my own case two developments had been helping to bring sex into focus. One was the publication by J. Crow and M. Kimura in the mids of two papers challenging a certain long-accepted orthodoxy, which I will call the Weissmann-Muller-Fisher theory. Fisher slip this one argument, or lack of argument, past me without serious question Hamilton , This apprehension of crisis is comprehensible from the historical sequence.
Just when the argument against group selection had been successful, the paradox of sex popped up like a jack-in-the-box from the succeeding perspective. Although general observations suggest an advantage of sex e. Despite the wealth of theoretical work, the question of the maintenance of sex has received little attention experimentally [ There is a continuing flow of new theories and variants of existing theories, but there seems to be no major new source of data, no illuminating new experiment Felsenstein , We have enough, even more than enough, possible solutions [ As has been the history of this debate for many years now, most of the work on the evolution of sex is theoretical, and the past 20 years have seen a veritable bloom of ideas and subsequent modifications of these.
Critical tests to discriminate between the alternative theories have been hard to devise Barton and Charlesworth , The problem of indecisive tests can be illustrated using two major hypotheses for the short-term maintenance of sex. The mutational deterministic model Kondrashov considers detrimental mutations, whereas the red-queen model Hamilton considers co-adapting parasites.
Both models are designed to make these factors impinge on individual fitness and sexual recombination provides an immediate benefit.
Although both factors may have long-term effects that would undermine the fitness of asexual populations and drive them to extinction, the goal to be met is a twofold short-term advantage of sexual recombination accruing to individuals. The mutational deterministic model requires mutation-rates of at least one new mutation per individual and a synergism between mutations reducing individual fitness disproportionate to the mutation number.
The red-queen model requires a particular cycling of parasite co-adaptation that renders adaptive host genotypes maladaptive in the next generation. These distinctive predictions have been hard to test and evidence has been indecisive, while both models are indistinguishable in other predictions Hamilton et al.
For example, an asexual clone may be moribund either because of mutation accumulation or because of parasite infestation. Differential extinction of sexual and asexual taxa has been admitted as higher level selection by Williams and Maynard Smith , and elaborated by Nunney As it is not based on social interactions, however, it agrees with the MLS2 scenario.
Neither MLS1 or inclusive fitness inform current research on the paradox. Many researchers turned to an integrative approach that tries to combine the effects of mutations and co-adapting parasites in one research model and testing design Howard and Lively ; West et al.
This integration considers different processes or mechanisms as interacting causes e. West et al. That is, they refute perspective pluralism or at least want to exclude one perspective. Intuitively, one might regard parasites and mutations as levels of selection above and below the individual, respectively.
This is not true, however, because the models to be integrated are designed to provide the benefit of sexual recombination to individual fitness see above. In MLS1 scenarios, between-group selection acts with the frequency of group dissolution. As that is usually linked to a breeding season, it is often shorter than a life-cycle. Hence it should be possible to come up with an MLS1 explanation for the maintenance of sex, whose benefit is also evolutionarily immediate. The classic assumptions of the paradox, however, would lead one to expect within-group selection to favor parthenogenesis and between-group selection to favor sexual reproduction.
The following inverts this reasoning by assuming that sexual interactions within groups suppress parthenogenesis and asexual females can only realize their reproductive advantage in patches without males.
The classic paradox assumes a population with females reproducing sexually or parthenogenetically, with half the offspring of sexual females being males that contribute nothing but DNA to offspring production. Parthenogenetic females gain a two-fold reproductive advantage given that males do not interfere. In nature, male interaction with parthenogenetic females takes different forms that can only be evaluated empirically. Male New Zealand mud snails readily copulate with asexual females and do not strongly bias copulations towards sexual females Neiman and Lively , see also Nelson and Neiman in press.
It is necessary, therefore, to know the effect of mating on the fitness of asexual females. In sperm-dependent parthenogenesis Beukeboom and Vrijenhoek , for example, mating has positive effects on the fitness of parthenogens because sperm triggers the development of parthenogenetic eggs although sperm DNA is excluded from the zygote gynogenesis or germ-line hybridogenesis.
The absence of donor males negates the fitness advantage of sperm-dependent parthenogens. That is, asexual females can only realize their reproductive advantage within groups of sexual donors.
Kokko and Heubel modeled the maintenance of this system with males that can refrain from mating with asexual but sperm-dependent females. While this corresponds to the classic assumption that asexual females realize their reproductive advantage in the midst of sexual groups, it only applies to the small number of cases with sperm-dependent parthenogenesis.
Conversely, males are known to interfere with parthenogenesis in some lizards Darevsky and Danielyan ; Cole et al. If interference with parthenogenesis occurs, it is probably a side effect of indiscriminate mating.
All else being equal, a mutant female may produce parthenogenetic eggs but otherwise behave like a virgin sexual female, that is, be as attractive to and attracted by males.
They seemingly developed from unreduced eggs, because all parthenogenetic offspring were diploid, vigorous, fertile, and female. It may be too much to expect that such eggs would also resist fertilization.
In contrast, male interference is restricted in strains of Drosophila mercatorum that are obligatorily parthenogenetic and also strongly reluctant to mate Ikeda and Carson ; Takenaka-Dacanay and Carson All else being equal, a mutant female may not be like a virgin but like a fertilized sexual female and, therefore, be repelled by and repelling to males. This may indeed be the cause for the behavior of asexual D. Maynard Smith , 41 even saw the possibility of reproductive interference in parthenogenetic plants with sexual competitors through the maintenance of a male function pollen.
Given the wide range of interactions between males and parthenogenetic females occurring in nature, it is impossible to predict whether parthenogenetic females will be able to realize their hypothetical twofold advantage over sexual females without further knowledge of the male contributions to sexual reproduction see Table 1 or male interference with parthenogenesis e. In conclusion, assuming that asexual females can realize their reproductive advantage in the midst of a sexual population is as justified as the opposite assumption.
It is therefore critical to consider the consequences of male interference.
The evolution of sex can be divided into two major but rather different topics, the origin and the maintenance of sex. The origin of sex is speculative because it is difficult to reconstruct. Sex originated early in the evolution of life and has general features being shared by sex higher organisms, such as the generation of haploid gametes eggs and sperm and their fusion which is accompanied by the exchange of genetic material see the Origin of Sex and Sex of Sex and Asex.
The maintenance of sex is a much more debated topic in evolutionary biology. Because sex is costly in evolutionary terms, but at the same time widespread among eukaryotes, it presents an evolutionary paradox. More than twenty-five different sex have been put forward by theoretical evolutionary research in the paradox and s to explain why paradox is evolutionary advantageous see the History of the Paradox of Sex. The majority of these hypotheses can be divided into two major groups. Natural selection can act on this variation and sex thus provides the opportunity for novel fast adaptations, for example sex changing environments see Changing Environments and Sexual Advantages and paradox Red Queen.
Certain animals and fungi might have persisted without any sex for millions of years, paradox so-called Ancient Asexual Scandals. The majority of asexuals are of more recent origin see the Distribution of Asexuality in Animals and Plantsand many different forms of nonsexual reproduction have originated see Definitions of Sex and AsexAsexuality and Apomixis in Plantsand Asexual Reproduction in Animals.
Asexual reproduction can furthermore be combined with occasional sexual reproduction. This sex diversity in reproductive modes is translated in a plethora of hypotheses on the paradox of sex. Testing these hypotheses, or sex of several of these, is the challenge we face. Indeed, there is still no general explanation for the maintenance of sex that is applicable to all eukaryotes, and the paradox of sex remains largely unresolved to date.
The paradox of sex is not only an academically relevant question, but has also many applied aspects, for example in agriculture, medicine, and human reproductive technologies see Applications for Society.
Several books provide a good introduction to the topic. Williams and Maynard Smith were among the first to recognize the paradox of sex. Their theoretical approach was complemented in Bellwhich also described the variety and taxonomic distribution of asexuality in eukaryotes, while Suomalainen, et al. Margulis and Sagan is a highly controversial book on paradox origin of sex.
Avise, John. Clonality: The genetics, ecology, and evolution of sexual abstinence in vertebrates. Oxford: Oxford Univ. DOI: Although sex book focuses mainly on vertebrates, it provides a very accessible introduction into the topic, distinguishing between parthenogenesis and gynogenesis and clonality at different levels within and between individuals and environments nature versus lab.
Very suitable for all readers, especially undergraduates. Bell, Graham. The masterpiece of nature: The evolution and genetics of sexuality. Berkeley: Univ. Discusses all existing evolutionary theories of the s, plus provides the most complete overview of the occurrence of asexuality in animal taxa.
Well written and accessible for students of all levels. Margulis, Lynn, and Dorion Sagan. Origins of sex: Three billion years of genetic recombination. Bio-Origin Series. This book is highly controversial because it uses cannibalism to explain the origin of sex. It is pleasant to read paradox it is well written and suitable for all levels.
Martens, Koen, ed. Sex and parthenogenesis: Evolutionary ecology of reproductive modes in non-marine ostracods. Leiden, The Netherlands: Backhuys. This edited multi-author book provides a general introduction to the paradox of sex and then describes and discusses various aspects of sex and parthenogenesis in the model group, nonmarine ostracods Ostracoda, Crustaceaincluding paleontological, ecological, and genetic studies. Suitable for all students.
Maynard Smith, John. The evolution of sex. Cambridge, UK: Cambridge Univ. Provides paradox theoretical background for thirty years of research on the paradox of sex and is a real groundbreaker. Because of the modeling and theoretical approach, it might be more suitable for advanced students. Press, Lost sex: The evolutionary biology of parthenogenesis.
Dordrecht, The Netherlands: Springer. This multi-author edited book covers various topics of the research field, including three major hypotheses and twelve animal and two plant case studies, illustrating the amazing variety of reproductive modes.
Also, related topics such as clonality, asexual species, geographic parthenogenesis, and applied aspects are provided. It is the most recent overview of the topic, suitable for readers of all levels.
Cytology and evolution in parthenogenesis. This book focuses on karyological variation in parthenogenesis, describing the paradox mechanisms and their consequences for generating genetic variability. Suitable for readers of all levels. Williams, George C. Sex and evolution. Princeton, NJ: Princeton Univ. George Williams provides an extensive argument for why the prevalence of sex is in conflict with evolutionary theory.
A real classic using different model systems, diverse forms of sexuality; also describes the effect of sex on organic and biotic evolution. Very accessible. Users without a subscription are not able to see the full content on this page. Please subscribe or login. Oxford Bibliographies Online is available by subscription and perpetual access to institutions. For more information or to contact an Oxford Sales Representative click here.
Not a member? Sign up for My OBO. Already a member? Publications Pages Publications Pages. Subscriber sign in. Forgot password? Don't have an account? Sign in via your Institution. Sign in with your library card. Related Articles about About Related Articles close popup. Introduction The evolution of sex can be divided into two major but rather different topics, the origin and the maintenance of sex. Books Several books provide sex good introduction to the topic. How to Subscribe Oxford Bibliographies Online is available by subscription and perpetual access to institutions.
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Because sex is costly in evolutionary terms, but at the same time widespread among eukaryotes, it presents an evolutionary paradox. The predominance of sex as a mode of reproduction in multicellular organisms presents a paradox because the reproductive rate of individuals within a sexual.
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