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Given any kind of living organism (plant, animal, silicate, gaseous, energy, etc.,) what logical reasons (natural, environmental, evolutionary selection, nurtured, artificial) could result in more than three genders?

There are a number of possible methods for single-gendered reproduction. See examples here....

Bi-gendered species are commonly known, won't get into that.

Possible tri-gendered combinations could be womb/sperm-donor/egg-donor, or perhaps womb&egg-donor/sperm-donor/post-birth-nurturer.

One possible quad-gendered combination could be womb/sperm-donor/egg-donor/post-birth-nurturer.

These combinations are, of course, simply swapping around the known existing gender parts from humans. I have not attempted to combine any of these with the asexual reproduction methods listed in the link, which could result in possible additional groupings and multiple genders (or perhaps heterogamy over time instead). What other possible combinations could exist, and what logical reasons could cause a species to have/develop more than three genders?

I have read this link, and it did not answer my particular angle.

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    $\begingroup$ Yesterday I read about a virus that has 5 separate parts that, alone, do nothing, but when 4 or 5 of them get together they infect the host. (The 5th portion is optional). It certainly doesn't explain a reason, but it may be useful for you to check into. And it might be useful to know that sometimes, nature offers up weird mistakes / variations like that and they survive anyway. $\endgroup$ – BrettFromLA Aug 26 '16 at 21:04
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    $\begingroup$ When you say "more than three" do you mean "at least four"? 'Cos I mean three is tricky enough. $\endgroup$ – Xplodotron Aug 26 '16 at 21:15
  • $\begingroup$ The three way options involve creatures nurturing a child that has none of their genes. There would be an advantage to killing the life off if it has none of your genes and you have to feed it. Such situations are unlikely to evolve. $\endgroup$ – Donald Hobson Aug 26 '16 at 23:00
  • $\begingroup$ Could it be that early life simply developed this way (perhaps as a natural population check on early microbes or even just by chance) and by the time anything close to an animal evolved, it was already hard-programmed and thus too late to lose? $\endgroup$ – Zac Walton Aug 27 '16 at 1:22
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    $\begingroup$ I think what you mean are three sexes and not three genders. Sex refers to reproductive entities and not sexual identities. Humans have a slew of sexual identities or genders, but only two egg-bearers. Basically a species where there are male and female sexual organs present in distinct individuals (most often). Technically humans are diecious ( or having two egg-carriers) for the purposes of sexual reproduction. The question should be about triecious species. Who will presumably have their own plenitude of genders. $\endgroup$ – a4android Aug 27 '16 at 12:37

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Trioecious reproduction

The definition of a sex is that each sex produces a unique type of gamete. If it does not produce unique gametes then it is not a sex. A caregiver that does not contribute genes is a caste or morph.

The advantage of being triploid and trioecious is that this reduces the risk of genetic bottlenecks and inbreeding depression.

The disadvantages are that three individuals are required to reproduce and, contrary to popular belief, reduce phenotypical diversity. Recessive alleles are fair less likely to display phenotypes because three copies would be required.

An environment where trioecious species outcompete dioecious species would need to place greater selection pressure on genetic redundancy over the costs of finding mates and reduced phenotypic diversity.

An example of a true trioecious species occurs in the speculative fiction novel Silent Runners (Source).

Hybridogenesis

Hybridogenesis is a form of sexual parasitism where one species is limited to a single sex and must parasitize the opposite sex of a closely related species in order to reproduce. The genes of the parasitized parent are discarded from the germline of their hybrid offspring, ensuring that the parasite species' purity is maintained.

Thus, species that engage in hybridogenesis may qualify as possessing multiple sexes on the demographic level. For example, the Iberian minnow consists of triploid males, triploid females and diploid males. The diploid male is a sexual parasite whose offspring are always diploid males, and ultimately requires both triploid males and females to reproduce.

Social Hybridogenesis

Social hybridogenesis is a form of hybridogenesis found in certain eusocial ants. (Source)

Symmetrical social hybridogenesis occurs when the parasitized species adopts the same strategy to parasitize the parasitize. The ant genus Pogonomyrmex, for example, has two species with four sexes on the demographic level. Each colony requires three individuals to reproduce: the female mates with males of her species and the other. Males are spawned asexually by females. Same species mating produces only reproductive females. Cross-species mating produces sterile hybrid females that form the worker caste. Collectively all four sexes, two males and two females of different species, are required for both species to survive.

To put this by analogy with cells in one organism: reproductive cells require gametes from the other species to produce somatic cells.

This would not work the same way on the organism level. There the analogy would be the female produces a sterile hybrid offspring which she parasitizes similar to the anglerfish. As far as I know this strategy does not occur in any known species.

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    $\begingroup$ You just made up about half of those words, didn't you? (JK - excellent answer! Upvote.) $\endgroup$ – BrettFromLA Aug 26 '16 at 21:06
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    $\begingroup$ It was fascinating reading, but it didn't actually answer the question, did it? (If it did, what was the answer? :) ) $\endgroup$ – GreenAsJade Aug 27 '16 at 12:16
  • $\begingroup$ @GreenAsJade An environment where trisexual species outcompete dioecious species would need to place greater selection pressure on genetic redundancy over the costs of finding mates and reduced phenotypic diversity. $\endgroup$ – HopelessN00b Aug 27 '16 at 15:09
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Extreme seasonal fluctuations

Imagine an area (or planet) with variable climatic conditions; varying combinations of length, precipitation, temperature, and an ecology similarly adapted to unpredictability.

An asexual surrogate sex could impart epigenetic information to the gestating fetus. Perhaps the surrogate's input determines whether it will develop into a male or female, or its size, or something else better suited to the environment the surrogate is exposed to. In boom times the surrogate births smaller offspring after shorter gestation so they can eat quickly and develope on their own; in lean times the surrogate puts the fetus(es) in stasis (perhaps for years) or gestates them longer so they're better able to cope.

How

An ovoviviparous species produces an egg that hatches wihin the host-mother. In this case the egg is transferred to the surrogate, where it hatches and an immature/underdeveloped fetus emerges to be incubated.

Other advantages

  • It would also mean the male and female can "impregnate" multiple surrogates each breeding season, resulting in more viable offspring without putting either at risk.

  • And a surrogate as part of the rearing process means three adults are protecting and gathering food for the offspring.

  • If the climate is particularly harsh or predation high, the death of either males or females won't result in catastrophic sex imbalances that disrupt future generations.

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As @Seeds and @John Feltz said above, you really can't logic evolution. The answer is always "because survival." But, below at least are some "reasons" how a species came to have multiple sexes:

Trisomy

Most life on Earth is diploid: each chromosome comes in pairs (X and Y are a famous variation on the rule). In "higher" species, having three of just one of your chromosomes leads to a syndrome of some sort. This is not good. Having three of every chromosome (triploidy) is always unviable. But maybe your creature evolved with three sets of chromosomes, meaning three gametes (be they egg or sperm). That could be a reason for three sexes (after all human males and females only vary by one single chromosome).

Chimeras/Mosaics

Chimeras are produced by the merger of multiple fertilized eggs. While it's considered a fluke from us diploid normal pants people, maybe your species has a reproductive mechanism that actually waits for eggs to be fertilized by more than one individual--who must be of different sexes themselves--then fuses them into a chimera.

Life Cycle-ish?

Maybe creatures with more complex life-stages might could be said to have an extra sex or two, depending on your story. Usually life-stages are quite different from each other, for example, Egg-->Caterpillar-->Chrysalis-->Butterfly are different life stages of a single individual. (I really like Egg-->Facehugger-->Chestburster-->Xenomorph!)

But to get to your question consider the hydra. Let's start with an egg. From that you get a polyp, which is basically a sessile non-sexual stalk that reproduces asexually by budding. When food is low it will bud off male and female medusa (which are free floating). The medusa spurt out gametes which join to form eggs. So, you sorta have three sexes: a non-sexual polyp, and male and female medusa.

Using this life-cycle idea, perhaps your made-up species could have three sexes in the adult stage, A,B, & C, which produce different life cycles depending on the pairing. A+B produce A or B or C. A+C produce a Type D warrior caste that can asexually clone itself (after all, Type D need genetic variation too). B+C produce a sterile Type E. A+B+C produce a trisomal crazy pants variant. And so on.

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I think I see a way you could actually evolve something akin to this:

Take a species with normal sexual reproduction, something happens to divide the population and they begin to diverge, although not quite to the point of becoming unable to interbreed.

Now the barrier is removed and they can now mix again. Their Y (or whatever equivalent they have) chromosomes have diverged substantially, however. With the population again mixed the females converge on a common genetic structure but the Y chromosomes don't mix and thus there is no convergence--females are common but there are two distinct varieties of male and they turn out to be complementary in the game of survival.

Thus you get the situation where the ideal family unit is a type A male, a type B male and a female, although any given offspring has genes from only two of the family members.

Once this situation has developed the Y chromosomes are free to evolve independently of each other, the type A and type B organisms drift apart.

You have a species with something close to three genders with a reasonable evolutionary path to get there.

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As a slight variation on your theme, you could have a species that is still split into male and female, but each of those is further split.

This is the case for the side-blotched lizard

There are 2 female "genders" and 3 male "genders", which are differently colored and have different behaviors.

The females are divided into "orange" and "yellow", with the orange laying many small eggs and being very territorial, and the yellow laying fewer, larger, eggs and are more tolerant of neighbors.

The males are divided into "orange", "yellow", and "blue". Oranges are larger and claim a large territory and many females and are very territorial. Yellows claim no territory or females but sneak into an orange's territory to mate with his females when he is in another part of his (too large to completely guard) territory. Blues claim a small territory and a single female, which he can defend from a sneaking yellow, but cannot prevent a larger orange from taking his female if it makes the attempt.

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    $\begingroup$ Upvote for the side blotched lizard! Intriguing critters. They still only need two to make a baby, however. $\endgroup$ – DrBob Aug 27 '16 at 10:30
  • $\begingroup$ I guess there's nothing that says that in a multi gender species, all genders have to contribute to any given baby is there? Different types of gamete combined in pairs might result in different sexes being born, which produce different again gametes, resulting in a potentially very interesting cycle... $\endgroup$ – GreenAsJade Aug 27 '16 at 12:18
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Each of the genders could contribute less than half of the genetic code needed to make a new critter. In animals on earth, males & females each contribute half of the genetic information (50% in the egg, 50% in the lucky sperm). In your critters, each could contribute 1/4 of the genetic material. Or 1/5th. Or anything you want.

Optionally, there could be one "female" in your species, carrying the egg with 50% of the genetic material and additional nutrients & stuff. There are lots of variations of the "male"; for example, each different type of "male" includes only 10% of the required genetic material in its sperm. A "female" needs to be impregnated by one of each of the 5 types of "males" in order to put together a complete genome. Somehow, by magical science and chemistry and biology, all 5 segments of "male" DNA arrange in the correct order and the correct orientation to create a complete genome.

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Gender is a matter of different cell types and genetic diversity. If there is only one gender, you find that the biodiversity can take a hit. Two genders tends to make sure that one organism can't breed with its (essentially) clones - this is better for the species.

Thus, I give to you the strange world known as Earth and the lowly slime mold.

Beneath the lilies lies a rotting log, home to a tiny organism that isn’t a plant or an animal or a fungus. It looks like a blob of scrambled eggs, but in fact it’s a slime mold (genus Physarum), an otherworldly creature with 29 variants of sex-controlling genes, dispersed among eight different types of sex cells. To ensure genetic diversity, each slime mold sex cell can only fuse with a sex cell that has completely different variants of genes than its own. If you calculate all the possible combinations of genes and sex cells, you will find that Physarum have more than 500 different sexes. Reproduction for slime molds may be complicated, but it’s never boring.

So there you have not sperm and egg, but eight different types of gametes. And not just "X" and "Y" but rather 29 genes.

And this gets you to, 500+ genders. Its not a matter of different binary, but just different being sufficient to have different genders. This isn't any threesome or strange biology. Just that you've got to have this thingy and that thingy be different, provide different genes and combine.

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Don't ask about logic, ask about environmental pressure. Some of the things you mention sound more like job descriptions than actual genders. Are the womb providers and/or nurturers actually structurally different that the ostensible male female roles? or are they just playing a part until their turn to be male/female. (are they even the same species?)

Aside from that though, all we know is DNA/RNA based life. There's two strands, which makes two sexes pretty easy. Not a lot of reason to have more, except maybe redundancy. Even then, though, say you have four participants in a given reproduction, if it comes down to three motile providers and one egg provider (plus any non-genetic roles) it's still just two sexes.

You could possibly work something where one being impregnates another, then that being impregnates another with the resulting cells, which undergo further modification/addition of material. Through a certain number of similar steps. But if all members of the species can perform any of the roles, there is really only one gender, just slightly different [changing] roles in reproduction.

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    $\begingroup$ I was thinking that if the food-producing organ (ie: breasts or teats) resided in a separate gender than the womb-possessing gender, which could again be physically separate from the sperm-donor and even egg-donor, this could create four physically different genders. The two donor types would possibly both be male-like, especially if the womb gender was female-like. The nurturer-type would be the one having the breasts. $\endgroup$ – nijineko Aug 26 '16 at 20:30
  • $\begingroup$ There are some issues with doing things that way, two of the participants are not contributing genetic material. From a "selfish gene" standpoint why are they even there? They might be a symbiotic species, or members of the species in question might change roles over time. How do the non-contributing members propagate THEIR genes? $\endgroup$ – Seeds Aug 26 '16 at 20:40
  • $\begingroup$ It could be that the womb type contributes genetic material in the uterus, and the nurturer-type also contributes genetic material via breast-feeding. This could be a case of one (or two) species hijacking a third, or something more odd. $\endgroup$ – nijineko Aug 26 '16 at 21:20
  • $\begingroup$ Ah, when I say "contribute genetic material" that typically means one half. Since DNA is a double helix, one half comes from each parent. $\endgroup$ – Seeds Aug 26 '16 at 22:04
  • $\begingroup$ Arguably bees fill this egg, sperm, nurturing 3 gender system described. Worker bees nurture larvae and can't reproduce. $\endgroup$ – Donald Hobson Aug 26 '16 at 23:04
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Why would it occur? Because it provides a survival advantage.

So why does it provide a survival advantage?

  • The more diversity in genetic material you have, the more likely you are to be able to survive and reproduce in a difficult environment: eat a different food, make an antidote to a poison, etc.

  • The more parents you have, the more likely you are to be well fed and cared for during infancy. Note that these have to be actual gene-contributing parents.

  • The more copies of genes you have, the more likely you are to survive in a highly mutagenic environment.

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  • $\begingroup$ However, isn't there the possibility that having a higher required number of genetic donors could potentially endanger the species, given some disaster reducing or eliminating a critical gender? $\endgroup$ – nijineko Aug 26 '16 at 20:26
  • $\begingroup$ @nijineko Yes, that's a drawback; that's an evolutionary min/max problem. You'd probably want to be able to 'bank' gametes to make it possible for fertilization to occur over a period of days or months instead of requiring all the parents to be present at one time. $\endgroup$ – John Feltz Aug 26 '16 at 20:29
  • $\begingroup$ if it's not efficient it doesn't mean it won't evolve... evolution is based on what works not what's best. $\endgroup$ – Charon Aug 26 '16 at 21:48
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If you have 4 or 5 genders you may have reproduction that requires something more complicated than a gamete-based method. For example, you could use role types versus gender types and there could group responsibility for nurturing the offspring until its capable of surviving on its own.

For the reproduction part, you would need gene-contributors (what humans would call egg and sperm donors) and you might need more than one of them due to something that would cause certain combinations to be successful and other to not be successful - and then perhaps you would need a "quickener" - a member of the reproductive group that contributes an energy that takes the gene contributions and makes them combine to form the proto-being. Lastly if these beings carry unborn offspring inside their bodies you would need an incubator. All of these factors taken together may result in a litter of offspring rather than one single offspring per reproductive cycle.

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One of my favorite numbers to use after three is four.

Reasons for four:

  • Two entwined species that can't live without one another, each with two genders.

  • Rather than dominate and recessive genes we could use a voting system. 3 of your parents have blond hair so you have blond hair.

  • Four parents with four biologically segregated roles mean there is less work for any one gender.

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  • $\begingroup$ Your first example is two entwined species, not a single species with 4 genders. $\endgroup$ – GreenAsJade Aug 27 '16 at 12:19
  • $\begingroup$ GreenAsJade yes it is. If we can play with gender we can play with species. $\endgroup$ – candied_orange Aug 27 '16 at 14:26
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I'm assuming you mean 'sex' (anatomy & biological function), rather than 'gender' (see Fayth85's answer).

Way back in the 90s, British Telecom was testing code which had 'sexes' and the ones with three sexes habitually came out best. The initial conditions favoured three sexes, so three sexes out-competed all other numbers of sexes.

Sex will evolve waaaaaay back in your planet's history, long before things like wombs and parental care have evolved. It'll happen when your animals and/or plants are just tiny things randomly dumping their equivalent of sperm and eggs into the ocean.

Here are a few science fiction examples which came up with reasons for 3 sexes. SPOILER ALERT - these examples give away plot twists.

The biochemistry rationale. In Vonda McIntyre's Dreamsnake, the snakes have a DNA triple helix (or alien equivalent thereof), so mate in threesomes. Each snake is providing one strand of the DNA when the snakes mate.

This version puts the three sexes at such a fundamental level - the biochemistry - then you could argue that everything on that planet which reproduces sexually has three sexes. It is the default setting for everything (apart from asexually reproducing things).

The quality control rationale. In Stephen Leigh's Dark Water's Embrace, the aliens have males, females and intersexes. The planet is bombared by radiation from its star, and the intersexes act as quality control to keep the mutation rate down. Male mates with intersex. Intersex filters his sperm to remove the defective ones. Intersex mates with female to pass on the good sperm.

This one has to evolve later, once internal fertilisation has been invented. So on Earth most fish and amphibians wouldn't have it (water protects them from radiation anyway?). But reptiles, birds and mammals could develop it.

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Some fungi have thousands of "sexes"

The answer to your question depends on why you mean by a sex. Male and female are dimorphic, and have separate roles in reproductive development that correspond at the fundamental level to micro- and macro- gametes. The female produces fewer, larger, less mobile gametes than contain more nourishment to get the development started; in contrast to the male producing many, small, typically mobile gametes. Everything about the distinction of sexes essentially follow from this starting point.

Fungi do it differently. There's no morphological distinction between the "sexes", or more properly, "mating types", instead cells of the two parents merge directly to form diploid cells that later divide. The importance of the mating types is simply to reduce inbreeding so two individuals of the same mating type can't interbreed. In some branches of the fungal tree species can have thousands of different mating types (i.e. Basidiomycetes).

So, the question of whether you can have more sexes depends on what roles you expect your sexes to take. Are you looking for a system in which >2 individuals are involved in each mating event? Do you require distinct morphology for your sexes?

The suggestions you put forward are implausible on the face of it - there must be individual advantage in every role for the system to persist. One possible way round this if the sexes rather than being fixed as in humans are life stages as in some fish (i.e. angler fish which start male and become female if they grow big enough) or if you imagine that the roles are more closely akin to castes in Eusocial species.

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Gender is a social construct. Sex is a biological contruct.

As is, in the vast majority of species you have a minimum of three sexes: male (sperm produders), female (egg producers), and a sex not readily identifiable as typically male or female (a combination of the two, whether on chromosomal (XX, XY, XXY, XYY, XO, etc.), gonadal (testes, ovaries, womb, etc), or external genetalia (vagina, penis).

Gender (identity) is a spectrum with at the two extremes thereof: masculine and feminine. For example, you could have feminine guys, masculine girls, or essentially any combination of biological sex and gender identity.

Now, having said that, what you are likely talking about is the biology; ergo, sex and sexual dimorphmism. There are dozens of unique approaches I can think of off the top of my head when it comes to procreation and raring offspring.

First, you have the posibilty of a 'slave sex'. Just like with the seahorse and some species of frog, that the egg producer either excretes the eggs (for or post fertilization), it isn't all that hard to believe there would be room in such a setup for a third sex to bring the eggs to term (i.e. guard them, offer nutrients). In fact, certain species of wasp use other species for this (burying the eggs in them so the hatchlings will have food after being born).

Another example? Well, we look again to nature: ants. Sterile females (in some subspecies with wings) versus fertile queens and worker/soldier ants (infertile males) versus drones (fertile males). There you have four biologically divergent sexes in one species, and how they help the species servive (even if the infertile ones cannot procreate).

Another way this can play out? Like with bees. In some subspecies (I'm tempted to say all, but I don't know for sure this is the case) females would suck/eat/drink a special substance (excretion) from the queen bee, keeping them infertile to not over populate the hive. This way, if anything happens to the queen, there is a host of females that can take over. It isn't as dramatic as the ant example, but it would work just the same.

Another, slightly off-topic, example is to show how the parents go about raising the offspring. Black swans help here. A percetage of them form homosexual couples and start a threesome with females just for the eggs. Once she lays, she's kicked out of the nest, and the happy couple go about their lives.

Then we get into 'gender differences' that could help with childrearing. For this we turn to Micronesian with Fa'afafine. The 'third gender'. They help take care of the home, help raise siblings, nieces/nephews, etc.

If you want a more religious reasoning (depends on your species) you can also go with the Hijra of India. They were believed to be holy, conserated to a god that demands the removal of male genetalia as an offering to her.

In the end, it's about what angle you want for these 'extra sexes/genders'.

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Consider two species (species A and species B) that initially develop as symbiotes, each with their own set of genders (let's say two each, A1, A2, B1, and B2). Let's also say that these species are in the vein of waterbears or similar species - good at gene transfer between species. Two such species that operate so closely could reasonably be expected to become very similar due to the gene transfer, but not so similar that they can no longer perform the functions that make them symbiotes.

As this pair of species evolves into higher life forms, their symbiosis might become less and less pronounced; it is presumably useful for an animal to be mostly independent. The one arena in which independence is not a virtue is in fetal development - a fetus has no hope of being independent anyway. So this pair of species might retain only the part of their symbiosis that relates directly to fetal development. Perhaps, for example, each species helps develop the other's immune system. The result might be that the mother, a female of species A, gives birth to a child of species A and another of species B simultaneously.

This isn't, scientifically speaking, a case of four genders; but it's a close enough mimic that I'd imagine that A1, A2, B1, and B2 would be treated as genders in any society that developed. It's also worth noting that if you started with a pair of species that had three genders each, the result would be a "composite" species with six "genders"; throw in a more complex, many-partner form of symbiosis and you could step it up even more.

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