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Old 11 Nov 2017, 10:54 PM   #679965 / #1
lpetrich
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Default Sex-Determination Mechanisms

What makes organisms one sex or the other? A lot of different mechanisms.

Prokaryotes reproduce asexually, and the closest that they come to sex is injecting genetic material into each other.

Many eukaryotes have a cell cycle of haploid - cell fusion - diploid - meiosis - haploid again. In the more primitive ones, a haploid one can only fuse with a haploid one with a different "mating type", a different version of some surface protein. Some species have large numbers of mating types. However, in many cases, organisms with different mating types all look alike: isogamy.

But mating types can get specialized, something called anisogamy. In animals and plants and some other organisms, it becomes oogamy, egg and sperm cells. One kind of cell gets specialized for food storage, the egg cell, and the other kind gets specialized for traveling to egg cells, the sperm cell.

But after that point, there are numerous mechanisms for deciding when to make egg cells and when to make sperm cells. PZ Myers has made a video about them: A little video about sex determination, and you can see more of what he has made at PZ Myers - YouTube - YouTube

He showed off a picture from Sex Determination: Why So Many Ways of Doing It?, a picture that illustrated the numerous different ways.

Tree of Sex: a database of eukaryotic sex determination systems… Currently with lists of sex-determination mechanisms for vertebrates, invertebrates, and plants.
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Old 11 Nov 2017, 11:48 PM   #679966 / #2
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That PLoS article has a picture that illustrates numerous mechanisms.

Some organisms are simultaneous hermaphrodites, having both sexes of reproductive organs. That's very common in flowering plants, gastropods, and earthworms.

Some are sequential hermaphrodites, or sex changers. Like many fish. Some, like clownfish, are male-to-female, while others, like many wrasses and gobies, are female-to-male.

The movie "Finding Nemo" anthropomorphized clownfish there. According to Clownfish Family Values, Coral was female because she was the biggest fish at her family's sea-anemone home. When she died, the next biggest, Marlin, changed sex to become Marla.

Environmental cues can also determine sex, like temperature. That is common among reptiles.

Marine spoonworms have a larval phase where they swim around looking for a place to stay. If they find one without other spoonworms nearby, they become female. But if there is another spoonworm nearby, they turn male. The females grow much bigger than the males, and the males live inside them, going further than deep-sea-anglerfish males.

There are a variety of genetic mechanisms. The XY one, with the male one different, is done by most mammals, most beetles, many flies, and some fish. The ZW one, with the female one different, is done by most birds, most snakes, most butterflies, and some fish. Sometimes the different chromosome drops out, giving the X0 and the Z0 mechanisms.

Mosses and liverworts have different sexes only in their haploid phases: U and V chromosomes. In some flowering plants and fish, like zebrafish, more than one gene determines sex, with the organism's sex being determined by which sex is a majority among them.

Among hymenopterans, wasps ants bees, a fertilized egg develops into a female while an unfertilized one develops into a male. This is haplodiploidy, and females are diploid and males haploid. Males of many species of scale insects knock out their paternal chromosomes as embryos, something rather Freudian.

Cytoplasmic elements determine sex in some cases. In many flowering plants, mitochondria determine sex, while in many insects, parasitic bacteria like Wolbachia do so.

Then monogeny. All offspring of some female are one sex or the other.


What these various mechanisms do for their users is not very clear, at leas tot me. The authors of the PLoS paper do not get into that very much, but they do discuss a little bit how organisms can change from one mechanism to another. They conclude that some mechanisms are essentially traps, difficult to evolve out of. Like XY and ZW and haplodiploidy.
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Old 12 Nov 2017, 12:05 AM   #679968 / #3
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It is worth noting that sex differences are almost always thought of (by us) with a bit of anthromorphization. With so many varieties on the reproductive mechanism, trying to split species into "male" and "female" or "hermaphrodite" based on whether they are currently capable of producing or fertilizing eggs (regardless of whether this might change at some other point in their lifespan) is more like an analogy to human sexual expression than a good descriptor of what the life of a frog or a fish or a chrysanthemum is really like.

Marlin isn't going to become a "mother" to Nemo and have "incestuous" children with him, as the article alleges, for instance; that is just replacing one kind of anthropomorphism with another, as neither of those terms would mean anything to a clownfish anymore than "depressed and anxious father" would. They do not have a large emotional range (beyond being seemingly more relaxed when safe, fed, and in the company of their kind) and the values of an American-style nuclear family have no particular place to attach to the facts their lives. They are simply very different from us, and were they able to express it in terms like our own, would probably do so in fundamentally different ways. I love books that take species differences seriously- Bakker's "Raptor Red" and Holling's "Pagoo" were favorites of mine as a youth.
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Old 12 Nov 2017, 01:16 AM   #679972 / #4
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A science video about gonad development by PZ on human gonads.
Also from PZ, Building a sex is harder than most people imagine using a diagram from Beyond XX and XY: The Extraordinary Complexity of Sex Determination - Scientific American -- about human sex determination, mostly.

A rather complicated cascade. It starts with XX vs. XY and the SRY gene being present or absent. That gene induces the gonads to develop in male fashion, and if absent, they develop in female fashion. The gonads then make hormones that shape the genitalia to be in one sex or the other, and also the secondary sexual features.

If something goes wrong somewhere, it can produce intersex features like ambiguous genitalia or development much like the "wrong" sex. For instance, someone with XY chromosomes but with defective androgen receptors may develop in mostly-female fashion.

An additional oddity is development of sexual preference and gender identity. How do we know which sex to have a sexual attraction to? Is it genetic? Is it some sort of imprinting? Is there something pheromonal involved? Likewise for which sex to identify as.
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Old 12 Nov 2017, 01:52 AM   #679973 / #5
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Quote:
Originally Posted by Politesse View Post
It is worth noting that sex differences are almost always thought of (by us) with a bit of anthromorphization. With so many varieties on the reproductive mechanism, trying to split species into "male" and "female" or "hermaphrodite" based on whether they are currently capable of producing or fertilizing eggs (regardless of whether this might change at some other point in their lifespan) is more like an analogy to human sexual expression than a good descriptor of what the life of a frog or a fish or a chrysanthemum is really like.
So you think that the terms "female" and "male" are too loaded with human stereotypes?

Would you prefer something like "egg maker" for "female", "sperm maker" for "male", and "both-gametes maker" for "hermaphrodite"?
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Old 12 Nov 2017, 02:48 AM   #679976 / #6
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Originally Posted by Politesse View Post
It is worth noting that sex differences are almost always thought of (by us) with a bit of anthromorphization. With so many varieties on the reproductive mechanism, trying to split species into "male" and "female" or "hermaphrodite" based on whether they are currently capable of producing or fertilizing eggs (regardless of whether this might change at some other point in their lifespan) is more like an analogy to human sexual expression than a good descriptor of what the life of a frog or a fish or a chrysanthemum is really like.
So you think that the terms "female" and "male" are too loaded with human stereotypes?

Would you prefer something like "egg maker" for "female", "sperm maker" for "male", and "both-gametes maker" for "hermaphrodite"?
I don't think the terminology is a problem per se; it seems very likely to me that such anthropomorphizations would occur no matter what you called them. Your students would be calling your sperm-makers "boys" and egg-makers "girls" and referring to them with gendered assumptions the second the connection occurred to them. I recall a study in which users spoke aloud to blue tinted iMacs with a completely different discourse than to pink tinted ones, owing to the perceived gender of the obviously inanimate and otherwise unmarked machines. Socialization and attributed stances are powerful cognitive forces.
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Old 13 Nov 2017, 02:54 PM   #680119 / #7
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Quote:
lpetrich:
Some organisms are simultaneous hermaphrodites, having both sexes of reproductive organs. That's very common in flowering plants, gastropods, and earthworms...

Mosses and liverworts have different sexes only in their haploid phases...
Very cool topic, I love explaining to students how many different ways there are to approach sexual reproduction. I have read that fungal infection can determine sex in some insects, but I cannot find a reference just now. Sex determination in the platypus deserves mention, there are ten sex chromosomes in the adult: males are XYXYXYXYXY while females are XXXXXXXXXX.

Note that, strictly speaking, typical flowering plants are not hermaphrodites. The familiar life history stage of a typical flowering plant is a sporophyte, it produces spores rather than gametes. These spores may grow into the gamete-producing stage, the gametophytes (pollen and embryo sac). This is actually the same pattern as mosses and liverworts have, except that in those plants the gametophytes are the dominant stage of the life history.

To pick another nit, while most terrestrial gastropods are hermaphrodites, this is not true of marine gastropods. Other groups of animals that are commonly hermaphroditic include flatworms (e.g., Planaria, flukes, tapeworms), Cnideria (sea anemones, coral, sea jellies), and sponges.

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Old 13 Nov 2017, 09:12 PM   #680149 / #8
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Note that, strictly speaking, typical flowering plants are not hermaphrodites. The familiar life history stage of a typical flowering plant is a sporophyte, it produces spores rather than gametes. These spores may grow into the gamete-producing stage, the gametophytes (pollen and embryo sac). This is actually the same pattern as mosses and liverworts have, except that in those plants the gametophytes are the dominant stage of the life history.
However, flowering plants' gametophytes are very vestigial, being only a few cells.

Back to that PLoS article, Sex Determination: Why So Many Ways of Doing It?, it lists some myths about sex determination and rebuttals to them.

Quote:
Myth 1: Sex is typically determined by X and Y chromosomes
...
Myth 1 Revisited—Sex-Determining Mechanisms Are Diverse and Can Evolve Rapidly
Some biologists seem to consider alternatives to XY to be aberrations.

Quote:
Myth 2: Sex is controlled by one master-switch gene

Myth 2 Revisited—Multiple and Various Genes Can Determine Sex
Has a big list of known ones. Some of them are recognizable as co-opted copies of other genes, though others' origins are obscure.

But some genes involved in gonad development are highly conserved, like doublesex-mab3 (DM) family genes in the animal kingdom. They can be found in mammals, birds, turtles, alligators, amphibians, fish, Drosophila fruit flies, hymenopterans, crustaceans, and mollusks.

So it's what's upstream of those genes that is so wildly variable.

Quote:
Myth 3: Sex chromosome differentiation and degeneration is inevitable

Myth 3 Revisited—Sex Chromosomes' Eternal Youth
Though many sex chromosomes have indeed degenerated, they can nevertheless be stable in that state.
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Old 14 Nov 2017, 12:41 AM   #680168 / #9
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Some possible adaptive value of having different mechanisms is as (Wikipedia)Reproductive isolation. Organisms have numerous mechanisms, and that article lists
  • Prezygotic -- before fertilization
    • Temporal or habitat isolation
    • Behavioral isolation
    • Mechanical isolation
    • Gametic isolation
  • Postzygotic -- before fertilization
    • Zygote mortality and non-viability of hybrids
    • Hybrid sterility
Many organisms use more than one mechanism.

So sex-determination changes could function as a postzygotic mechanism.

Sex variation with temperature suggests that this may be some cue for producing one sex or the other, as is convenient. Thus, it may be a form of environmental sex determination.

I've found Evolution of Haplodiploidy: Models for Inbred and Outbred Systems describing various theories. It's easiest to make haplodiploidy in very inbred populations.

I've also found Evolution of Haplodiploidy in Dermanyssine Mites (Acari: Mesostigmata) on JSTOR
Quote:
Haplodiploidy, a widespread phenomenon in which males are haploid and females are diploid, can be caused by a number of different underlying genetic systems. In the most common of these, arrhenotoky, males arise from unfertilized eggs, whereas females arise from fertilized eggs. In another system, pseudoarrhenotoky, males arise from fertilized eggs, but they eliminate the paternal genome at some point prior to spermatogenesis, with the consequence that they do not pass this genome to their offspring. In 1931 Schrader and Hughes-Schrader suggested that arrhenotoky arises through a series of stages involving pseudoarrhenotokous systems such as those found in many scale insects (Homoptera: Coccoidea), however, their hypothesis has been largely ignored. We have used a phylogenetic analysis of 751 base pairs of 28S rDNA from a group of mites (Mesostigmata: Dermanyssina) that contains arrhenotokous, pseudoarrhenotokous, and ancestrally diplodiploid members to test this hypothesis. Neighbor-joining, maximum-parsimony, and maximum-likelihood methods all indicate that the arrhenotokous members of this group form a clade that arose from a pseudoarrhenotokous ancestor, rather than directly from a diplodiploid one. This provides unequivocal support for the hypothesis of Schrader and Hughes-Schrader. The wider implications of this result for the evolution of uniparental genetic systems are discussed.
Diplodiploid = "normal": both sexes diploid

Pseudoarrhenotokous = a male deactivates or destroys his father's chromosomes, something that seems to me much like the Oedipus complex in Freudianism.

Arrhenotokous = males are haploid

Stephen Jay Gould once wrote an essay called "Death Before Birth, or a Mite's Nunc Dimittis", about how some mites have a phase of living inside their mothers. In the mite genus Adactylidium, an impregnated female finds a thrips egg and lives off of it for the rest of her life. She has about 5 to 8 female offspring and only 1 male one. They all grow up inside her, and the male fertilizes his sisters. They emerge, with the females looking for thrips eggs and the male soon dying.

Acarophenax tribolii has a similar lifestyle, but the single male lives out his entire life inside his mother, alongside some 14 or 15 sisters.
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Old 14 Nov 2017, 04:04 PM   #680243 / #10
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Note that, strictly speaking, typical flowering plants are not hermaphrodites. The familiar life history stage of a typical flowering plant is a sporophyte, it produces spores rather than gametes. These spores may grow into the gamete-producing stage, the gametophytes (pollen and embryo sac). This is actually the same pattern as mosses and liverworts have, except that in those plants the gametophytes are the dominant stage of the life history.
However, flowering plants' gametophytes are very vestigial, being only a few cells.
Indeed, and this makes them easy to miss.

Quote:
Back to that PLoS article, Sex Determination: Why So Many Ways of Doing It?, it lists some myths about sex determination and rebuttals to them.

Quote:
Myth 1: Sex is typically determined by X and Y chromosomes
...
Myth 1 Revisited—Sex-Determining Mechanisms Are Diverse and Can Evolve Rapidly
Some biologists seem to consider alternatives to XY to be aberrations.

Quote:
Myth 2: Sex is controlled by one master-switch gene

Myth 2 Revisited—Multiple and Various Genes Can Determine Sex
Has a big list of known ones. Some of them are recognizable as co-opted copies of other genes, though others' origins are obscure.

But some genes involved in gonad development are highly conserved, like doublesex-mab3 (DM) family genes in the animal kingdom. They can be found in mammals, birds, turtles, alligators, amphibians, fish, Drosophila fruit flies, hymenopterans, crustaceans, and mollusks.

So it's what's upstream of those genes that is so wildly variable.

Quote:
Myth 3: Sex chromosome differentiation and degeneration is inevitable

Myth 3 Revisited—Sex Chromosomes' Eternal Youth
Though many sex chromosomes have indeed degenerated, they can nevertheless be stable in that state.
Are any of those really myths, or is the idea that they are myths actually a myth? I would grant that it is not uncommon for non-biologists to think that "XY" is the standard for males while "XX" is the standard for females, but these are usually the people who don't know that females are generally larger than males, or that sexual reproduction does not require males or females, etc. I certainly don't remember a biologist ever thinking that the "XY" vs "XX" sex determination system is usual outside of mammals.

Regarding a 'master switch gene', I suspect that a 'master switch chromosome' is more likely to be blamed for sex determination. A more likely myth is that the sex chromosomes carry only (or even mainly) genes related to sex determination.

Although concerns have been raised about the potential for the loss of the "Y" chromosome over evolutionary time, I don't know how widespread this myth might be.

There is definitely no shortage of myths about sex and sex determination, then there is genetics...

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Old 16 Nov 2017, 01:34 AM   #680366 / #11
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Stephen Jay Gould wrote "Quick Lives and Quirky Changes", collected in "Hen's Teeth and Horse's Toes", about an odd lifestyle that gives a clue to the origin of haplodiploidy.

Histiostoma murchiei mites parasitize earthworm egg cocoons, and they have some interesting life-cycle features. A female spends much of her life as a "hypopus", a phase where she looks for egg cocoons to parasitize and grow up in. When she first lays eggs, they hatch into some 2 to 9 males. These males then mate with her and then soon die. Then she lays some 500 eggs, and they hatch into females -- and hunt for more cocoons.

The difference? These mites are haplodiploid, and a mite first lays unfertilized eggs, which become male, and then fertilized eggs (fertilized by her sons!), which become female.

SJG proposed that haplodiploidy evolved because it is helpful for "colonizers", organisms whose preferred habitats are good food sources but widely scattered and hard to find. Once one finds a food source, one must reproduce as fast as one can, even if it means doing Oedipus-style reproduction.

This then led to eusociality evolving several times in hymenopterans, because it makes worker ones more closely related to each other than to their mothers. Meaning that workers have a genetic tendency to help produce more workers.

Eusociality has also evolved in termites, but termite workers and termite resident reproducers come in both sexes. Among hymenopterans, they are all female, with the males only trying to mate with queens.

When a drone or male honeybee mates with a queen one, he sticks an "endophallus" into her, semen and all. When he tries to fly off, it rips his internal organs so much that he soon dies. The next male must then remove that endophallus before continuing, making it a sort of chastity belt. Queen honeybees can store the semen that they acquire for a long time, producing lots of workers with it.
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Old Yesterday, 03:09 PM   #680497 / #12
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Stephen Jay Gould wrote "Quick Lives and Quirky Changes", collected in "Hen's Teeth and Horse's Toes", about an odd lifestyle that gives a clue to the origin of haplodiploidy.

Histiostoma murchiei mites parasitize earthworm egg cocoons, and they have some interesting life-cycle features. A female spends much of her life as a "hypopus", a phase where she looks for egg cocoons to parasitize and grow up in. When she first lays eggs, they hatch into some 2 to 9 males. These males then mate with her and then soon die. Then she lays some 500 eggs, and they hatch into females -- and hunt for more cocoons.

The difference? These mites are haplodiploid, and a mite first lays unfertilized eggs, which become male, and then fertilized eggs (fertilized by her sons!), which become female.

SJG proposed that haplodiploidy evolved because it is helpful for "colonizers", organisms whose preferred habitats are good food sources but widely scattered and hard to find. Once one finds a food source, one must reproduce as fast as one can, even if it means doing Oedipus-style reproduction.

This then led to eusociality evolving several times in hymenopterans, because it makes worker ones more closely related to each other than to their mothers. Meaning that workers have a genetic tendency to help produce more workers.

Eusociality has also evolved in termites, but termite workers and termite resident reproducers come in both sexes. Among hymenopterans, they are all female, with the males only trying to mate with queens.

When a drone or male honeybee mates with a queen one, he sticks an "endophallus" into her, semen and all. When he tries to fly off, it rips his internal organs so much that he soon dies. The next male must then remove that endophallus before continuing, making it a sort of chastity belt. Queen honeybees can store the semen that they acquire for a long time, producing lots of workers with it.
Thanks for sharing that, those mites are new to me.

The relatedness of bee females is interesting and important in the evolution of eusocial systems, but it is not the relatedness of workers to each other that matters per se. This is an example of kin selection, in which selection favours sacrifice for another organism if the benefit outweighs the sacrifice. The benefit depends on two factors, the relatedness of that organism, and the increase in reproduction by that organism due to the sacrifice made. Since worker bees do not reproduce, sacrificing for them yields no benefit regardless of the relatedness. However, the workers are also related to the same degree with sisters who are new queens, so each queen that the colony produces carries more of a worker's DNA than their hypothetical daughter would.

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Old Yesterday, 08:47 PM   #680528 / #14
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All that Oedipal mating would give conservative Christians conniptions! I also came across Wasmannia auropunctata (electric ant, little fire ant): This is a small social ant in which the workers are diploid sterile females that develop from fertilized eggs. Queens develop from unfertilized diploid eggs that form from the fusion of two 'immature eggs' (secondary oocytes or ootids). Haploid drones develop from fertilized eggs in which the maternal chromosomes have been destroyed, leaving only paternal DNA in the nucleus.

Peez

PS: Bonellia viridis (green spoonworm) larvae are sexually undifferentiated. If they contact a female they becomes male, if they land on the sea floor they become female.
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