Science My transgender hobbyhorse

[lpetrich]

The New Tree of Eukaryotes: Trends in Ecology & Evolution - eukaryote phylogeny has been a difficult subject, with eukaryotes' overall family tree getting a major revision in the late 1990's from having to work around "long-branch attraction", an effect that can cause confusion in molecular-phylogeny attempts. But the family tree of eukaryotes has stayed broadly stable since then.

To look at a feature related to the evolution of sex, it's rather obvious from that family tree that multicellularity was invented several times among eukaryotes, from lineages of multicelled ones being nestled inside of lineages of one-celled ones. That is also evident from the details of the multicellularity. Reversion to unicellularity happened a few times, however, like in yeast. It's a fungus which has reverted to protist-like status. BTW, "protist" is a "wastebasket taxon", and is only used for convenience, like "reptile" (non-avian, non-mammalian amniote) or "amphibian" (non-amniote tetrapod) or "fish" (non-tetrapod vertebrate) or "invertebrate" (non-vertebrate animal).

A familiar feature of sexual reproduction is sexual dimorphism, differences between the sexes - but that is absent from most one-celled eukaryotes. Instead, they have  Isogamy - What do isogamous organisms teach us about sex and the two sexes? | Philosophical Transactions of the Royal Society B: Biological Sciences - the sexes looking alike and acting alike. Some multicellular organisms also have isogamy, notably fungi and some algae.

Isogamous organisms may have more than two "sexes" or  Mating type - some of them have a large number of them, though only two different ones need to get together.

So the ancestral eukaryote has several lookalike "sexes".

 Anisogamy - differences between the gametes and always only two sexes - evolved several times, and is usually associated with multicellularity. Some gametes store a lot of food - eggs - and some gametes swim to them - sperm. That often leads to larger-scale differences between the sexes:  Sexual dimorphism

 

[lpetrich]

Having separate sexes is for each individual making only one kind of gamete, and is variously called  Gonochorism and  Dioecy. When an individual produces both kinds, it is a  Hermaphrodite

Some flowering plants make separate sexes of flower on the same plant:  Monoecy - male, stamen-only flowers and female, carpel-only flowers. The most familiar state is hermaphrodite flowers, with both stamens and carpels in them.

Changing sex is  Sequential hermaphroditism

Separate sexes and hermaphrodites in a population:  Trioecy

Separate hermaphrodites and males in a population:  Androdioecy

Separate hermaphrodites and females in a population:  Gynodioecy

Hermaphrodite and male flowers on the same plant:  Andromonoecy

Hermaphrodite and female flowers on the same plant:  Gynomonoecy

Hermaphrodite and both sexes of flowers on the same plant:  Hermaphrodite

 

[lpetrich]

In summary, organisms can have populations of

• h
• h, m
• h, f
• h, m, f
• m, f

Flowering plants can grow all these combinations of flowers on the same individual plant. Gymnosperms, like conifers, cycads, and ginkgo, are more limited, with separate sexes of cones, either on the same plant (monoecy) or on separate sexes of plant (dioecy).

Plants also have alternation of generation, between haploid "gametophytes" and diploid "sporophytes". That is most evident in relatively primitive plants, like mosses and ferns. The easily-visible part of a fern is the sporophyte, getting its name from making spores. When a spore lands on suitable ground, it germinates and makes a small green blob that never grows very high, the gametophyte. They reproduce by releasing sperms and eggs, and sperms swim through the ground to the eggs. Once fertilized, the eggs grow into sporophytes.

Seed plants have made the gametophytes very small, only a few cells. Male ones are released as pollen, and female ones stay put. That's a clear case of convergent evolution that could not quite make it to the animal-kingdom state of only the gametes being haploid.

 

[lpetrich]

•  Sex-determination system
• Sex Determination: Why So Many Ways of Doing It? - PMC
• Tackling the diversity of sex determination - PMC

Oodles of them.

The "Why So Many Ways of Doing It?" article mentioned three myths about sex determination:

"Myth 1: Sex is typically determined by X and Y chromosomes" -- as if other mechanisms are aberrations. Although XY is common, other mechanisms are also common. They themselves say: "Myth 1 Revisited—Sex-Determining Mechanisms Are Diverse and Can Evolve Rapidly"

From one of "Why So Many Ways of Doing It?" picture captions,

• Hermaphroditism
  • Simultaneous (both sexes of sex organs together): most flowering plants, gastropods, and earthworms
  • Sequential (sex changers): many fish, some gastropods and plants:
    • Male to female: clownfish
    • Female to male: many wrasses and gobies
• Environmental:
  • Temperature of egg incubation: turtles and some other reptiles
  • Daytime length: marine amphipods and some barnacles
  • Social factors: sexually undifferentiated larvae of the marine green spoonworm (Bonellidae) that land on unoccupied sea floor develop into females (and grow up to 15 cm long), while larvae that come into contact with females develop into tiny males (1–3 mm long) that live inside the female.
• Genotypic
  • Male heterogamety (XX - XY, X0): almost all mammals and beetles, many flies and some fish.
  • Female heterogamety (ZZ - ZW, Z0): birds, snakes, butterflies, and some fish.
  • Haploid-only separate sexes (UV): mosses and liverworts
  • Polygenic (multiple genes): some flowering plants and fish, like zebrafish
  • Haplodiploidy: in hymenopterans (wasps, bees, ants): females are diploid, from fertilized eggs, males are haploid, from unfertilized eggs
  • Paternal genome elimination: males of many scale insects inactivate or lose their paternal chromosomes
  • Cytoplasmic elements:
    • Intracellular parasites: many insects have Wolbachia bacteria that kill, sterilize, or make female their male hosts
    • Mitochondria: many flowering plants
  • Monogeny: in some flies and crustaceans, all offspring of a particular individual female are either exclusively male or exclusively female

Distinct sex chromosomes (Y or W) can range from being hard to distinguish from their counterparts (X or Z) to being greatly degenerated to disappearing outright. Also, environmental features can override genetic features.

In fact, sex determination is a rapidly evolving trait in many lineages (Figure 3), and sometimes closely related species, or populations of the same species, have different modes of sex determination [3],[4],[38]. Houseflies, for example, normally have XY sex chromosomes, but dominant masculinizing and feminizing alleles on other chromosomes exist in some populations that override sex determination by the XY chromosomes [39]. This variety has stimulated investigation into what evolutionary forces drive the turnover of sex determination mechanisms, what molecular mechanisms underlie the different modes of sex determination, and why sex determination is labile in some taxa and not in others.

The authors note that genotypic vs. envrionmental sex determination is a function of what is adaptive.

ESD is favored over GSD when specific environments are more beneficial to one sex [3], selecting for sex-determining mechanisms that match each sex to its best environment. ... The reverse transition, from ESD to GSD, is thought to be favored when the environment is unpredictable or not variable enough, in which case ESD could produce strongly skewed sex ratios or intersex individuals [3].

Also,

In species with genotypic sex determination, the chromosome pair that determines sex can change rapidly over time. Transitions are particularly likely when the ancestral sex chromosome exhibits little genetic differentiation, since WW or YY combinations are then less likely to be lethal (Figure 5).

 

[TomC]

You know @lpetrich

If I understood your last 5 posts I'd probably understand a big question I've had for decades.

"How did sexual reproduction become a thing?"

Obviously*, it explains a ton of stuff. Evolution. Sentient life. Speciation.

Life as we know it.

Life as we know it couldn't exist without sexual reproduction. Single celled organisms could keep fissioning into clones. But they aren't going to do anything important, like develop nervous systems that develops into brains that develops sentience, that develops religion and science and literature and political ideology and...


Tom
* By sexual reproduction I'm talking about the reproductive system where half the population are unnecessary as individuals. They only function as the spreaders of the genes among the organisms that can actually make babies.

 

[blastula]

That some people are born with abnormalities does not change the sex binary

You keep saying "the sex binary" like a religious creed or incantation.

 

[lpetrich]

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

Sex determination in model species suggests that a master-switch gene (e.g. Sry in mammals, Sxl in D. melanogaster, and xol-1 in C. elegans) acts as the main control element to trigger either male or female sexual development. Changes in the sex determination pathways across taxa are assumed to involve adding a new master-switch gene to this molecular pathway (as in some fly taxa; [9]), with little change to downstream elements of the sex determination pathway [10]. A few genes are thought to have the capacity to take on the role of sex determination genes, and these have been co-opted as master-switch genes independently in different lineages (for example, dmrt1 in several vertebrates [11]–[14] and tra in insects [15]–[17]).

The next question is how conserved is the rest of the sex-determination mechanisms.

They themselves note "Myth 2 Revisited—Multiple and Various Genes Can Determine Sex"

The pathways that control sexual development have been well characterized at the molecular level in D. melanogaster, C. elegans, and mammals. All three involve a master-switch sex-determining gene, which led to the birth of Myth 2. Although the simplicity of a single master-switch is alluring, this archetype of sex determination is clearly not universal. Below we discuss systems where sex is determined by multiple genes, recent molecular data on the nature and evolution of sex-determining genes, and how sex determination can vary in different parts of the body.

...
In some species, sex determination is polygenic. For example, in zebrafish (Danio rerio), a key developmental model organism, sex is not controlled by a single master regulator but is instead a quantitative threshold trait with either a male or female outcome, which is determined by multiple regions in the genome [69]–[71]. While some of those regions contain genes known to play a role in sex determination in other organisms [70], there is an enduring mystery as to how these multiple loci and the environment interact to control downstream sexual differentiation in zebrafish.

...
Some taxa have master-switch sex-determining genes that are highly conserved, such as the Sry gene in nearly all mammals [77]. In other lineages, such as fish from the genus Oryzias [78]–[80], the master-switch gene differs among closely-related species (Table 1).

...
These data suggest that there are constraints on the types of genes that can be co-opted as master sex determination genes [81]. Nevertheless, there are several cases of switch genes with no homologs in closely related taxa.

...
No master sex determination gene has been identified in dioecious plants, but genes that affect flower sex determination have been found [86],[87]. Indeed, many genes may serve as potential targets for sex determination in plants, given that male or female sterility can evolve in various ways [86].

For example, rice plants have 227 known male-sterility genes, at least one on each of the plant's 12 chromosomes.

 

[lpetrich]

In sharp contrast with the diversity of primary sex-determining signals, some key regulatory genes play conserved roles in the molecular pathways leading to male or female gonad development across invertebrates and vertebrates, such as the doublesex-mab3 (DM) family genes [89],[90].

Where these genes are found:

• Mammals, birds, turtiles, alligators -- aminotes
• Amniotes, frogs -- tetrapods
• Tetrapods, teleost fish -- bony vertebrates -- vertebrates -- chordates -- deuterostomes
• Coleoptera (beetles), Strepsiptera, Hymenoptera (wasps, bees, ants), Diptera (flies, mosquitoes), Lepidoptera (moths, butterflies) - Endopterygota (four-stage insects)
• Endopterygota, Hemiptera, Phthiraptera (lice) -- Neoptera (folding-wing insects) -- Insecta
• Insects, Branchiopoda (Daphnia shrimp), Malacostraca (Chinese mitten crab, Eriocheir sinensis) -- Pancrustacea -- arthropods
• Arthropods, nematode Caenorhabditis elegans -- Ecdysozoa (molting animals)
• Gastropods (Pacific abalone, Haliotis asinina), bivalves (Pacific oyster, Crassostrea gigas) -- mollusks -- Lophotrochozoa
• Ecdysozoa, Lophotrochozoa -- Protostomia
• Deuterostomia, Protostomia -- Bilateria
• Bilateria, Cnidaria (coral Acropora millepora) -- ParaHoxozoa

Comb jellies also have gonads, but sea sponges don't.

Plants have gonads, even if they are not usually called gonads. For flowering plants, the male gonads are anthers, at the outer ends of stamens, and the female gonads are ovules, inside of pistls. For more primitive plants, the gonads are antheridia (male) and archegonia (female).

 

[bilby]

By sexual reproduction I'm talking about the reproductive system where half the population are unnecessary as individuals.

No such system exists.

All of the population are unnecessary as individuals, unless you take a humanistic view, in which case, none are.

Reproduction doesn't even come in to it. it has exactly zero to do with individual worth, or individual necessity.

 

[lpetrich]

Sex determination can also differ with respect to where in the body sex is determined. In humans, sex is determined in the developing gonad, and gonadal sex hormones in turn trigger sex determination and differentiation in nongonadal tissues. By contrast, in birds, Drosophila, and nematodes [106]–[109], sexual differentiation is a cell-autonomous process, although secreted signaling molecules can play a role in generating sexual dimorphism in somatic tissues. Studies in Drosophila have shown that only a subset of cells express the genes of the sex determination cascade and have a sexual identity [106]. Cell-autonomous sex determination can result in the formation of gynandromorphs—individuals that contain both male and female characteristics, found in birds and many insects, including butterflies and beetles. Sex determination can also be regulated differently in the soma versus the germ line of the same species [110],[111]. In houseflies [112] and some frogs [113] and fish [114]–[116], transplantation experiments indicate that the sex of germ cells depends on the surrounding soma, i.e., XX germ cells transplanted into male soma form sperm, and XY germ cells transplanted in a female soma form oocytes. In contrast, germ cells in Drosophila [117] and mammals [118] receive signals from the surrounding somatic gonad, but they also make an autonomous decision during germ line sexual development; this may also be true for chickens [107]. In these animals, the “sex” of the soma must match the “sex” of the germ cells for proper gametogenesis to occur. If XX germ cells are transplanted into male soma they do not form sperm, and XY germ cells transplanted into female soma fail to form oocytes.

Then a lot of discussion of what can happen to sex chromosomes. Do they become distinct? Do they degenrate or disappear? Sometimes, sometimes not to both questions.

 

[lpetrich]

 Sexual differentiation in humans
Sexual Differentiation - Endotext - NCBI Bookshelf

Human embryos develop as female unless the SRY gene is present. This is the sex-determining Y gene or the testis-determining factor gene. SRY's protein causes the gonads to become male, and the gonads then make the rest of the body follow. An SRY gene can jump onto an X chromosome, making XX males, and an absent SRY gene will cause XY females,

This mechanism is shared by other therians (placentals + marsupials) and had its form for the last 160 million years. Monotremes have a different system. The duckbill platypus has five XY pairs, none of them homologous with the therian one, and without SRY. The echidna or spiny anteater has the same system, but with only four Y's. The remaining Y is fused with one of the others. The multiple sex chromosomes of platypus and echidna are not completely identical and several share homology with the avian Z | Genome Biology | Full Text

A brief review of vertebrate sex evolution with a pledge for integrative research: towards ‘sexomics’ - PMC

Sturgeon fish are the champion with a presumed ZW system that is 180 million years old, older than the therian XY system. Birds are all ZW, but the oldest divergences in them are a little before the K-Pg mass extinction. The branching of the bird family tree before that disaster was:

• Palaeognathae - ratites (ostrich, emu, rhea, cassowary, kiwi, tinamous, moa*, elephant bird*)
• Neognathae
  • Galloanserae - Galliformes (chicken, turkey, pheasant, quail, ...) and Anseriformes (goose, duck, swan, ...)
  • Neoaves - all other present-day birds: they had some fast divergence after that disaster

* = recently extinct

Their closest living relatives are crocodilians, splitting around 250 million years ago, not long after the P-Tr mass extinction. Crocodilians' sex determination is by temperature: below 30 C, all female, above 34 C, all male. On the effect of temperature-dependent sex determination on sex ratio and survivorship in crocodilians | Proceedings of the Royal Society of London. Series B: Biological Sciences

Sex determination and sexual differentiation in the avian model - PubMed - chickens, the most-researched bird species

Most recently, it has been shown that sex is cell autonomous in birds. Evidence from gynandromorphic chickens (male on one side, female on the other) points to the likelihood that sex is determined directly in each cell of the body, independently of, or in addition to, hormonal signalling. Hence, sex-determining genes may operate not only in the gonads, to produce testes or ovaries, but also throughout cells of the body.

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