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Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnan

Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnant.

dmos62 6y ago
Sorry for asking a simple question, but how do we distinguish sexes? If male seahorses can be pregnant, it's obviously not by which sex gives birth.
- goalieca 6y ago
  I am not a biologist but what I remember from biology is that female seahorses have the eggs. Sexual reproduction happens when two haploid cells combine and the sex is determined based on this system.
- chongli 6y ago
  It’s entirely determined by the gametes. Males produce sperm, females produce eggs.
  - jessriedel 6y ago
    And the thing that distinguishes eggs from sperm is what? The relative size? (Are there no species where they are roughly the same?) Or that the mitochondria come from the egg? Edit: Looks like it's merely relative size, and when neither gamete is bigger biologists throw up their hands. Sort of a bummer that there isn't a more robust definition. https://en.wikipedia.org/wiki/Isogamy https://en.wikipedia.org/wiki/Isogamy
    - chongli 6y ago
      The relative size but also the structure and the process of fertilization. The sperm enters the egg, not the other way around. Mitochondria come from the egg, yes. Also note that sperm vary in shape. Mammalian sperm have the familiar tadpole shape with a flagellum for swimming. Other species, such as nematodes, have ameboid sperm which move along surfaces rather than swimming freely.
      - jessriedel 6y ago
        > The sperm enters the egg, not the other way around. I think this is wrong. My understanding is that the sperm cell membrane fuses with the egg cell membrane, forming a single container. The haploid nucleus of the sperm and the haploid nucleus of the egg then fuse. This processes is by no means fully symmetric (the egg has a tough coating on its cell membrane that must be dissolved by the sperm through the Acrosome reaction), but there is also no time when the sperm cell (delineated by its membrane) is inside the egg cell. https://en.wikipedia.org/wiki/Acrosome_reaction https://en.wikipedia.org/wiki/Acrosome_reaction My point was not that there aren't many correlated asymmetries between eggs and sperm in particular parts of the phylogenetic tree (like mammals) that can be used for distinguishing them. My point was that there seems to be no fundamental asymmetry that applies to all two-sex sexually reproducing organisms; rather, there is a continuum of degree of gamete dimorphism that ends at organisms where there is no principled distinction between sperm and egg (isogamy).
        f6v 6y ago
        I red this in some book I forgot the name of, so I can’t cite anything. But it was explained that female strips everything except for the DNA from sperm. This way there’s minimal risk of infecting the egg with the pathogens the sperm can carry. I’m not sure if it’s true just for humans or other organisms as well.

Seahorses are also interesting in this regard, not only do they also have internal pregnancy and a placenta-like structure, it's also the males that get pregnant.

dmos62 6y ago

Sorry for asking a simple question, but how do we distinguish sexes? If male seahorses can be pregnant, it's obviously not by which sex gives birth.

goalieca 6y ago
I am not a biologist but what I remember from biology is that female seahorses have the eggs. Sexual reproduction happens when two haploid cells combine and the sex is determined based on this system.
chongli 6y ago
It’s entirely determined by the gametes. Males produce sperm, females produce eggs.
- jessriedel 6y ago
  And the thing that distinguishes eggs from sperm is what? The relative size? (Are there no species where they are roughly the same?) Or that the mitochondria come from the egg? Edit: Looks like it's merely relative size, and when neither gamete is bigger biologists throw up their hands. Sort of a bummer that there isn't a more robust definition. https://en.wikipedia.org/wiki/Isogamy https://en.wikipedia.org/wiki/Isogamy
  - chongli 6y ago
    The relative size but also the structure and the process of fertilization. The sperm enters the egg, not the other way around. Mitochondria come from the egg, yes. Also note that sperm vary in shape. Mammalian sperm have the familiar tadpole shape with a flagellum for swimming. Other species, such as nematodes, have ameboid sperm which move along surfaces rather than swimming freely.
    - jessriedel 6y ago
      > The sperm enters the egg, not the other way around. I think this is wrong. My understanding is that the sperm cell membrane fuses with the egg cell membrane, forming a single container. The haploid nucleus of the sperm and the haploid nucleus of the egg then fuse. This processes is by no means fully symmetric (the egg has a tough coating on its cell membrane that must be dissolved by the sperm through the Acrosome reaction), but there is also no time when the sperm cell (delineated by its membrane) is inside the egg cell. https://en.wikipedia.org/wiki/Acrosome_reaction https://en.wikipedia.org/wiki/Acrosome_reaction My point was not that there aren't many correlated asymmetries between eggs and sperm in particular parts of the phylogenetic tree (like mammals) that can be used for distinguishing them. My point was that there seems to be no fundamental asymmetry that applies to all two-sex sexually reproducing organisms; rather, there is a continuum of degree of gamete dimorphism that ends at organisms where there is no principled distinction between sperm and egg (isogamy).
      - f6v 6y ago
        I red this in some book I forgot the name of, so I can’t cite anything. But it was explained that female strips everything except for the DNA from sperm. This way there’s minimal risk of infecting the egg with the pathogens the sperm can carry. I’m not sure if it’s true just for humans or other organisms as well.