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Biology: Why does nature prefer two sexes?

Introversion

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Some species have the equivalent of many more than two sexes, but most do not. A new model suggests the reason depends on how often they mate.

Quanta Magazine said:
We tend to think about two biological sexes: male and female. But before the evolution of eggs and sperm — before sex cells began to diverge in size and form — organisms couldn’t be classified by sex. The same holds true for many fungi, algae and protozoans today. Instead of sexes, these species have mating types, with sex cells that differ at the molecular level but not anatomically. And those mating types don’t necessarily come in pairs.

Take the social amoeba Dictyostelium discoideum, which has three: Each type can mate with members of the other two. Coprinellus disseminatus, a white-capped mushroom, has 143, each able to find a partner among the 142 others. The hairy, fan-shape fungus Schizophyllum commune boasts more than 23,000 mating types (though its more intricate reproductive strategy means that not every type can mate with every other).

Yet most species still have only two mating types. George Constable, a research fellow at the University of Bath, and Hanna Kokko, an evolutionary biologist at the University of Zurich, wanted to know why. In a paper published last month in Nature Ecology & Evolution, they developed a model that predicts how many mating types will emerge in a species based on just three fundamental ecological elements: the mutation rate (which introduces new types), the population size and — perhaps most surprisingly — the frequency of sex. Their work not only provides insights about the basic biology of these kinds of organisms, but could also contribute to our understanding of how the male and female sexes ultimately evolved.

Many scientists believe mating types evolved early in life’s history as a barrier against behaviors like inbreeding that might be harmful to a population or species. If an organism has sex with an incompatible mating type (including its own mating type), then the union generally produces no offspring.

That restriction aside, logic suggests that species should benefit from having as many mating types as possible. With two types, only half the population is eligible as a mate for any individual. With three, that rises to two-thirds — and so on as more mating types join the mix. Should a mutation lead to the appearance of a new type, it wouldn’t be stuck with the problem of finding a rare match for itself in the population; instead, it would be able to mate with everyone else, thereby producing offspring more quickly and growing its numbers.

“The intuitive expectation is that this should happen for larger and larger numbers of mating types, until you have thousands of them,” Constable said.

To date, the hypotheses about why the number of mating types only rarely soars to enormous heights revolve around considerations of stability. Maintaining just two types may be the better way to go: It allows for simpler, more efficient pheromone-signaling networks, and for an easier sorting system when it comes to passing on organelles from parent to offspring cells. But these theories don’t account for a slew of exceptions.

Then something occurred to Constable. “I realized that we’d been assuming that these species have sex all the time,” he said. That assumption made a huge difference in his predictions about how populations would evolve, because during periods without sex, mating type becomes a neutral trait: Chance events dictate the dominance of some types and the disappearance of many others.

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DrDLN

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Bacteria, viruses and even unicellular organisms have not distinct male and female as in advanced forms. Species in advanced form breed with its own and maintain distinction....lol
 

shadowsminder

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The model in the article is interesting as a test of how well a binary classification generally applies.

“Researchers [can be] a bit myopic when it comes to understanding diversity. Not all life obeys the most familiar rules,” Kokko wrote in an email.

This is a good point, too. I wonder how much data researches consider when in discussions of sex classifications. For humans, we see more variances of the sex chromosomes the more data that's compiled. Seeing our species described as if there are only two sexes despite modern discussions of sex classification is somewhat strange. Intersex (of course, not all) people are capable of reproducing. Horizontal genetic transfer is a thing. Spontaneous sex change in higher animals such as chickens is well documented, and the result at times is an animal that can produce semen while retaining its original female-classified genetics.

At least there's a great deal of room for studies in reproductive biology. That's good for scientists and sci-fi writers. :Hug2:
 

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Why Two, Not One? Two is more effective at creating an exchange of genetic material than single-cell reproduction, and this appears to have a significant benefit for multicellular organisms.

Why Two, Not More? Well, lets say it took three organisms to produce a viable offspring. That would mean that for a species to spread, it would need to have at least three members to start with, and produce an optimum number of each sex to keep the species going. This is possible, but the math quickly becomes complicated, and you're stuck with many occasions where offspring *could* have been created, but one of the three types just wasn't available. To put it plainly, it's much easier to arrange a get-together for two than for three.

So adding a third, fourth or fifth sex wouldn't have *helped* with genetic diversity much, and it would have *hurt* by reducing the chance that a successful mating could occur. To evolution, it would have just been an unnecessary complication.

(I recall the TV show "Alien Nation" had the Newcomers with a third sex, which was relatively rare but required for mating, and it did get into how if these individuals were targeted and removed from the gene pool, the other two would be helpless to continue the race. Dreamsnake has a similar situation.)
 

indianroads

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Have you read The Gods Themselves by Asimov?

I believe this be his only novel with aliens in it... and predictably it’s amazing IMO. 3 sexes required to produce offspring is far from the most interesting thing about them. Beautifully written with incredible characters. Highly recommended if you’ve missed it.
 

SandyH

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That's part of evolutionary advancement. Primitive life forms can do without two sexes.... Unicellular organisms don't need any sexual partner.....
 

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Sure. But if two are better than one, why not three? Or four? That was the interesting thrust of the article, to me.
 

nickj47

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I think it was Isaac Asimov in The Gods Themselves who had an alien race with three sexes. IIRC, it took all three sexes to procreate.

Richard Dawkins has described the development of the two sexes in some of his books on evolution. It didn't make a lot of sense, but it's Richard Dawkins, so it must be true.
 

lpetrich

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Prokaryotes and viruses all reproduce asexually. It is eukaryotes that have a sexual cycle, and even they reproduce asexually most of the time, though for multicelled ones, it's at the cellular level.

This cycle is an alternation between haploid (one set) and diploid (two sets) chromosomes.

Diploid: (XX)
Ordinary reproduction (mitosis): (XX) -> (XXXX) -> (XX) + (XX)

Meiosis: (XX) -> (XXXX) -> (XX) + (XX) -> (X) + (X) + (X) + (X)

Haploid: (X)
Ordinary reproduction (mitosis): (X) -> (XX) -> (X) + (X)

Cell fusion: (X) + (X) -> (XX)

Diploid again.

The mating-type mechanism blocks cell fusion in some cases, presumably preventing inbreeding. It is also common for organisms with different mating types to look alike: "isogamy". Looking different, "anisogamy", is a derived state, and eggs and sperms ("oogamy") is an extreme case of it.
 

Helix

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Prokaryotes and viruses all reproduce asexually. It is eukaryotes that have a sexual cycle, and even they reproduce asexually most of the time, though for multicelled ones, it's at the cellular level.

This cycle is an alternation between haploid (one set) and diploid (two sets) chromosomes.

Diploid: (XX)
Ordinary reproduction (mitosis): (XX) -> (XXXX) -> (XX) + (XX)

Meiosis: (XX) -> (XXXX) -> (XX) + (XX) -> (X) + (X) + (X) + (X)

Haploid: (X)
Ordinary reproduction (mitosis): (X) -> (XX) -> (X) + (X)

Cell fusion: (X) + (X) -> (XX)

Diploid again.

The mating-type mechanism blocks cell fusion in some cases, presumably preventing inbreeding. It is also common for organisms with different mating types to look alike: "isogamy". Looking different, "anisogamy", is a derived state, and eggs and sperms ("oogamy") is an extreme case of it.

But it's not even as clear cut as that. Prokaryotes swap genetic material all the time. Single-celled eukaryotes have multiple mating types. Many, many multicelled eukaryotes reproduce asexually, either facultatively, if conditions favour/necessitate it, or as a matter of course. Plants and fungi are weird af, but animal reproductive strategies are pretty varied too. Some animals undergo alternation of generations, switching between sexual and asexual reproductive phases. There are hermaphrodites of different sorts -- simultaneous, sequential, serial -- which involves changes in the pattern of gamete production.