if there were 3 sexes male,female and ____

[LJBrown88]

I had some friends who engaged in trinities. Not threesomes, but 3-way relationships.Needless to say, most of these realtionships ended in tears, 3 always became 2 in the end, as one party would always get jealous or feel left out. Three finally became two for the last time, as the last two left have finally tied the conventional two-way not.

 

That wasn't meant to be a tongue twister! But as much as it seems a nice idea in theory, I don't believe three way relationships, let alone three way reproduction is feasible. human beings are far too complicated enough as it is already. It's already hard enough putting two together!

[Daecon]

Maybe it would be XX, XY and YY?

[Mokele]

Slime molds have way more than 3 (507, to be precise). Or one, depending on your POV. They have 3 genes that determine sexual compatability: two have 13 alleles, one has 3. In order to be compatible, two gametes must differ at each locus, so any given slime mold is compatible with 288 out of 507 of its kin.

 

However, it's *still* only the union of two gametes, not 3. Three is problematic, because the organism would have 3 sets of chromosomes, making cell division difficult (though some organisms have coped). Four may be more likely - there are plenty of tetraploid animals.

[visceral]

Like what would be the evolutionary benefit for a species with 3 sexes?

 

Hmmm...

 

Say this 3-sex species were all massively enhanced mutants.

 

Would their superior strength, intelligence, etc. compensate for the difficulties of 3-way mating? Or would they die out?

[snezanaulbrich]

It seems hard to imagine. But humans (and most other species) are perfectly adapted to living with only 2 sexes. If there were another sex, then there would be alot of competition and eventually one sex would die out and there would only be 2.

[Fuzzwood]

No one can make a right or wrong statement here as we cannot imagine how 3 sexes would be like, simply because we do not HAVE 3 sexes.

[iNow]

No one can make a right or wrong statement here as we cannot imagine how 3 sexes would be like, simply because we do not HAVE 3 sexes.

 

You're right. We actually have closer to six or seven.

http://en.wikipedia.org/wiki/Aneuploidy

[Fuzzwood]

Different chromosome numbers != sexes. This thread goes about penises, vaginas and something else.

[iNow]

Those chromosomal conditions do generally result in different genitalia. For example: http://en.wikipedia.org/wiki/Intersexuality#Ambiguous_genitalia

[Martin]

Slime molds have way more than 3 (507, to be precise). Or one, depending on your POV. They have 3 genes that determine sexual compatability: two have 13 alleles, one has 3. In order to be compatible, two gametes must differ at each locus, so any given slime mold is compatible with 288 out of 507 of its kin.

 

However, it's *still* only the union of two gametes, not 3. Three is problematic, because the organism would have 3 sets of chromosomes, making cell division difficult (though some organisms have coped). Four may be more likely - there are plenty of tetraploid animals.

 

 

Since species are the result of evo, an interesting question is how this setup of more than two sexes would have evolved.

 

I think it may have to do with there being a fitness advantage of avoiding intermarriage or inbreeding or whatchacallit...when two close relatives mate...

 

These slime people clone a lot. So say one colony sends out a spore and it lands somewhere and begins to grow and spread and clone, but you dont want the clones inbreeding with themselves because that's incest (leads to degenerate halfwits and such). Incest. That's the word I was trying to think of!

 

So evo fixes it so there are N sexes.

For simplicity let's picture N = 5, say, instead of N = 507---a nice small number like 5.

What is the advantage?

 

What does this arrangement do for the slime molds that makes them happier and better people? Or more evolutionary successful? This is what I want to know.

 

Well now a colony of these people has to consist of at least two genetic distinct individuals in order for them to mate and make spores, which float off on the summer breeze. And if there were only 2 sexes then, since these slime people are sedentary and don't move around very much there is a big chance you get to copulate with your sister. We've all wanted to, right?

 

Why does having 5 sexes (lets call them A, B, C, D, E ) make it less likely that somebody will mate with his sister?

 

Mokele you better have a good explanation for this.

Edited June 7, 2009 by Martin

[Mokele]

What is the advantage?

 

Well, first, there's the advantage of sex over cloning, namely increased disease resistance.

 

However, our version of sexual reproduction, with two gametes of massively different sizes, is a derived trait - at first, gametes were the same size. This means any two gametes can combine, and raises the possibility of self-fertilization and the problem of inbreeding.

 

This sort of 'genetic match' test, where organisms sharing alleles at a gene locus cannot fertilize each other, is a way to prevent inbreeding, and is found in some flowering plants, too - pollen tubes cannot form if the pollen is the same allele as the flower it lands on. This not only prevents self fertilization, it also prevents breeding with close relatives.

 

So the question that remains is, why so many alleles? Simply put, the more alleles, the more spores you can combine with. If there are only 2 alleles, a given spore can only combine with 50% of other spores. If there are 3 alleles, the percentage rises to 66%, and at 5 alleles, a given spore can combine viably with 80% of other spores.

[bascule]

 

We now return to your regularly scheduled thread...

[Martin]

 

We now return to your regularly scheduled thread...

 

Cribbed directly out of Plato Symposium. Mighty fine performance!

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Merged post follows:

Consecutive posts merged

Well, first, there's the advantage of sex over cloning, namely increased disease resistance.

 

However, our version of sexual reproduction, with two gametes of massively different sizes, is a derived trait - at first, gametes were the same size. This means any two gametes can combine, and raises the possibility of self-fertilization and the problem of inbreeding.

 

This sort of 'genetic match' test, where organisms sharing alleles at a gene locus cannot fertilize each other, is a way to prevent inbreeding, and is found in some flowering plants, too - pollen tubes cannot form if the pollen is the same allele as the flower it lands on. This not only prevents self fertilization, it also prevents breeding with close relatives.

 

So the question that remains is, why so many alleles? Simply put, the more alleles, the more spores you can combine with. If there are only 2 alleles, a given spore can only combine with 50% of other spores. If there are 3 alleles, the percentage rises to 66%, and at 5 alleles, a given spore can combine viably with 80% of other spores.

 

I read somewhere that evolution of 2 or more sexes also had to do with mitochondrial conflict resolution.

 

The male is the one whose mitochondria selfdestruct. when two gametes get together you have to have some pre-arranged plan or convention to decide whose mitochondria rule. Because mito have their own genes which arent predivided so they can't take part in the choreography. So one persons mito have to win. And you don't want to waste time and energy arguing.

 

So among our imagined slime people with five sexes A,B,C,D,E there would have to be some established rule about whose mito dominate and whose mito self-destruct. Like maybe it could be alphabetical order.

A has mitochondria that survive in union with B and all the rest. B has mito that survive except when mating with A and so on. I'm not clear about this.

I may have misunderstood, or the source may have been in error.

[Mokele]

You know, I'm not actually sure *how* slime molds deal with the mitochondria issue. Or whether it's an issue for all organisms.

 

In googling, I found this paper (free) that discusses the topic and mechanisms, the punchline of which seems to be that there's no simple answer, and some organisms do quite well with a mix of organelles from each parent.

[CharonY]

I think in many cases only certain recombinations are possible indicated by mating types or fertility groups. The mitochondria of one of those groups get lost subsequently (my memory is rather vague in details, though).

However, regarding the advantages of sexual recombination. While creating diversity has been often cited, I recall that there was actually little evidence supporting that, given the two-fold cost of sexual reproduction. Another theory was that it evolved as a form of genomic parasitism (a review in which this was one of the subtopics was written by Rose and Oakley, 2007... I think).

[changizi]

There is a well-known problem in computer science of this form. For two sexes, there is a fast algorithm that can take a bunch of males and a bunch of females, and sort them into pairs such that everyone is happy. But as soon as n>2 the problem switches into something disproportionately more computationally complex. It becomes something called "NP-hard", roughly meaning that you can't solve it fast. So, one *could* say that having greater than two sexes never happened because of the inherent computational complexity, relative to the comparative ease for two sexes.

 

[Whether this computational argument is *really* the right one here is another question. For example, even if there were a magic oracle that would have helped some species handle this computational problem were it to evolve three sexes, it may be that no species ever would have had any selective benefit to having three sexes. But I dunno.]

[Mokele]

Time isn't really a relevant variable. If it takes more time, but there's a reproductive advantage in it, it'll evolve.

[changizi]

Time isn't really a relevant variable. If it takes more time, but there's a reproductive advantage in it, it'll evolve.

 

While I agree with the second sentence you said in this quote, it does not imply that time is irrelevant. If something takes much more time, then the odds go down that it will still be the optimal solution. And if it takes exponential time, so that it require the age of the universe to get a feasible solution, then it potentially makes it impossible for it to be an optimal solution. (That said, most of these NP-hardness arguments are only if one wants exact solutions. Biology doesn't need exact solutions, which is why they're probably a bad way of reasoning about the three-sex question.)

[iNow]

And none of that is relevant to the discussion at hand. Thanks though.

[changizi]

The original question included the following:

What would be the life like? More complex? difficult?

 

My point is that there are computational complexity considerations that may bear on whether species with three sexes might have things more complex and difficult. (Whether they are valid considerations is another thing.)

[iNow]

SUGGESTION: Start a new thread if you want to explore that. This thread is nearly 5 years old, so the most recent posts are surely much more relevant.

[changizi]

SUGGESTION: Start a new thread if you want to explore that. This thread is nearly 5 years old, so the most recent posts are surely much more relevant.

 

Oh. Sorry. I didn't notice how old it was...