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One of the respondents in this thread suggested a 7-sex system with the following features:

An idea I came up with years ago was a variation on the male/female/carries-offspring idea. This species has three "base genders"; males, "egg layers" that are fertilized by males, and brooders who carry the eggs to term. To make things more complicated however any individual can have anything from just one to all three sets of genitals, resulting in seven possible genders.

Male

Egglayer

Brooder

Male/Egglayer

Male/Brooder

Egglayer/Brooder

Male/Egglayer/Brooder

The post was unclear about whether brooders contribute genetic material or just a womb/pouch/whatever to raise the young in. So, for the sake of argument, let's assume that the offspring receive genetic material only from the male and egglayer, and not the brooder. (However, combination brooder + another sex would probably primarily brood their biological offspring.)

My two questions are:

What would be a plausible genetic sex-determination system for this species?

Given your suggested system, what would a stable population distribution of the 7 sexes be?

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plausible genetic sex-determination system

Two or three sexual cromosomes, one of them must be X - the largest, carries enough genes to count. Any non-X genome is unviable.

  • XX or XXX - brooder
  • XY or XYY - male
  • XZ or XZZ - egglayer
  • XXY - male/brooder
  • XXZ - egglayer/brooder
  • XYZ - male/egglayer/brooder

what would a stable population distribution of the 7 sexes be?

I don't believe this can be answered on the genetic determination only, it depends on too many factors.

See also epigenetics - genes switched on-off in response to the env conditions.

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Pure brooders would be rare. They cannot pass their genes to a new generation. Consequently, a genotype which causes an individual to have many pure brooder descendants will be selected against, in favor of genotypes which do not.

In fact, there will probably be significant selective pressure against having brooder parts at all. Time spent being a brooder means time not spent laying eggs, or fertilizing them, and that means your genes don't spread as fast.

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    $\begingroup$ Unless brooder relationships are generally incestuous, then they’re ensuring the survival of their family genes quite effectively. $\endgroup$
    – Joe Bloggs
    Mar 27, 2020 at 8:50
  • $\begingroup$ Hm. You raise a good point, I hadn't considered that. $\endgroup$
    – Ton Day
    Mar 27, 2020 at 22:06
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Stable is not a thing in natural populations

Let's consider herd animals to start with.

If a male is successful, that male will have lots of offspring every year, or perhaps for only one year. It's a high risk strategy, an unsuccessful male might have no offspring ever. A female might have a maximum of one offspring each year, but will have that offspring every year.

Female looks like a safe bet, right?

Not so, if there are a lot of females then there's a vast opening for more males, and hence males will be more successful. But now there are lots of males, so the reliability of a female's regular breeding carries the flag.

This creates an on going oscillation in the populations of the two sexes, but at no point is the situation stable.

So where does the advantage come for each of your sexes?

  1. The strongest is Male/Egglayer/Brooder. This obviously fills all slots, will breed every season but could breed multiple times with other egglayers or egglayer/brooders.

  2. Next is egglayer/brooders, these will breed successfully every season at the baseline of the herd female.

  3. The male/brooder is an interesting one, there are two examples in nature, seahorses and ostriches. It's clearly a valid strategy, they're likely going to attract egglayers, though other egglayer combinations are going to seek them out for a bit of added seasonal insurance.

  4. The female/brooder is the classic female bird (or herd animal) we're all aware of. I put this below male/brooder because laying eggs requires more resource investment than producing sperm.

  5. Male and egglayer are at a disadvantage, they either need to find a combination of {other}/brooder or both an {other} and a brooder. The egglayer also suffers the higher investment of laying eggs. These are the equivalent of the herd male, high risk strategy but could be really successful as they don't have to invest in actual brooding or raising of offspring.

  6. Brooder. A genetic dead end in its own right. There would need to be a very strong family group system for them to be anything other than a burden on the parents to raise.

But what if they're small birds rather than herd animals?

Now you get the advantage of the brooder being a third parent. If you can get one that is. They're still going to be the neglected child if there's any risk of one starving during them being raised. Brooders aren't going to be common.

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  • $\begingroup$ "Male and egglayer are at a disadvantage" No, they only need a brooder, because they can fertize their own eggs. $\endgroup$
    – Julian Egner
    Mar 27, 2020 at 11:46
  • $\begingroup$ @JulianEgner, as per OP I've used the slashes to denote the combos. Male and Egglayer are the two single aspect versions. $\endgroup$
    – Separatrix
    Mar 27, 2020 at 12:50
  • $\begingroup$ oh, I see - then your statement is true, of course. $\endgroup$
    – Julian Egner
    Mar 29, 2020 at 23:43

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