What is a plausible sex determination system with males, females and multiple hermaphroditic sexes?

Question

EDIT: I unexpectedly had a possible solution to this come to me, and posted it as answer here. Please look it up and give feedback on it.

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I'm trying to come up with a plausible sex determination system that has the characteristics listed below. It's intended to be general-use, i.e. the subject species could be a modified version of Homo sapiens, humanoid aliens, a nonhuman animal, etc.; for convenience, though, I'll be assuming that the species is human(oid).

1. There are five sexes: female, male, and three kinds of simultaneous hermaphrodites (berm, merm and ferm, collectively known as herms).
2. Sex determination involves three sex chromosomes: X (tied to females and ferms), Y (tied to males and merms), and Z (tied to herms in general).
3. All of the hermaphrodite sexes are fairly androgynous in facial features and fertile in both the male and female organs; berms have the most androgynous-looking, and their physical development follows a pattern that is intermediate between males and females (e.g. on average they're between both in height), while merms and ferms parallel males and females in their dimorphism (e.g. merms develop heavier jaws, greater height, a "V-shaped" torso, and find it much easier to achieve muscle hypertrophy).
4. All possible pairings between the five sexes can produce a child of any sex.
5. The sex system is stable and is in no danger of having any of the sexes being outcompeted within the foreseeable future on an evolutionary timescale.

I've looked into the various existing systems, and couldn't find anything that appeared to be sufficient for my needs. For example, an XYZ system didn't work, even if I employed a second pair of sex chromosomes (like the white-throated sparrow appears to be slowly evolving to), because every version resulted in either males/females or herms being unable to produce the other.

Answer 1

Carrier Chromosomes

The species is hybrid diploid-triploid. Most alleles come in pairs, but there are some behind the scenes helper chromosomes that come in triples instead.

In particular everyone has XX,YY,ZZ pairs that do not manifest anything, but are used to store data. This is needed to get the same ratio of female, male, unviable, merm, ferm, and berm.

The child's ses is determined by the manifesting pair XX*,XY*,YY*,ZX*,ZY*, or ZZ*. Here * is a third helper chromosome that just says manifest this one guys. Otherwise the entire chromosome is recessive.

These manifested pairs correspond to female, male, unviable, merm, ferm, and berm respectively. The nonexistent YY* pair is unviable. The embryo fails to attach to the uterine wall and is flushed out with next month's menstruation.

This is how reproduction works. A male and merm with male and merm children.

Each child gets one chromosome from each parent's XX,YY,ZZ pairs. I have not drawn the arrows for YY and ZZ to avoid cluttering the image. They work the same way as the XX pairs. Your homework is to draw the missing arrows.

The above gives the child data storage pairs. Each parent also contributes one chromosome from a randomly chosen data storage pairs to the child's manifesting pair. This ensures an even ratio of sexes. The child also gets one of the parent's * chromosomes.

The XYZ in the manifesting chromosomes do not pass to the child.

Answer 2

Sex determination by chromosome ordering

All humans have all three chromosomes, but they are encapsulated in a protein structure that preserves ordering. The ordering determines which phenotype is manifested.

When two humans mate the new egg draws random chromosomes from both parents, but the protein that collects the chromosomes always collect one of each type. It uses a special nucleation protein that has three slots angled at 120 degrees, one for each type. Once it has captured three chromosomes it is inserted into an encapsulation. It can be rotated or up-down inverted before it is inserted, but once encapsulated it is fixed. Creating 6 possible orderings of chromosomes.

The orderings are:

1. X Y Z
2. X Z Y
3. Y X Z
4. Y Z X
5. Z X Y
6. Z Y X

If Z is first the type is always berm. The order of the X and Y after it are not important.

If X is first more female characteristics will be manifested. Whether it is a female or a ferm is determined by the second chromosome. If the second is an Y it enforces the female characteristics making a female, if it is a Z it enforces herm characteristics making a ferm.

The same for the male Y chromosome. If the second is an X it enforces the male characteristics making a male, if it is a Z it enforces herm characteristics making a merm.

How the proteins that make this happen exactly work, I can't tell. I'm not a biologist but a software engineer.

Answer 3

HandWavium

Assuming your humans aren't 100% like us,the mating system might introduce a certain randomness to it, you can even say that it isn't (fully? if necessary) understood. Or that Hermaphrodites can choose what Chromosome to pass on due to a complicated system of chemicals in their brain, pretend chromosomes are like features you pass when you have a child, you don't choose if they keep your stellar physic or your poor eye sight, let fate have a hand on it.In your "universe"/"world" it could be a normal things, just how in our the Chromosome dictates sex, in your world it could not mean anything.

The blob comes to mind when talking about this subject.

Answer 4

You may be describing Trioecy

Trioecy is a sexual system that contains both males, females, and hermaphrodites, and is found in both plants and animals. Now, as a disclaimer; on Earth, it is more common in plants, and none the animals that it occurs in are chordates, ie. human or any kind of humanoid/tetrapod/sci-fi stand in, however, you have a bit of leeway here when it comes to alien planets.

One drawback of the system is it tends to be unstable on Earth, gradually shifting into dioecy (male and female) or gynodioecy (female and hermaprodite). But all you really need is a plausible reason for the system to stay in place, particularly in science-fiction, the sky's literally the limit.

One factor to keep in mind is that sex and mating complexity tends to increase the longer and more reliably a species lives. Species with very low life expectancy or in sparse or uncompetitive environments will want to produce asexually as frequently as possible. It was only when ecosystems were filled with life, and particularly with a threat of being eaten and need to evolve, that taking care of one's young and more elaborate sexual systems were created (there are definitely more environmental factors that can come into play; this is a very oversimplified rundown but for now we'll leave it there).

So one particularly reason the system could develop and stay in place is that a more complex mating system meant that children are more likely to reach maturity and need to be more adaptable with higher genetic variation. And the reason trioecy may dominate is because it gives a higher percentage of the population a chance to pass on their genes than more typical dominance hierarchies that we see on Earth.

An important thing to remember is, when it comes to Life in the Universe, we have a sample size of one. There's nothing to stop you from saying that Trioecy just became a dominant form of sexual system in your world.

Answer 5

XO + YO + ZO system

Some species follow an XO sex-determination system, where they inherit one X chromosome from their mother, and zero or one X chromosomes from their father. A double-X (XX) individual becomes female, and a single-X (XO) individual becomes male.

My proposal is a variation on that system. Instead of inheriting only one type of chromosome in this fashion, your species inherits three: X, Y, and Z. Having more X than Y makes you female or ferm, while more Y than X results in male or merm. Herm-ness requires a double Z.

To give the full set:

• XO + YO + ZO: non-viable
• XX + YO + ZO: female
• XO + YY + ZO: male
• XX + YY + ZO: non-viable
• XO + YO + ZZ: berm
• XX + YO + ZZ: ferm
• XO + YY + ZZ: merm
• XX + YY + ZZ: berm

(Note that berms are twice as common as any other sex. If that is undesirable, change "XO + YO + ZZ" to "non-viable".)

Because all individuals have at least one copy of all three chromosomes, their children can be any biological sex.

Answer 6

Wasn't expecting myself to provide a (possible) answer, but here I am.

Someone elsewhere drew my attention to the fact that microorganisms and fungi have an analogue to sexes, called mating types. Reading about it, especially the protozoan Tetrahymena thermophila and how it can essentially "choose" its mating type, led me to formulate the following sex-determination system.

There are two sex chromosomes: X⁰ and X^*. The X⁰ chromosome is "neutral" with respect to sex determination (i.e. has no influence) but does carry multiple genes that play important roles in the individual's reproductive biology, and is shared by all sexes. The X^*chromosome, meanwhile, contains a supergene locus that determines the individual's sex via five different alleles: X^F (female), X^M (emale), X^H1 (berm), X^H2 (ferm), and X^H3 (ferm).

Stem and germ cells exhibit nuclear dimorphism: They have a somatic macronucleus, which contains the individual’s genome and governs the cell’s behavior; and a germline micronucleus, which contains the DNA information for encoding the five alleles of the X^*chromosome. During gametogenesis, the gamete’s macronucleus only retains one of the five possible alleles of the sex-determining locus on the X^*chromosome by way of a stochastic process of DNA rearrangement and deletion, while an identical copy of the micronucleus is passed on.

During the embryonic stage of prenatal development, somatic cells typically possess micronuclei, but gradually begin to lose them after the gonads begin to form, such that by the end of last trimester, only the germ cells and the stem cells retain micronuclei.

How plausible is this model?

Answer 7

If we are talking about sentient species: It is possible that the sentient genus was split in two and left in that state for a prolonged period of time, during which one of the species were left with XY and the other evolved into XZ system, with Z a different descendant of X chromosome acting as Y but also functional in absense of X. Now there can be XX XY XZ YZ ZZ (YY does not work but ZZ does) now when the ocean is crossed and they started cross-breeding again.

In one subspecies, only one of sex chromosomes were active in females, as it is in humans (of XX only first or second X gets to be active) - so XZ may appear as ferm but also as fully typical female, while potentially producing herm offspring due to having a Z.

Of course, evolution would prefer to straighten this system out, but this is where sentience with its reproductive medicine comes into play, ensuring that even a specific combination is at reproductive disadvantage in field conditions, it is overcome with medicine.

I guess that some pairings will still not produce all kinds of sexes, because obviously XX and ZZ (or XZ) cannot produce anyone who has Y.