Are there lifeforms that have more than 2 sexes?

Question

Are there any lifeforms on earth that have more than male and female biological sexes?

If so, are all three mates required for reproduction or is there something else going on?

If not, why not? There are species that have one sex and there are species with 2 sexes. I get some of the basics of why there are two instead of one, but would those benefits not be even greater if there were three sexes?

NOTE: I am of course only talking about sexual reproduction. I have no intention or desire to talk about transgender or LGBT issues nor am I wanting to make any points on the issue. This is solely a question about sexes in biology and not human social constructs.

Also, I am a computer programmer, not a biologist. Please forgive me for not knowing any of the terminology.

Answer 1

There exists something like multiple sexes in fungi, but it's a bit complicated.

First of all fungi don't actually have anything that represents the two classical animal sexes, phenotypically their reproductive cells are all equal (they have isogamic reproduction).
However there can be certain alleles in some fungi species (sometimes multigenic, sometimes not), that restrict sexual reproduction to other fungi with a different variant of that allele, these alleles are therefore called mating types. It is however important to note, that almost all fungi can also reproduce asexually (by basically cloning themselves).
In general all of the mating type systems always require only 2 individuals to (sexually) reproduce, it just can't be 2 individuals with the same type.

In their simplest form mating types resemble the male/female sex we often see in animals: for some fungi (e.g. yeast) there exist exactly 2 forms of the mating type allele (called a and $\alpha$), so basically they have '2 sexes' (though genetically/biologically the difference is MUCH smaller).

There are however also much more complicated mating type systems: Some species of the Basidiomycota or Agaricomycotina groups have a tetrapolar mating type, which is composed of 2 independent alleles (with 2 variants per allele this would already results in 4 different 'sexes'). Additionally they can have many more than 2 forms for each of the alleles (up to hundreds), which leads to (ten-)thousands of different mating type combinations (or 'sexes').

Answer 2

It depends on just what you mean by "required", but insects such as bees and ants sort of meet the criteria.

First, the double helix of DNA is inherently binary, so at the lowest level you need two and only two individuals for sexual reproduction. However, the actual process in the world may require more types.

Take bees for instance. There are basically three types of individuals in a hive: one female queen, which lays eggs, male drones which fertilize a queen (or try to), and then die, and neuter workers, which feed the queen, raise the eggs & larvae, and determine whether a particular egg will become a new queen or drones.

So for bees, only two types of individuals actually participate in sex, but all three types are required for the hive to survive and reproduce.

Answer 3

Yes in many different way. No for a species in which all three are required at once.

1. there are species that have more than one type of male or female, each with drastically different body plans, this is called Alternative mating strategy. (African cichlid fish, cuttlefish)

2. Bacteria with up to 7 sexs are known to exist, each sex can mate with any of the other 6 but not itself. Tetrahymena thermophila technically these are called mating types.

3. Worms which have males, females, and hermaphrodites exist. Auanema rhodensis this is probably the closest you will find.

But there are no species that have three sexes that all need to participate. Part of the issue is logistics it is just harder to get three animals together than two, and there are far more ways to split a genome in half than into thirds for exchange and evolution is often a lazy tinkerer. There is also an issue with mitochondria since they mutate faster and differently they do not do as well with genetic exchange than nuclear DNA. it is better for a mating creature to only use mitochondria from one parent, a logistically that makes for two gender one with one without. There is also a statistical issue that three genders are not stable and drift can turn them into two genders rather easily.

Answer 4

There are some single celled organism that are considered to have more than 2 sexes but biologist generally call them mating types rather than sexes. One good example is Chlamydomonas. They use flagellas for mating and once their chromosomes is mixed in the fused cell, the cell undergoes meiosis to produce the next generation of haploid daughter cells. (See this Quora post).

A mushroom Schizophillum commune has two mating type genes, with one having over 300 possibilities and the other 64. This leads to a documented 23,328 distinct mating types. A S. commune individual would be capable of reproducing with just under 23,000 of these types. (See this Reddit post for a detailed discussion on this topic).

Clam shrimp have a male as well as two varieties of hermaphrodite, which either self-fertilize or mate with males but cannot mate with each other. The protozoan Tetrahymena thermophila has seven distinct "sexes" that can hook up and swap genes (See here).

See this biology.SE question and this worldbuilding.SE question for more information.