I Love a Good Oddball

Carrying on the theme from the lovable loopiness of Kary Mullis...

Here's a bit from a posting by Olivia in her NY Time blog about oddballs in the unicellular world:

When it comes to sex and reproduction, mammals are ultra-orthodox and, frankly, rather dull. Individuals are either male or female, no one changes sex and there are never more than two sexes in a species. No mammal reproduces asexually — by budding off a small piece of itself, say, or by splitting down the middle and growing a new individual from each half. Nope: among mammals, offspring are always produced by sex. That is, an egg fuses with a sperm to produce a child that is genetically distinct from both parents.

...

And if you’re a fact jock, you’ll like this detail. Almost all organisms on the planet read their DNA with the same language, so that if you put a jellyfish gene into a pig, you get exactly the same product in both organisms. Ciliates, however, don’t play strictly by the rules: they have repeatedly evolved small variations on the normal way of reading genes. (They also, mysteriously, have rather a lot of genes: some have as many as 30,000, which is several thousand more than we have.)

...

Many ciliates have more than two sexes (or “mating types”) and some — Stylonychia mytilus, for example — have as many as 100. This doesn’t mean that 100 individuals have to gather for sex to take place. Rather, it means that you can mate with anyone not of the same mating type as yourself. In principle, it gives you more choice: with more mating types, more individuals are eligible mates.

I won't spoil the mystery of ciliate sex. Go read Judson's posting to get all the juicy -- and surprising -- details!

Here's the standard explanation for just two sexes:

Once going down the road of sexual reproduction, the next question is, Why are there only two sexes? Why not 3 or 10 or 100? Although there are a few rare exceptions (a 13-sex slime mold, for example), most higher organisms have only two sexes. Low explains this as the natural outcome of the two competing tasks gametes must accomplish to form a successful zygote: They must find another gamete, and they must form a well-endowed and ultimately successful zygote. Small gametes perform the first task well, large gametes the second. Medium-size gametes do neither well. This leads to a bimodal distribution of gametes into small, abundant, low-cost ones (male sperm) and large, high-cost, scarce ones (female eggs).

Notice that slime mold breaks the "mold" with 13 sexes.

But if you, like me, think that 13 sexes is just plain too limiting...

Take heart! There is a fungus with 28,000 sexes: Schizophyllum commune. Go read all the gory scientific details here (as well as look at some very nice snaps of a specimen of this super-sexy organism).