Olivia Judson: Weird Ciliate Sex

Dr. Olivia Judson

Dr. Olivia Judson

THIS is Valentine’s Day, so we had to post about a column by the splendidly-evolved Olivia Judson — an evolutionary biologist and a research fellow in biology at Imperial College London. This article, part of Dr. Judson’s series in the New York Times, is titled: Unorthodox. Here are some excerpts, with bold added by us:

When it comes to sex and reproduction, mammals are ultra-orthodox and, frankly, rather dull.

Olivia, just wait until you meet your Curmudgeon! Let’s read on:

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.

Uh, Olivia … did you have something else in mind? If so, tell us.

Take ciliates — which get my vote for Life-form of The Month: February.

Most ciliates are single-celled organisms covered with tiny hair-like structures known as cilia — which is Latin for eyelashes. The cilia are used for a variety of activities, including swimming and feeding. Some ciliates even use them to scuttle across surfaces.

Why should we care about ciliates? Let’s see what Olivia says:

Ciliate sex is peculiar in several ways. For one thing, reproduction and sex do not happen together. When a ciliate reproduces, it does so asexually, typically by splitting in half and growing a complete new individual from each piece. So: where there was one individual, there are now two.

Not very thrilling. Tell us more.

In and of itself, asexual reproduction is not especially strange — many organisms, from aphids to sea anemones, do it at least from time to time. The weird stuff happens when ciliates get sexual.

Weird stuff? You’ve got our attention, Olivia.

In ciliate sex, two individuals arrive, and two individuals leave: no eggs are fertilized, no offspring are produced. But by the time the two individuals go their separate ways, a massive change will have come over both of them: they will both have acquired a new genetic identity.

Here’s what happens. Each ciliate has something called a micronucleus; this contains two complete versions of its genome. During sex, the micronucleus divides in such a way that each individual keeps one version of its genome for itself; it then gives an exact copy of this version to its partner. Afterwards, each individual fuses the two genomes (the one it kept and the one it got) to make a new micronucleus.

That is weird! What’s it all about?

This has three odd consequences. The first is that, by the end of sex, the two individuals have become genetically identical. It’s as if you and your mate began coitus as yourselves and finished as identical twins.

The second odd consequence is that, partway through its life, a ciliate can radically alter its genetic make-up; genetically speaking, the transformation is so extreme that it’s as if you changed into one of your children. Talk about being reborn.

Which brings me to the third odd consequence: after sex, the organism undergoes a profound remodeling.

Olivia, you are our inspiration in such things. Tell us more.

As if that wasn’t enough strangeness, here’s one other peculiar detail. Many ciliates have more than two sexes (or “mating types”) and some — Stylonychia mytilus, for example — have as many as 100.

Egad! How does all this end?

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. In my next life . . . .

Your current mammalian life looks pretty good to us, Olivia. Happy Valentine’s Day from your Curmudgeon!

Copyright © 2010. The Sensuous Curmudgeon. All rights reserved.

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2 responses to “Olivia Judson: Weird Ciliate Sex

  1. Since the creationists are always up in arms about “sex and religion”, I think she has the first part very well covered. Well done, Olivia!

  2. It seems that with the ciliates, an individual actually CAN evolve into a new form.