You’re an alien on another planet and you walk into the local joint hoping that maybe you’ll meet someone to

You’re an alien on another planet and you walk into the local joint hoping that maybe you’ll meet someone to partner with. And you see 7 different genders and the only one you can’t successfully mate with is one that’s the same as you!

Nice! Lots of choices!

Next scenario.

You’re not an alien and you’re not walking into a bar. You’re a single-celled creature cavorting around in some pond water on planet Earth. And there’s 7 different genders and the only one you can’t successfully mate with is one that’s the same as you!

Nice! Lots of choices!

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But then this question arises.

How does a single-celled creature know who's who and who it can mate with?

And that’s a very important question! Because one of the basic questions that all organisms need to answer is who are my peeps and who isn’t?

The creature featured in this article offers us the opportunity to gain new insights into how such recognition comes about.

Welcome to the fascinating world of Tetrahymena, a single-celled ciliated* organism that has been studied by biologists since the 1800s.

 * FYI, cilia are tiny hair-like structures that stick out from cells.  In the feature photo of Tetrahymena above, they are all the little green strings that look like hair.  Many other organisms have cells that are ciliated, too, including we humans.  I describe one such bunch of our cells in this article.
In today’s article, we’ll look at several of the reasons, including its seven sexual systems, why Tetrahymena continues to be a valuable organism to study. We’ll see how studying a simple creature that is about as far removed from us on the Eukaryotic evolution tree as you can get, can help us understand some of the fundamental molecular processes that govern how we age, how we recognize infectious foreign invaders and lots more.

So let’s dive into that pond and get intimate with some of these Tetrahymena cells.

First, a little history and basic Tetrahymena biology.

Personal disclaimer: Back in the early 1990s, I did my Ph.D. research under the mentorship of Dr. C. David Allis, one of the world’s foremost Tetrahymena investigators. So I have more than a little bit of personal history doing experiments with this organism!
Discovery
It is very likely that Tetrahymena was first seen in the original microscopic meanderings of Anthonie van Leeuwenhoek back in the 17th century.

Because the different species of Tetrahymena (abbreviated T. when the species name is included) all look the same physically, in the past they were originally lumped into one species called T. pyriformis. The species name, pyriformis, referred to its shape.

Pyriform, means pear-shaped (from Latin pirum “pear” and forma “shape”) and is sometimes spelled piriform.
T. pyriformis was first described in 1830 by Ehrenberg under the name Leucophrys pyriformis.

Since the initial finding and naming of T. pyriformis, we have found 22 other species. Of those, T. thermophila has become the one most commonly used in modern research labs. In fact, it even has its own genome database page!

If you’re getting impatient, you know basic Tetrahymena biology, and you just want to get to the sexy stuff then skip this next section and head on down to Sex Cells below.

But, maybe you might wanna continue reading because…

Tetrahymena is a truly fascinating critter and you won’t really understand all that sex cells stuff if you don’t acquaint yourself with some important features of the beast’s biology.

Biology and Life Cycle
Physical Features
How did Tetrahymena get its name?