Single Cell
Imagine a creature that changes gender to avoid same-sex mating. Or how about a creature that, when starved, thickens its skin and stays motionless for a month. These strange creatures are de rigeur for biologists.
Scientists count on model organisms to study all manner of biological processes and diseases. For example, the common ancestry of mice and men means that mice can be studied to learn about human cancer. Mice and rats are familiar model organisms, but scientific research has some lesser-known heroes.
One such hero, favored by developmental biologists, is C. elegans. This tiny roundworm is only one millimeter long. It is ubiquitous in soil, especially near rotting vegetation, and its favorite snack is E. coli bacteria. And by human standards, it’s very weird.
First of all, there are no female C. elegans. There are males, and there are hermaphrodites. The hermaphrodites can fertilize their own eggs, because they have special sperm holding tanks between their ovaries and uteruses. As the eggs make their way down to the uterus, they pick up sperm and are fertilized.
This would be ideal, except for the fact that a hermaphrodite has far more eggs than sperm. Left to its own devices, the hermaphrodite can fertilize a few hundred eggs, but its leftover gametes go to waste.
So the hermaphrodite takes a lover. A male C. elegans has a special fan-shaped tail that he uses to deposit some rather unconventional sperm.
These sperm aren’t swimmers, like the kind used by civilized humans. Instead, C. elegans sperm are amoeboid blobs that drag themselves by their pseudopods up to the hermaphrodite sperm tanks. Once there, they out-compete the hermaphrodite’s own sperm to fertilize up to 1000 eggs.
This works out just fine for most worms. Unfortunately, a few have certain awkward mutations that make reproduction a bit difficult. Some of the first mutations in C. elegans were discovered when scientists noticed two so-called “vulvar mutants.”
One is a horrifying mutant called multivulva. (You can probably imagine what it looks like.) Frightening as multivulva worms are, vulvaless worms have an even bigger problem. When they get pregnant, their offspring are trapped inside. These babies develop inside their parent, wriggling around nauseatingly as they eat their way out.
A less disgusting aspect of C. elegans biology is its life cycle. The worm goes through four larval stages, getting bigger and bigger until it reaches adult size.
If things go wrong, however, the worm can opt out of development and move into suspended animation. In response to lack of food, temperature changes, or overcrowding, C. elegans changes into “dauer larvae.”
Dauer larvae are small, and they don’t move around much. They can’t eat, because their mouths are sealed shut. “Dauer” in German means “enduring”, and indeed, the worms can survive in this state for more than two months, until conditions improve and they resume normal development.
C. elegans is so hearty that it was the only living creature to survive the Columbia space shuttle disaster. When the shuttle broke up, it was carrying more than 60 scientific experiments, with animals including bees, fish and spiders. The other animals perished, but the C. elegans were found in the rubble, swimming in their Petri dishes inside a metal locker, totally oblivious to the tragedy.
Another model organism may not have survived any shuttle crashes, but if you like kegstands, you should appreciate it.
Saccharomyces cerevisiae is the kind of yeast responsible for the fermentation of beer. But the organism is even more valuable to scientists. A yeast cell is remarkably like a human cell, and many human cell cycle proteins were discovered by first finding their counterparts in yeast.
Yeast has two mating types, called “a” and “a” The fungus can reproduce by pinching off smaller clones, but it also likes to mix up its genetic material. Special mating pheromones let “a” cells know when “a” cells are near, and vice versa.
Then, each cell grows a special projection called a shmoo (no joke) in the direction of the opposite-gendered cell. The cells eventually fuse, exchanging DNA.
Yeasts have emphatically rejected the gender binary. Haploid yeast cells are either type “a” or type “a”, but they can switch if they need to.
An “a” cell has the “a” gene, but it has also has a silent copy of the “a” gene. When it wants to swap sexes, special enzymes chop up the DNA where the “a” gene was, and replace that gene by copying the DNA from the waiting “a” gene. Ingenious, right?
As strange as we might find them, humans are not as removed from these creatures as we might think. Scientists recently uncovered 150 genes involved in C. elegans fat storage, and 100 percent of those had direct counterparts in humans.
And human cancer research has an ally in yeast cells, which accumulates DNA damage much like human cells. After all, we share 35 percent of our genes with the lowly roundworm, and 23 percent with the yeast.
Unfortunately, we missed out on their sweet powers, like suspended animation and gender-swapping. Suddenly, I think humans got the raw end of the evolutionary deal.
