One of the most widely-studied worms in the world is the tiny, humble vinegar worm, known to science as C. elegans. And this worm has a really interesting way of reproducing. It's called selfing.
Image via Frontiers in Aging Neuroscience
Over at This View of Life, Carl Zimmer has a terrific article on these worms, and what they've taught us about evolution. First, he explains how they reproduce:
In humans and many other animal species, individuals are typically either males or females. The males produce the sperm, and the females produce the eggs. In C. elegans, individuals can either be males or hermaphrodites.
The biology of the male worms is straightforward enough: they have sperm, which they can insert into a mate. But the biology of the hermaphrodites is unquestionably strange. They start out life essentially as males, producing sperm that they store in a special chamber deep inside their body. Later in life, their gonads undergo a radical transformation: now they only make eggs.
The hermaphrodite never develops an organ for delivering sperm into other worms. And so it can only use its sperm to fertilize its own eggs. When an egg is ready to develop, it swims past the sperm chamber, picks up a sperm, and then continues on to the worm's uterus, where it can develop into a larva. This self-fertilization is called selfing.
For a male worm to become a father, he has to interrupt the selfing going on inside a hermaphrodite. After mating, the male's sperm swim to the chamber, where they dump out the hermaphrodite's own sperm and swim inside to take their place. When an egg travels past the chamber, it picks up the male's sperm for fertilization.
This is exactly what I will now imagine when I hear the word "selfie."
Read more about these amazing worms, and why they still need males, over at This View of Life
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