Nearly everybody has seen one at some point, and maybe even played with one. Though many people have an aversion to arthropods, these little critters seem to get a pass, and their common names of pillbug and roly-poly demonstrate their appeal: When disturbed, they roll up into a ball like tiny armadillos, entertaining countless bored kids throughout the years- at least until cell phones came out, that is.
But that defensive behavior is far from the most interesting detail about these animals. First of all, pillbugs aren’t bugs. They’re terrestrial crustaceans, more closely related to animals like shrimp, lobster, crayfish, and crabs than the beetles and grasshoppers that you may find in their vicinity. And though crabs may inhabit the shore, the isopods are the only group of crustaceans to fully conquer the land.
No, I’ve never eaten one. But I hear they taste like shrimp.
More relevant to this month’s topics is what’s quietly happening on the inside. Like many animals- but not all- pillbugs have sex chromosomes, which trigger development of physical sex characteristics. But that’s not all. There’s actually a hitchhiker calling the shots.
In 1924, scientists working with mosquitoes isolated a bacterium which they named Wolbachia. Though it didn’t seem particularly important at the time, others returned to this research years later. Scott O’Neill, founder of the World Mosquito program, began studying Wolbachia in 1980 and mosquito-borne illnesses such as Dengue fever in 1991, eventually designing a program using the bacteria to stop disease replication in the mosquitos, and thus to reduce its transmission.
So what does this have to do with isopods? Turns out they carry Wolbachia too- in fact, an estimated half to two-thirds of arthropods have it- and its relevance is found in how it affects their offspring. As this bacteria is passed maternally, it is better suited in female hosts. So it makes sure to have ample.
Wolbachia affects the development of pillbug eggs, causing the extra-mini miniature armadillo imposters to develop as female regardless of their sex chromosomes. Over time, some populations have entirely lost the functionality of those chromosomes to depend on the bacteria instead; just as fascinating is that it has further modified their genes to skew toward female even without active bacterial infection.
This adds an interesting wrinkle to the idea of biological sex. What counts? In this case, a pillbug may have the sex chromosomes for a male, but the physical characteristics of a female. But they’re making it work.
And that’s okay too.
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