January 3, 2017
By Liz Clift
When was the last time you really thought about the roly poly (Armadillium vulgare)? You know the little detritivore that primarily consumes dead plant materials
As a kid, I collected roly polies and kept them in jars. I didn’t know that they were a type of crustacean . I liked that these isopods came in browns and greys and near-black. I liked that they curled up in my hand and I could gently roll them around on my palm. I liked that when I flipped them over on their backs, their little legs wiggled around. I grew up in the south—an environment with lots of moisture and decaying plant matter, their preferred habitat—and so I’d find them under logs and rocks and in piles of pine needles—but also crawling into the swimming pool and walking steadily along the side of the wall under water.
It wasn’t until I was doing restoration work, and installing a native plants garden, on a previous employer’s property, that I ever saw a roly poly that wasn’t an “earth tone.” The one I found was blue. This blue coloration was caused by something caused iridovirus, which is deadly to the roly poly (not to humans). The virus can also cause the isopods to appear purple.
Roly polies are also susceptible to environmental stressors, such as the addition of nitrogen-based fertilizers, which are commonly applied to field crops, and insecticides. The permethrin-based insecticides have proved especially fatal to roly polies, which is unfortunate since permethrin-based insecticides are pretty widespread and roly polies do not appear to have mechanisms for detecting and avoiding them.
As if all of this wasn’t enough of a struggle, roly polies are also impacted by a microbe called Wolbachia. This microbe alters the development of hormone-producing glands, which means that a genetically male roly poly (ZZ chromosomes) who is infected with Wolbachia grows up to be female. Eventually genetically female (ZW chromosomes) roly polies disappear from a population (and all the roly polies appear with ZZ chromosomes).
Scientists have been studying this phenomenon for forty years, and in the 1980s, some scientists showed that some populations that are not infected with Wolbachia still have only ZZ individuals. They had no way of proving their hypothesis: that the microbe had left a piece of its DNA behind and that was influencing the chromosomal make-up, but as of this year, science is close to proving they were right.
The long and the short of it is this: ZZ individuals who are female always have a trace of Wolbachia hanging out in their DNA; those who are male don’t. Ever. Wolbachia turns one of the roly poly’s other chromosomes into a new sex chromosome that behaves like the disappeared W chromosome.
The science is still out on this research around roly poly DNA and how it may or may not be influenced by Wolbachia, at least for now. Some other researchers are waiting for further data to come in, and wonder if perhaps other chromosomes impact sex. All of this research is important—even though, perhaps, most of us wouldn’t think twice about the biological sex of a roly poly or how it feels in say a flooded environment or one laced with permethrins—because it demonstrates the intricacy of natural systems and how small shifts can have a major impact.
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