“My name is Legion, for we are many”: according to the Gospel of Matthew, that’s how a man possessed by demons answered when Jesus asked his name. Demonic possession is supernatural claptrap, but science has nevertheless revealed no end of proofs that we are not alone in our own skins. Bacteria swarm through our mouths and intestines; fungi and yeasts take hold where they can in our moist, warm places; parasitic animals and microbes homestead in our blood and tissues.
As explained so vividly by Carl Zimmer and others, the chemical signals that these organisms release into our bodies can also have much more subtle effects, not just on our health but on our behavior. Toxoplasma gondii parasites make their host rats less afraid of cats, which Toxo must also infest to complete their life cycle. No need to stop there, however. Certain bacteria change the sexual preferences of their fruit fly hosts, intestinal flora alter brain development in mice, and yogurt-culture bacteria can seemingly reduce their host’s stress behaviors—to choose just a few discoveries that Ed Yong has described recently.
The sources of those chemical signals don’t have to stop at our skins, however, and they needn’t have anything to do with symbiosis, as an intriguing paper from September underscored. In Cell Research, scientists at Nanjing University in China reported that some of the regulatory molecules called microRNAs found in foods can survive digestion and change gene expression in the creatures that eat them—including humans.
Rice genetically engineers us
Veronique Greenwood at 80beats has a neat summary of the work, but to recap the recap: The scientists were looking at microRNAs, small RNA molecules that are not translated into proteins as messenger RNAs are. MicroRNAs turn out to have an important function as regulatory elements: they bind complementary messenger RNAs and prevent their translation. The Nanjing University group showed that microRNAs produced by rice plants turn up in the blood of people and mice that eat rice, and that in our bodies these microRNAs lower the production of low-density lipoprotein (LDL), a molecule that scavenges cholesterol. In short, the rice was altering the physiology of the rice-eaters—not just nutritionally but by tampering with the animals’ gene expression.
Bear in mind, too, that the microRNAs are not just haphazardly swept along with the mash of other chewed-up vegetable matter. The rice packages these microRNAs inside tiny membrane bubbles (vesicles) excreted from the plant cells. That packaging is what allows the microRNAs to survive digestion in the animal guts, and it may mediate how efficiently the microRNAs get into animal cells to stymie LDL production.
Because this finding is so new, the significance of microRNAs circulating between species is still unclear. Whatever impact the microRNAs in our foods have on human health should basically already be a component of what epidemiologists and nutritionists already see resulting from various diets, so in that respect, it’s nothing new. (But no doubt it will contribute to future debates, rightly or wrongly, about the effects of eating genetically modified foods, heavily processed foods, and so on.)
The more interesting question to me is what the evolutionary impact of interspecies microRNA transfers through food might have been. To what extent might the reliable presence of these regulatory elements in food have reshaped how organisms’ genomes worked?
We are what we eat
Consider the extreme example of what happened in the evolution of mitochondria, the organelles that provide chemical energy to cells. Two billion years ago, the forerunners of mitochondria were free-living, independent cells resembling bacteria. They struck up a symbiotic way of life with other cells inside which they lurked. Over time, the proto-mitochondria lost many of their original genes: they were redundant with ones in the host cell, so the proto-mitochondria could just take many of the proteins it needed from its surroundings rather than make them. Today, mitochondria still have a distinct genome but it encodes almost nothing more than what they need for energy transduction.
My guess is, in humans we’ll find that microRNAs in food haven’t been much of an evolutionary factor because our diets shifted too much as our ancestors spread around the globe. But in animals that have enjoyed much more stable who-eats-whom relationships over tens of thousands of years, we might find a lot of subtle interplay once we start to look for it.
Indeed, interspecies factors keep showing up again and again in nature. Many bacteria can pass genes to one another, even across the bacterial equivalent of species lines. Some can also pick up DNA from their environment. Gut flora in the intestines of sushi-eating people in Japan have lifted their genes for seaweed-digesting enzymes from a marine bacterium.
A tide of evidence runs against the traditional definitions of our biological and psychological selves as distinct, self-contained units. Rather than being isolated, we’re deeply intertwined with other living things through a miasma of chemistry that can makes it hard to distinguish where we leave off and where our surroundings (or neighbors) begin. We are never alone in the web of life. And though we may not be possessed by demons, the voices whispering within us are legion.
Reference: Zhang, et al. Exogenous plant MIR168a specifically targets mammalian LDLRAP1: evidence of cross-kingdom regulation by microRNA. Cell Research, (20 September 2011) | doi:10.1038/cr.2011.158
Image: Khalid Mahmood, via Wikimedia Commons
John Rennie, a freelance science writer based in New York City, was for 15 years the editor in chief of Scientific American. He contributes regularly to a variety of publications today, including the PLoS Blogs network, for which he writes “The Gleaming Retort.” Learn more about him at johnrennie.net and follow him on Twitter as @tvjrennie.