Lifeform of the week: Naked mole-rat genome exposed
In July, 2011, the naked mole-rat joined the illustrious ranks of creatures like the giant panda, the chimpanzee and DNA pioneer Robert Watson when scientists at the University of Liverpool unveiled its newly sequenced genome. Science has good reason to be excited about these bizarre-looking animals. Naked mole-rats aren’t your typical rodents, or your average mammals for that matter. With an insect-like social structure, a reptile-esque metabolism, imperviousness to pain and a maximum lifespan that would be considered long for a large dog, much less a tiny rodent, they are anomalous in almost every way imaginable.
Their striking longevity is the peculiarity that most intrigues geneticists. Some of these animals have been observed to live as long as 30 years (well beyond the lifespan of other mouse-sized rodents) and they appear to be graced with a resistance to tumors. Cancer has never been detected in this species, and researchers are eager to learn how they manage to avoid the disease. Somewhere in the genome of this homely little rodent may lay the key to health and longevity for our own species.
The rat that builds its own maze
Naked mole-rats are native to the arid grasslands of East Africa. But you’re not likely to see them there, as they spend most of their lives underground. They dig elaborate, branching burrows, sometimes up to 4km (2.5 miles) in total length, in an organized assembly-line fashion. Rodents in the front chisel through the dirt as those behind them sweep it away. The burrows function as both living quarters and tunnels to food sources. Naked mole-rats subsist on root vegetables growing beneath the surface, obtaining all their necessary water from these tubers as well.* In order to maximize nutrient uptake from limited resources, they also regularly consume their own feces. Let’s hope this particular habit doesn’t turn out to be their secret to a long and healthy life.
These animals’ outlandish bodies (they’ve been nicknamed “saber-toothed sausages”) are a perfect fit for their subterranean lifestyle. Much of the burrowing is accomplished with their prominent front teeth and strong jaw muscles. The lips of a naked mole-rat are located behind their teeth, allowing them to dig without filling their mouths with dirt. Their low metabolic rate helps them make do with the limited oxygen supply available underground. The downside of their metabolism is that they cannot maintain a stable body temperature like normal mammals. Instead, they regulate this behaviorally by huddling together when environmental temperatures are too low, and retreating into cooler portions of their burrows when they get too warm.
Like bees and ants, naked mole-rats live in a eusocial colony: a highly-structured, cooperative society in which only one female – the queen – bears offspring, and the rest of the clan functions as workers, helping raise the pups, maintain the lodging and gather food for the group. Hormonal activity tightly controls the reproductive caste system, with worker females not being “allowed” to ovulate. The queen in a naked mole-rat colony spends most of her time and energy cranking out babies. She can give birth to up to five litters a year – the gestation period for each is about 70 days. If you do the math, there isn’t much time left over to take up a hobby. Despite its demanding schedule, females in the colony aspire to this role. If a queen dies or is ousted from her throne, several ambitious females fight it out (often quite violently) to decide who will be the next queen. Once a new queen is installed, she actually grows in length even though she has already reached adulthood. The trick to this is expanding the spaces between her vertebrae. Nobody said the life of royalty was an easy one.
Have you ever chopped spicy peppers and been left with that maddening burning finger sensation? Or perhaps you’ve accidentally scorched your skin with some acidic chemical? Well, if you were a naked mole-rat, you would have been completely unfazed by such incidents. These animals lack a neurotransmitter called substance P, which is responsible for relaying pain signals to the brain when the skin makes contact with such irritants. For humans, this wouldn’t be such a fabulous adaptation. The presence of pain is often useful. For instance, it functions as a gentle reminder to take your hand out of that jar of undiluted sulfuric acid. But the world of mole rats is different than ours. The narrow underground environment they inhabit is low on oxygen and high on carbon dioxide (that stuff mammals exhale as a byproduct of breathing), which is converted to an acid when it makes contact with skin. If naked mole-rats had not evolved their inability to feel this type of pain, they would suffer the rather impractical sensation of constant skin soreness.
Age before beauty
The common rat (the kind you don’t want scrabbling through your attic) can live four years, if it’s lucky. And yet naked mole-rats can survive as long as 30 years, which is highly unusual for such a small animal (in general, bigger mammals live longer than small ones). And it’s not like they’re slow to reach maturity either. Naked mole-rat pups are put to work on the family farm at the young age of four weeks. Despite toiling their whole lives in poorly-ventilated environments, these pale rodents don’t easily succumb to diseases associated with aging, particularly cancer, which has yet to be seen in the species despite decades of scrutiny. With the genome sequenced, scientists hope to get some insight into these rodents’ disease resistance. They can now examine DNA repair mechanisms and the genes associated with them in the naked mole-rat and compare these to the ones in their shorter-lived cousins.
The sequencing was done in collaboration with The Genome Analysis Centre (TGAC†) and the “rough draft” genome is available online to researchers wishing to have a go at discovering the naked mole-rats’ secret to success. They certainly aren’t getting by on their good looks.
* Since they consume the roots from underground, naked mole-rats can be a major problem to farmers of such vegetables.
† I’d just like to applaud the folks at TGAC for having an acronym composed of the four DNA nucleotide letters (even if they did sort of cheat by using “the” to yield one of the letters). Clever, clever.
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