An international team comprising more researchers than the arthropod has legs (106 researchers) has sequenced the genome of Strigamia maritima, a Northern European centipede, and found that its genome, while less than a tenth the size of a human’s, has around two-thirds the number of genes, distributed across one pair of large chromosomes and seven pairs of tiny ones, including X and Y sex chromosomes. The results are published today in the journal PLOS Biology.
Arthropods are the most species-rich group of animals on Earth. There are four classes of arthropods alive today: insects, crustaceans, chelicerates (which include spiders and scorpions) and myriapods. This latter class, which includes centipedes, is the only class for which no genome has yet been sequenced.
Myriapods arose most likely from marine ancestors that invaded the land more than 400 million years ago. All myriapods have a large number of near-identical segments, most bearing one or two pairs of legs. However, despite their name, centipedes never have a hundred legs. Strigamia maritima, which lives in coastal habitats, can have from 45 to 51 pairs – but the number of pairs is always odd, as it is in all centipedes.
The team found that the centipede genome is more conserved than that of many other arthropods, such as the fruit fly, with less gene loss and scrambling. This suggests that the centipede has evolved slowly from their common ancestor and should allow researchers to draw comparisons between very different animals, which are not obvious when working with fruit flies or other fast evolving insects. For example, the researchers found parallels in the way that the brain is patterned between centipedes and other very distantly related animals such as marine worms. Such comparisons will enable scientists to build an overall picture of how genetic changes underlie the diversity of all animals.
“With genomes in hand from each of the four classes of living arthropod, we can now begin to build a picture of the genetic make-up of their common ancestor,” says Dr Frank Jiggins, of the Department of Genetics at the University of Cambridge, one of the researchers involved “For example, by comparing flies and mosquitoes with centipedes, we have shown that the innate immune systems of insects are much older than previously appreciated.”
One of the most surprising findings is that these centipedes appear to have lost the genes encoding all of the known light receptors used by animals, as well as the genes controlling circadian rhythm, the body clock.
“Strigamia live underground and have no eyes, so it is not surprising that many of the genes for light receptors are missing, but they behave as if they are hiding from the light. They must have some alternative way of detecting when they are exposed,” says Professor Michael Akam, Head of the Department of Zoology at Cambridge and one of the lead researchers. “It’s curious, too, that this creature appears to have no body clock – or if it does, it must use a system very different to other animals.”
The centipede’s genome sequence is of more than just scientific interest, argues Professor Akam. “Some of its genes may be of direct use. All centipedes inject venom to paralyse their prey,” he explains. “Components of venom often make powerful drugs, and the centipede genome will help researchers find these venom genes.”
Reference
Chipman, AD et al. The First Myriapod Genome Sequence Reveals Conservative Arthropod Gene Content and Genome Organisation In the Centipede Strigamia maritima. PLOS Biology; 25 Nov 2014
What it lacks in genes, it certainly makes up for in legs: the genome of the humble centipede has been found to have around 15,000 genes – around 7,000 fewer than a human.
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