A multinational group of genetic researchers has sequenced the genome of a maritime centipede called Strigamia maritima, enabling them to reconstruct many features of the genetic make-up of the ancestral arthropod that lived about 500 million years ago.
Strigamia maritima, a female and eggs. Image: © Carlo Brena.
Arthropods are the most species-rich group of animals on our planet. There are four classes of arthropods alive today: insects, crustaceans, chelicerates (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.
The centipede 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.
This species is favored by scientists for the accessibility of its nests, from which embryos can be gathered for study – making it an ideal candidate for obtaining the first genome sequence from a myriapod, and opening the door to new understandings of the developmental biology and ecology of these secretive animals.
The team found that the genome of Strigamia maritima has around 15,000 genes – around 7,000 fewer than a human – and 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 scientists to draw comparisons between very different animals, which are not obvious when working with fruit flies or other fast evolving insects.
For example, the team 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. 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,” said Dr Frank Jiggins of the University of Cambridge, who is a co-author of a paper published in the journal PLoS Biology.
One of the most surprising results is that Strigamia maritima appears 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.
“The Strigamia maritima’s genome sequence is of more than just scientific interest. Some of its genes may be of direct use. All centipedes inject venom to paralyze their prey. Components of venom often make powerful drugs, and the centipede genome will help researchers find these venom genes,” said co-author Prof Michael Akam of the University of Cambridge.
“Despite being closely related to insects, the centipede lacks the olfactory gene family used by insects to smell the air, and thus developed its own air-sniffing ability by expanding other gene families not present in insects,” added lead author Prof Ariel Chipman of the Hebrew University of Jerusalem’s Alexander Silberman Institute of Life Science.
“These centipedes live underground and have lost their eyes, together with almost all vision genes and genes involved in the body’s internal clock. They maintain enhanced sensory capabilities enabling them to recognize their environment and capture prey.”
“The centipede genome has proved to be particularly valuable in deducing the content of important gene families in the ancestral arthropod, this ancestor then being the starting point for the evolution of the huge diversity of arthropods that we currently see today,” said co-author Dr David Ferrier of the University of St. Andrews.
“There has been a high turn-over in arthropod gene and genome organization, with lots of rearrangements and plenty of gene losses during the evolution of animals like the insects.”
“The sorts of reconstructions that have been made possible by this new myriapod genome provide a foundation for delving more deeply into the biology of these genetic changes to see how they were linked to the diversification of the incredible range of body forms and modes of life that we now find in the arthropods.”
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Chipman, Ariel D. et al. 2014. The first myriapod genome sequence reveals conservative arthropod gene content and genome organisation in the centipede Strigamia maritima. PLoS Biology 12 (11): e1002005; doi: 10.1371/journal.pbio.1002005
