VERTEBRATES might not be so special after all. Though we and our relatives were once thought to be the pinnacle of genomic complexity, the lowly sea anemone 鈥� one of the simplest animals on the planet 鈥� turns out to have a genetic vocabulary that is almost as rich.
Vertebrate genomes have far more genes coding for the signal molecules that direct embryonic development than other animals such as fruit flies or worms. This was assumed to reflect the unusual complexity of vertebrate development, so simpler animals ought to have correspondingly simpler genomes.
At least, this is what researchers expected to find when they started sorting through the genomes of animals from the phylum cnidaria, an anatomically simple group including sea anemones and corals that have only two embryonic tissue layers instead of the three other animals have. Having three layers allows development of complex structures such as internal organs. Cnidaria also lack such basic features as a flow-through gut.
Advertisement
But when researchers started identifying active genes in the coral Acropora millepora (pictured) and the sea anemone Nematostella vectensis by cataloguing RNA transcripts that are the first step in expressing genes, they got a surprise. 鈥淲e estimate they have somewhere between 20 and 25,000 鈥� the same ballpark humans are in,鈥� says team member Eldon Ball, a developmental biologist at the Australian National University in Canberra. The cnidarians also have genes for signalling molecules that were once thought to be exclusive to vertebrates (Trends in Genetics, vol 21, p 633).
If so, the common ancestor of cnidarians and higher animals that lived more than half a billion years ago must have been much more sophisticated than we thought. It also means that animals we know have simpler genomes, such as worms and fruit flies, must have lost many of the genes their ancestors once possessed.
鈥淲e鈥檝e been thinking so much about the origin of new genes that we haven鈥檛 put enough emphasis on gene loss,鈥� says Peter Holland, an evolutionary developmental biologist at the University of Oxford. 鈥淭his work highlights gene loss as something important.鈥�
