Leeches, despite the yuck factor, have captured the hearts of two University of California, Berkeley, scientists who are part of a team that this week is publishing the leech's complete genome sequence.
"This genome has revolutionized our studies," said David Weisblat, a UC Berkeley professor of molecular and cell biology who lobbied for inclusion of the leech in a genome sequencing initiative that has targeted a variety of animals in order to learn what they have in common with one another and with humans.
The initiative is being led by Daniel Rokhsar, UC Berkeley professor of molecular and cell biology and of physics and program lead for eukaryotic genomics at the Department of Energy's Joint Genome Institute (JGI) in Walnut Creek, Calif. His multinational team published the genomes of the leech and two relatives, the limpet and the marine worm, or polychaete, in the Dec. 20 issue of the journal Nature. The publication comes after five years of efforts analyzing mountains of information provided by the initial sequencing effort.
"Through comparison of these diverse genomes with each other, we can learn what animals have in common with each other, which in turn tells us about the features that their common ancestors had," said Rokhsar. "That is a big driver for comparative genome sequencing - it is one of the few ways we have of looking back in deep time."
For these organisms, deep time means more than 500 million years ago, toward the end of the Precambrian era, when they split off from animals that eventually evolved to become vertebrates - animals with backbones - such as humans.
These three genomes are important because they represent a group of animals that makes up between one quarter and one-third of all marine creatures. Called lophotrochozoans, they range from clams, snails and octopuses to segmented worms and even earthworms, most of which develop from an egg to an intermediate larval stage called a trochophore before metamorphosing to their final form.
"These trochophores are small, free-swimming ciliated larva, first described over 100 years ago, and this mode of embryological development underlies a tremendous amount of animal diversity in the oceans," Rokhsar said.
"It's incredible to think that over half a billion years ago, there was an animal with a trochophore larva that was swimming around in the Precambrian seas - and that animal gave rise to so much modern diversity," he said.