Of leeches and men
Genomic analysis of previously neglected species opens up new avenues for the study of biodiversity, development and evolution
Credit: Ajna S. Rivera/UC-Berkeley
The genomes of the freshwater leech Helobdella robusta (pictured), a marine snail and an ocean-dwelling worm are among the first sequenced genomes from a vast, understudied swath of the animal kingdom known as Lophotrochozoans.
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Researchers from EMBL in Heidelberg, in collaboration with colleagues from the Rice University, Texas, the University of Berkeley-California, and the Joint Genome Institute (JGI),California, have more than doubled our knowledge on the genomes of Lophotrochozoans. Never heard of Lophotrochozoans? It may be because that branch of the animal kingdom has often been neglected by scientists. However it contains a wide variety of species, mostly marine, that are widespread all over the world. The results, published in Nature in December, describe the genomes of three Lophotrochozoan species, and their evolutionary relationships to the other branches of the animal kingdom. These findings set the frame for future research on the biodiversity of this group, as well as on its evolution from our last common ancestor.
600 million years ago, in the oceans, lived Urbilateria, the common ancestor to all bilateran animals - animals whose right and left sides are mirrors of each other. With time (a lot of time), Urbilateria evolved into two separate groups of animals: a group called Deuterostomia (that includes humans and other vertebrates, as well as sea urchins and ascidians), and the much bigger group of the Protostomia (encompassing most of the invertebrate species). Protostomia are further divided into two sub-groups, one of which is the Lophotrochozoan clade.
“We analysed the genomes of a leech, of an ocean-dwelling worm and of a limpet that are representatives of different phyla of Lophotrochozoans,” explains Oleg Simakov, Oleg Simakov, who led and conducted large part of the bioinformatic analysis at EMBL. “We looked at both the genome’s sequence and organization: we compared them to the two other Lophotrochozoan genomes we know, and to other invertebrates and vertebrates, to find their place in the evolutionary tree.”
Comparing genomic features across different animal kingdoms can give scientists crucial hints on what the genome of Urbilateria looked like, but also on how evolution took place. “The three species that we analysed have very ancient features, such as the structure and the sequence of the genes, that don’t seem to have evolved too much since Urbilateria,” explains Detlev Arendt, one of the co-authors from EMBL. “However the study also identifies significant differences in some specific genes that highlight how complex evolution can be when it affects different features in different ways.”
The genes that help leeches ‘smell’ blood is one example of such very specific evolution. Another example is the Hox cluster: a very important group of genes that controls the direction of the embryonic development, defining what becomes the front and back end of an animal. This cluster of genes is present in some form in all animals: it is similar between the limpet and the marine worm but is completely disintegrated in the leech.
This result might give scientists some clues on how the embryonic development of the leech differs from that of the limpet or the marine worm. But more importantly, it highlights the fact that evolution is not a linear, simple process. Its complexity requires that we sequence and analyse a much larger number of organisms in order to understand marine life and the Lophotrochozoans and, further down the road, how evolution works in general.
Insights into bilaterian evolution fromthree spiralian genomes - Oleg Simakov, Ferdinand Marletaz, Sung-Jin Cho, Eric Edsinger-Gonzales, Paul Havlak, Uffe Hellsten, Dian-Han Kuo, Tomas Larsson, Jie Lv, Detlev Arendt, Robert Savage, Kazutoyo Osoegawa, Pieter de Jong, Jane Grimwood, Jarrod A. Chapman, Harris Shapiro, Andrea Aerts, Robert P. Otillar, Astrid Y. Terry, Jeffrey L. Boore, Igor V. Grigoriev, David R. Lindberg, Elaine C. Seaver, David A. Weisblat, Nicholas H. Putnam & Daniel S. Rokhsar - Published online in Nature on the 19 December 2012