The acoel flatworm Symsagittifera roscoffensis.
The green colour is due to the presence of a symbiotic microalgae living inside the adults' tissues.
credit photography : Sarah Nahon (Ile Callot - 20/09/2005).
Symsagittifera roscoffensis is an acoel flatworm that belongs to the Acoelmorphs’ phylum which occupies a basal phylogenetic position in bilaterians (see phylogenetic tree). This basal position makes this species particularly interesting concerning the study the evolution of body plans.
The phylogenetic relationships of basal metazoans
Acoelomorphs are located at the base of the bilaterians (in light green), right after the Cnidarians phylum that is part of the Radiata (in dark green).
Document extracted from: The development of metazoans axial properties, Mark Q. Martindale, Nature Reviews Genetics 2005
Today, 99 % of animals on earth are bilaterians meaning their bodies are organized around a bilateral symmetry axis.
Bilateral symmetry
credit: http://en.wikipedia.org/wiki/Symmetry_(biology)
Most of 1% left animals have a radial symmetry and it is generally assumed among scientists that all bilateral animals come from a common ancestor that had a radial symmetry. This transition from radial to bilateral symmetry seems to have been strongly selected during the course of evolution. Because of the capital step and the evolutionary success the transition to bilateriality represents, studying how this transition happened is of great importance to scientists.
Developmental biologists are trying to understand what kind of event led to bilaterality and more generally what are the great steps in evolution which have permitted to observe the extant biodiversity.
The aim of the researches carried out on Symsagittifera roscoffensis is to compare the development of this species to the development of other bilaterians or radial organisms like jelly fishes. The comparison of the genes and gene networks involved in embryogenesis between species on both sides of the radial/bilateral transition may enable scientists to understand the mechanisms underlying this historical event.
Symsagittifera roscoffensis, is studied more specifically as it has some interesting advantages as it is a local species and therefore found in large amounts on the Atlantic and the English Channel coasts including Britanny coasts. Its culture is also performed in controlled conditions in the laboratory and allows to access to all developmental stages. Eggs and young individuals can be recovered at different ages which enable scientists to extract genetic material at different steps in development. Scientists should therefore be able to identify gene expression and then compare it to those present in other species and see which gene expression is similar.
2500 ESTs have been already sequenced during a pilot project for the flatworm Convoluta pulchra which was studied before Symsagittifera roscoffensis (see transcriptomics). These two species are closely related and data on C. pulchra can give information on S. roscoffensis.
Functional genomics will be used to understand how the embryo is developing and which and when genes are used in the development. EST libraries have been constructed representing which genes are expressed (e.g. RNAs) at different stages in the development of the organism. In situ hybridization technique is also used to see where and when genes are expressed during development. The way genes are expressed in acoel flatworms will then be compared to what happens in radial and bilateral organisms to see how gene expression patterns occur.
Within the network Marine Genomics Europe, Pedro Martinez of the University of Barcelona (partner #35) and his team are collaborating with Xavier Bailly of the Biological station of Roscoff (partner #2). Collaborations on this subject of study also exist outside the network with Andreas Wanninger of the University of Copenhagen (Department of Cell biology and Comparative Zoology). However, many partners of MGE are working on the evolution of regulatory networks in bilaterians and are focused on other developmental models as the sea urchin Strongylocentrotus purpuratus or the amphioxus (a small fish) Branchiostoma floridae, etc.
The acoel flatworm, Symsagittifera roscoffensis, is also interesting to study as it has a strong capacity of regeneration. When a piece of the worm is cut off, one could suspect and assume that the program of development which had already been used during embryogenesis may be launched again in the regeneration process. Understanding this phenomenon is of great importance for research in human health as it could enable scientists to propose basic molecular regeneration models which could be useful to understand mechanisms of regeneration of tissues that have been damaged for example.
Glossary:
Bilateral symmetry: Symmetrical arrangement, as of an organism or a body part, along a central axis, so that the body is divided into equivalent right and left halves by only one plane.
http://www.answers.com/topic/bilateral-symmetry
