Spadella cephaloptera

The determination of the phylogenetic position of the chaetognaths is very important, because it will then be possible to polarize the major traits used to define the Deuterostomes. In the two scenarios presented above, deuterostomy and enterocoelous development of the coelom would thus be ancestral (plesiomorphic) traits of the bilaterians, inherited in common by the chaetognaths and the Deuterostomes, and lost at a later time in the Protostome branch. However, the less probable hypothesis of convergent acquisition of deuterostomy in both chaetognaths and Deuterostomes cannot be excluded.

New genomic data, combined with the first sequences of genes used for phylogeny (18S RNA, intermediate filaments), have advanced the study of the position of the chaetognaths in the Bilaterian tree. The mitochondrial genomes of two species of chaetognaths - Spadella cephaloptera (Papillon et al., 2004) and Paraspadella gotoi (Helfenbein et al.) - have recently been sequenced. These have turned out to be the smallest mitochondrial genomes known in animals, with only 13 and 14 genes respectively. After examining these sequences, the two groups have concluded that the chaetognaths are closely related to the Protostomes, and thus rejected the scenario of branching before the Protostome-Deuterostome divergence.

The phylogenetic position of the chaetognaths must be clarified by the study of new sequences. The sequencing project undertaken by Genoscope on the initiative of a consortium of three laboratories consists in sequencing a collection of expressed sequence tags (ESTs). This should produce a large number of molecular markers which should provide phylogenetic information. The congruence of the trees obtained with these different markers will be verified. The ESTs will have other uses as well: the cDNAs were prepared from different embryonic stages of Spadella cephaloptera, and will certainly yield new developmental genes (a Hox gene with an unusual type of structure has already been studied (Papillon et al., 2003)). Spadella cephaloptera is well-adapted to the methods of in situ analysis of gene expression, because the eggs and the embryos are totally transparent. This species lives in

Posidonia oceanica seagrass meadows, in shallow water not far from the shore, and is therefore easy to collect. Furthermore, the reproductive cycle of Spadella cephaloptera is fully mastered by one of the three groups, and the different pre- and post-hatching developmental stages of this animal have been well characterized. Analyzing the expression of genes of interest at each of these stages will help explain the origin of the molecular mechanisms which control the development of metazoans and their evolution, in relation to the genesis of the different body plans in the bilaterians (the “evo-devo” paradigm).

At the national level, the participant teams have produced a library of BAC clones within the framework of a "scientific interest group“(GIS) named”Genomique marine". This library will be useful in the isolation of the cluster of Hox genes of Spadella cephaloptera, which will provide new information on the evolution of body plans. The three groups are also members of a marine genomic network in the framework of the sixth PCRD, where they are promoting the chaetognath model.

Bibliography

• J.Y. Chen & D.Y. Huang, Science (2002) 298, 187
• D. Papillon et al., Dev. Genes Evol. (2003) 213, 142-148
• D. Papillon et al., Mol. Biol. Evol. (2004) 21, 1-8
• K.G. Helfenbein et al., Proc. Natl. Acad. Sci. USA (2004) 101, 10639-10643
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