Expression pattern of Brachyury in the mollusc Patella vulgata suggests a conserved role in the establishment of the AP axis in Bilateria

International audienceWe report the characterisation of a Brachyury ortholog (PvuBra) in the marine gastropod Patella vulgata. In this mollusc, the embryo displays an equal cleavage pattern until the 32-cell stage. There, an inductive event takes place that sets up the bilateral symmetry, by specifying one of the four initially equipotent vegetal macromeres as the posterior pole of all subsequent morphogenesis. This macromere, usually designated as 3D, will subsequently act as an organiser. We show that 3D expresses PvuBra as soon as its fate is determined. As reported for another mollusc (J. D. Lambert and L. M. Nagy (2001) Development128, 45-56), we found that 3D determination and activity also involve the activation of the MAP kinase ERK, and we further show that PvuBra expression in 3D requires ERK activity. PvuBra expression then rapidly spreads to neighbouring cells that cleave in a bilateral fashion and whose progeny will constitute the posterior edge of the blastopore during gastrulation, suggesting a role for PvuBra in regulating cell movements and cleavage morphology in Patella. Until the completion of gastrulation, PvuBra expression is maintained at the posterior pole, and along the developing anterior-posterior axis. Comparing this expression pattern with what is known in other Bilateria, we advocate that Brachyury might have a conserved role in the regulation of anterior-posterior patterning among Bilateria, through the maintenance of a posterior growth zone, suggesting that a teloblastic mode of axis formation might be ancestral to the Bilateria

Characterisation of two snail genes in the gastropod mollusc Patella vulgata. Implications for understanding the ancestral function of the snail-related genes in Bilateria

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The new animal phylogeny: Reliability and implications

DNA sequence analysis dictates new interpretation of phylogenic trees. Taxa that were once thought to represent successive grades of complexity at the base of the metazoan tree are being displaced to much higher positions inside the tree. This leaves no evolutionary “intermediates” and forces us to rethink the genesis of bilaterian complexity

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This article confirmed also some earlier trees showing some `pseudocoelomate ' phyla (nematomorphs, priapulids) emerging with the arthropods 18 . Aguinaldo et al. noted that this new clade of metazoans comprises taxa that all undergo molting of their cuticle and so they named it `Ecdysozoa'. Similarly, a slowly evolving platyhelminth also rose in the tree to a position with the lophotrochozoans. These results have been confirmed in two independent papers presenting very broad metazoan rRNA phylogenies 20,21 . Shortly before, Balavoine obtained two other results that fitted remarkably the new position of flatworms: the flatworms contain an almost full set of Hox genes. This surprising complexity of the cluster in an unsegmented, `simple' looking organism provided a first hint that it might have been wrongly positioned as a primitive bilaterian. In addition, and even more significantly, careful analysis of amino acid and peptide signatures within and close to the homeodomain showed several similarities with protostome genes and, more specifically, to lophotrochozoan ones. These combined results appear to signal the demise of acoelomates and pseudocoelomates as early grades of metazoans, pre-dating the protostome/deuterostome split and suggest instead that they might correspond to secondarily simplified organisms 22,23 . It should be stressed that, in spite of these modifications in the trees, the resolution within the two large protostome clades did not improve: in particular, neither annelids nor molluscs appeared as monophyletic groups within lophotrochozoans, while branching orders remained basically unresolved within Ecdysozo