Arthropod-like expression patterns of engrailed and wingless in the annelid Platynereis dumerilii suggest a role in segment formation.

The origin of animal segmentation, the periodic repetition of anatomical structures along the anteroposterior axis, is a long-standing issue that has been recently revived by comparative developmental genetics. In particular, a similar extensive morphological segmentation (or metamerism) is commonly recognized in annelids and arthropods. Mostly based on this supposedly homologous segmentation, these phyla have been united for a long time into the clade Articulata.

The expression of a caudal homologue in a mollusc, Patella vulgata.

We cloned and analyzed the expression of a caudal homologue (PvuCdx) during the early development of the marine gastropod, Patella vulgata. PvuCdx is expressed at the onset of gastrulation in the ectodermal cells that constitute the posterior edge of the blastopore, as well as in the paired mesentoblasts, the stem cells that generate the posterior mesoderm of the trochophore larva. During larval stages, PvuCdx is expressed in the posterior neurectoderm of the larva, as well as in part of the mesoderm.

The segmented urbilateria: a testable scenario.

The idea that the last common ancestor of bilaterian animals (Urbilateria) was segmented has been raised recently on evidence coming from comparative molecular embryology. Leaving aside the complex debate on the value of genetic evidence, the morphological and developmental evidence in favor of a segmented Urbilateria are discussed in the light of the emerging molecular phylogeny of metazoans. Applying a cladistic character optimization procedure to the question of segmentation is vastly complicated by the problem of defining without ambiguity what segmentation is and to what taxa this definition applies.

Expression patterns of fork head and goosecoid homologues in the mollusc Patella vulgata supports the ancestry of the anterior mesendoderm across Bilateria.

We have characterised orthologues of the genes fork head and goosecoid in the gastropod Patella vulgata. In this species, the anterior-posterior (AP) axis is determined just before gastrulation, and leads to the specification of two mesodermal components on each side of the presumptive endoderm, one anterior (ectomesoderm), and one posterior (endomesoderm). Both fork head and goosecoid are expressed from the time the AP axis is specified, up to the end of gastrulation.

A lophotrochozoan twist gene is expressed in the ectomesoderm of the gastropod mollusk Patella vulgata.

The twist gene is known to be involved in mesoderm formation in two of the three clades of bilaterally symmetrical animals: viz. deuterostomes (such as vertebrates) and ecdysozoans (such as arthropods and nematodes). There are currently no data on the spatiotemporal expression of this gene in the third clade, the lophotrochozoans (such as mollusks and annelids).