Cranial sensory placodes are focused areas of the head ectoderm of vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactory/adenohypophyseal placodes of vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in vertebrates, but evolved from pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of vertebrates and tunicates.

The conservation of developmental functions exerted by Antpclass homeoproteins in protostomes and deuterostomes suggested that homologs with related functions are present in diploblastic animals. Our phylogenetic analyses showed that Antp-class homeodomains belong either to non-Hox or to Hox͞paraHox families. Among the 13 non-Hox families, 9 have diploblastic homologs, Msx, Emx, Barx, Evx, Tlx, NK-2, and Prh͞Hex, Not, and Dlx, reported here. Among the Hox͞paraHox, poriferan sequences were not found, and the cnidarian sequences formed at least five distinct cnox families. Two are significantly related to the paraHox Gsx (cnox-2) and the mox (cnox-5) sequences, whereas three display some relatedness to the Hox paralog groups 1 (cnox-1), 9͞10 (cnox-3) and the paraHox cdx (cnox-4). Intermediate Hox͞paraHox genes (PG 3 to 8 and lox) did not have clear cnidarian counterparts. In Hydra, cnox-1, cnox-2, and cnox-3 were not found chromosomally linked within a 150-kb range and displayed specific expression patterns in the adult head. During regeneration, cnox-1 was expressed as an early gene whatever the polarity, whereas cnox-2 was up-regulated later during head but not foot regeneration. Finally, cnox-3 expression was reestablished in the adult head once it was fully formed. These results suggest that the Hydra genes related to anterior Hox͞paraHox genes are involved at different stages of apical differentiation. However, the positional information defining the oral͞aboral axis in Hydra cannot be correlated strictly to that characterizing the anterior-posterior axis in vertebrates or arthropods. T he discovery of structural and functional homologies between regulatory genes used by Drosophila and vertebrates during their development led to the hypothesis that animals would share a common set of genes for defining the head, trunk, and posterior regions at early developmental stages (1-6). The proposed genes were homeobox genes belonging either to the Antp class, like empty-spiracle (emx), even-skipped (evx), Hox genes, or to the Prd class, like orthodenticle (Otx), goosecoid. Phylogenetic analyses performed on a vast amount of Hox homeodomain (HD) sequences, including representatives from all classes of homeobox genes from animals, protozoa, fungi, and plants, confirmed the monophyly of the Antp class as well as its position as a sister group to the Paired class (7). Within the Antp class, the Hox gene organization is distinctive and enigmatic: the genes map in clusters, and the order of individual genes within a cluster correlates with their temporospatial expression pattern along the anterior-posterior body axis during development (8). Recently, it was proposed that the common bilaterian ancestor of protostomes and deuterostomes had at least seven Hox genes (9). However, the question of the composition of the ancestral HOX cluster remains open. Analysis of Hox homeobox sequences (10) suggested that the conserved HOX cluster emerged early in the evolution of metazoans from an original cluster harboring three ancestral g...

No abstract

A survey against the draft genome sequence and the cDNA/EST database of Ciona intestinalis identified a number of genes encoding transcription factors regulating a variety of processes including development. In the present study, we describe almost complete sets of genes for Fox, ETS-domain transcription factors, nuclear receptors, and NFkappaB as well as other factors regulating NFkappaB activity, with their phylogenetic nature. Vertebrate Fox transcription factors are currently delineated into 17 subfamilies: FoxA to FoxQ. The present survey yielded 29 genes of this family in the Ciona genome, 24 of which were Ciona orthologues of known Fox genes. In addition, we found 15 ETS genes, 17 nuclear receptor genes, and several NFkappaB signaling pathway genes in the Ciona genome. The number of Ciona genes in each family is much smaller than that of vertebrates, which represents a simplified feature of the ascidian genome. For example, humans have two NFkappaB genes, three Rel genes, and five NFAT genes, while Ciona has one gene for each family. The Ciona genome also contains smaller numbers of genes for the NFkappaB regulatory system, i.e. after the split of ascidians/vertebrates, vertebrates evolved a more complex NFkappaB system. The present results therefore provide molecular information for the investigation of complex developmental processes, and an insight into chordate evolution.