Abstract
Hox genes are regulators of animal embryonic development. Changes in the number and sequence of Hox genes as well as in their expression patterns have been related to the evolution of the body plan. Lophotrochozoa is a clade of Protostomia characterized by several phyla which show a wide morphological diversity. Despite that the works summarized in this review emphasize the fragmentary nature of the data available regarding the presence and expression of Hox genes, they also offer interesting insight into the evolution of the Hox cluster and the role played by Hox genes in several phyla. However, the number of genes involved in the cluster of the lophotrochozoan ancestor is still a question of debate. The data presented here suggest that at least nine genes were present while two other genes, Lox4 and Post-2, may either have been present in the ancestor or may have arisen as a result of duplication in the Brachiopoda-Mollusca-Annelida lineage. Spatial and temporal collinearity is a feature of Hox gene expression which was probably present in the ancestor of deuterostomes and protostomes. However, in Lophotrochozoa, it has been detected in only a few species belonging to Annelida and Mollusca.
Highlights
Hox genes, a subfamily of homeobox genes, encode transcription factors containing a highly conserved 60 amino acid homeodomain characterized by a helix-turn-helix motif [1]
Internal relationships within the Lophotrochozoa clade are still controversial [18,19,20,21,22], we focused on works concerning the presence of Hox genes in 12 phyla, and the evolution of the genes composing the Hox cluster was discussed in relation to the evolutionary relationships between lophotrochozoan phyla
This review provides clear evidence that insight into Hox cluster composition and expression patterns is limited to a few phyla and that information is completely lacking for about half the taxa belonging to Lophotrochozoa
Summary
A subfamily of homeobox genes, encode transcription factors containing a highly conserved 60 amino acid homeodomain characterized by a helix-turn-helix motif [1] The members of this gene subfamily are regulators of animal embryonic development and play a role in the patterning of the anterior-posterior body axis of Bilateria [2]. One of the major groups of bilaterian organisms is Protostomia, which is subdivided into two clades: Ecdysozoa and Lophotrochozoa. The peculiarity of the former is the ability to undergo ecdysis under the hormonal control of ecdysteroids. The data obtained allowed different hypotheses to be delineated regarding the evolution of the Hox gene subfamily within Lophotrochozoa and its implications on development
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