Hox genes are found in all metazoan phyla. They are active in distinct domains along the main body axis and direct the morphogenesis of segment-specific structures via the activation of downstream target genes. Hox genes are important factors in the evolution of animal body plans. They share three key traits [1]: (1) they are basically organized in a cluster in the genome, (2) there is a correlation between the 3'-5' order of the genes in the genome and the anterior to posterior order of expression of the genes, and (3) the protein encoded by each of the genes contains a homeobox, a highly conserved 60 amino acid sequence that is a DNA binding motif [2].
The Hox genes primarily are involved in providing the embryo with positional information. This is most obvious from experiments with mutants that lack a particular Hox gene or from embryos in which a particular Hox gene is misexpressed. Such embryos produce structures at the incorrect position, as the affected cells seem to misunderstand their location within the embryo. For instance when a particular Hox gene is absent or is misexpressed in the fruit fly Drosophila melanogaster, the affected segments get the identity of another segment [e.g. [3,4]]. Famous examples are the four-winged Drosophila fly, in which the halteres on the third thoracic segments are transformed to wings, or the flies with legs at the position of the antennae. This is homeosis, and the mutations are homeotic transformations. The Hox genes thus act as selector genes that select one anterior-posterior identity over another along the main body axis in the embryo, while their downstream target genes actually act as realizator genes that make the structure specific for each location [summarized in [5,6]].
Due to the widespread sampling of Hox genes from a large variety of metazoans, the evolution of Hox genes is well characterized. Gene duplications played an important role in the evolution of the Hox genes. Recent data on cnidarians [7] suggest that the last common ancestor of the cnidarians and bilaterians had a Hox cluster consisting of two anterior genes (a Hox1/2 and a Hox3 gene), and that the Hox cluster subsequently expanded via internal duplications in the lineages leading to the cnidarians and the bilaterians. The last common ancestor of the bilaterians (animals with a bilateral symmetry) presumably still contained such a cluster of three genes as seen in today's acoel flatworms, which may represent the closest approximation of the ancestral bilaterian [8]. The last common ancestor of the other bilaterians (the protosome/deuterostome ancestor) at least contained seven different Hox genes, maybe even nine or more [9], implying several Hox gene duplication events in this lineage after the divergence of the acoel flatworms [8]. The different genes in the Hox complex are most likely the result of tandem duplications followed by sequence divergence [9,10]. In vertebrates the complete Hox cluster has been duplicated twice, presumably via whole genome duplications, resulting in four clusters in tetrapods, while in teleost fish additional duplication events took place [11].
Arthropod Hox genes can basically be assigned to ten different classes and seem to be present in a single Hox cluster [12]. In the chelicerates (spiders, scorpions, mites, ticks, horseshoe crabs) however there are examples of duplicated Hox genes. In a PCR survey Cartwright et al. [13] found 28 different small homeobox fragments of Hox genes in the horseshoe crab Limulus polyphemus. They could identify one to four representatives for each Hox gene class suggesting the presence of multiple Hox clusters in an invertebrate. Additional data for duplications of Hox genes come from two spiders, Achaearanea tepidariorum and Cupiennius salei. Two copies of the Deformed (Dfd) gene have been described for Achaearanea [14], and a duplicated Ultrabithorax (Ubx) gene has been described for Cupiennius [15]. In addition to these chelicerates there is one example of a duplicated Hox gene in a myriapod; a duplicated Dfd gene has been described for the geophilomorph centipede Pachymerium ferrugineum [16].
In the present paper we describe four new Hox genes from the spider Cupiennius salei: a proboscipedia gene (Cs-pb), a second Dfd gene (Cs-Dfd-2), and two Sex comb reduced (Cs-Scr) genes. Our data shows that at least three Hox genes (Dfd, Scr, and Ubx) are duplicated in the spider C. salei. Furthermore, pb and Scr orthologs have not been described from C. salei before. In previous work we described the sequence and expression of orthologs of eight classes of Hox genes from the spider C. salei: labial (Cs-lab), Hox3 (Cs-Hox3), Deformed (Cs-Dfd-1), fushi tarazu (ftz), Antennapedia (Cs-Antp), Ultrabithorax (Cs-Ubx-1 and Cs-Ubx-2), abdominal-A (Cs-abdA), and Abdominal-B (Cs-AbdB) [15,17-19]. With our new data on pb and Scr, the Central American wander spider Cupiennius salei becomes the first chelicerate for which orthologs of all ten arthropod Hox genes have been described.
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