Abstract
BackgroundHox genes are key elements in patterning animal development. They are renowned for their, often, clustered organisation in the genome, with supposed mechanistic links between the organisation of the genes and their expression. The widespread distribution and comparable functions of Hox genes across the animals has led to them being a major study system for comparing the molecular bases for construction and divergence of animal morphologies. Echinoderms (including sea urchins, sea stars, sea cucumbers, feather stars and brittle stars) possess one of the most unusual body plans in the animal kingdom with pronounced pentameral symmetry in the adults. Consequently, much interest has focused on their development, evolution and the role of the Hox genes in these processes. In this context, the organisation of echinoderm Hox gene clusters is distinctive. Within the classificatory system of Duboule, echinoderms constitute one of the clearest examples of Disorganized (D) clusters (i.e. intact clusters but with a gene order or orientation rearranged relative to the ancestral state).ResultsHere we describe two Hox genes (Hox11/13d and e) that have been overlooked in most previous work and have not been considered in reconstructions of echinoderm Hox complements and cluster organisation. The two genes are related to Posterior Hox genes and are present in all classes of echinoderm. Importantly, they do not reside in the Hox cluster of any species for which genomic linkage data is available.ConclusionIncorporating the two neglected Posterior Hox genes into assessments of echinoderm Hox gene complements and organisation shows that these animals in fact have Split (S) Hox clusters rather than simply Disorganized (D) clusters within the Duboule classification scheme. This then has implications for how these genes are likely regulated, with them no longer covered by any potential long-range Hox cluster-wide, or multigenic sub-cluster, regulatory mechanisms.
Highlights
Hox genes are key elements in patterning animal development
Hox11/13d and Hox11/13e are novel Posterior Hox genes Hox11/13d was first detected during an attempt to find orthologues of known echinoderm Hox genes in the ophiuroid Ophiothrix spiculata
Subsequent searches confirmed the existence of a Hox11/13d gene distinct from Hox11/13b and c in all other echinoderm genomes available at the time of study: the sea urchin Lytechinus variegatus, the sea cucumbers Parastichopus parvimensis and Apostichopus japonicus, the sea stars Patiria miniata and Acanthaster planci, and the crinoid Anneissia japonica
Summary
Hox genes are key elements in patterning animal development. They are renowned for their, often, clustered organisation in the genome, with supposed mechanistic links between the organisation of the genes and their expression. Within the classificatory system of Duboule, echinoderms constitute one of the clearest examples of Disorganized (D) clusters (i.e. intact clusters but with a gene order or orientation rearranged relative to the ancestral state). Echinoderms (which along with hemichordates constitute the Ambulacraria, see Fig. 1) occupy a key position in studies on the evolution of Hox gene organisation, because of the amenability of these organisms to molecular genetic research, and because they provided the clearest example of intact but Disorganized (D) clusters, according to the classification system of Duboule [4]. The unconventional Hox cluster of sea urchins turned out to be a lineage-specific oddity, with both enteropneust
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