Comparative molecular embryology of arthropods:: the expression of Hox genes in the spider Cupiennius salei

In order to get a better understanding in the evolution of developmental processes, we have studied the expression of segmentation genes in a spider. The chelicerates, to which the spiders belong, form a basal arthropod group. Shared embryological features between chelicerates and insects therefore presumably represent ancestral characters for the arthropods. The comparative molecular embryology of insects and spiders will show us the degree of conservation or divergence of the processes underlying arthropod development. Shared molecular mechanisms between spiders and insects may eventually allow us to define a molecular archetype for the phylum Arthropoda. This in itself will be a necessity for comparing different phyla. The data we present here are an analysis of Hox genes in the spider Cupiennius salei. The Hox complex of chelicerates presumably consists of nine Hox genes. In addition to orthologues for the eight Hox genes as recognized in insects, the spider possesses an orthologue for the chordate Hox3 gene. This is probably the most direct evidence that the ancestral arthropod Hox-complex consisted of nine Hox-genes. Analysis of the expression patterns of the anterior Hox genes revealed a conserved mode of head segmentation in arthropods. This contradicts the traditional view that the deuterocerebral segment is missing in chelicerates. The conserved anterior expression borders for the Hox genes suggest that the Hox gene expression register as found in spiders and in insects may reflect to a great extent the ancestral mode of segmental subdivision in arthropods. This strengthens the argument for a monophyly of the arthropods.