Conservation of the segmentation cascade in arthropods.

The well-studied segmentation cascade of Drosophila melanogaster represents a derived mechanism compared with that of other arthropods. The degree of conservation of genes that function in successive steps of the segmentation cascade are represented by the width of the hourglass. The earliest stage of the cascade, axis determination by maternal gradients, has diverged significantly between arthropod groups. Gap-gene homologues can be found in all arthropods, but their function in segmentation is variable. Pair-rule patterning has been described in several arthropods, but it is not clear whether this is an ancestral feature or one that has evolved convergently. The best-conserved stage is the one in which segmental boundaries are defined by the interaction of segment polarity genes. The expression of these genes coincides with the so-called 'phylotypic stage' of arthropods — the segmented germ band. Later on in the cascade, when axial identity is conferred by Hox genes, arthropod groups diverge again, with genes of the Hox family being expressed at different axial levels in different species.