Larval And Adult Body Axes In Echinoderms

The echinoderm body plan is amazing and unique among metazoans, with pentameral (fivefold) symmetry as adults. Fossil records indicate that most of the extinct species also had non-bilateral shapes. Five classes of extant echinoderm species show diverse morphologies incorporating the pentameral symmetry. The anterior-posterior axis of most living echinoderms is not obvious, and it appears to be radially symmetrical as well. Instead, an oral-aboral axis and a proximal-distal axis have been assigned. Moreover, the body axes are different in an embryo/larva when compared to an adult. Embryos and larvae are bilateral with animal-vegetal, anterior-posterior, left-right, dorsal-ventral (oral-aboral) axes present. The larvae metamorphose into pentameral adults, and together with the change in structure, the body axes change. This remarkable transition in morphology makes their study unique for understanding how body axes are formed and how they change in evolution. Molecular studies of the development of axes in embryos and larvae have shown that the factors and genes involved in axis formation in echinoderms are shared with other bilaterians and these mechanisms appear to be conserved. However, the development into a pentaradial adult remains a mystery. Recent findings based on analyses of hox genes suggest that the oral-aboral axis of the adult body is equivalent to the anterior-posterior axis of other bilaterians. In this chapter, we will discuss the correlation between the hox gene expression and transitions in this unique change in body axes.