Dorsal or ventral: similarities in fate maps and gastrulation patterns in annelids, arthropods and chordates.

The idea that chordates, during their evolution, have inverted their dorsoventral body axis has recently gained substantial support. It has been shown that various dorsoventral patterning genes that are evolutionarily conserved between insects and vertebrates are expressed dorsally in insects, and ventrally in vertebrates, or vice versa. The ventral body side of insects thus seems to correspond to the dorsal body side of vertebrates, and these are nerve cord-bearing, neural body sides in both groups. In order to exclude that the inverted polarity of gene patterning activity is purely accidental, we compare here vertebrate and invertebrate blastula fate maps and their gastrulation patterns in the framework of early gene expression. From this comparison it appears that the neural body sides, 'ventral' in annelids or arthropods, and 'dorsal' in chordates, develop at similar positions with respect to the initial egg asymmetry. In addition, the formation of the neural body sides involves similar movements during gastrulation. We further suggest that the deuterostome gastrulation seen in today's chordates can be derived from a more ancestral gastrulation pattern seen in today's annelids and arthropods, and that the ventral midline cells of insects correspond to the dorsal midline cells of vertebrates.