EvoD/Vo: the origins of BMP signalling in the neuroectoderm

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Abstract
Signalling by bone morphogenetic proteins (BMPs) acts in a conserved all-or-none fashion to repress the expression of all neural genes in the epidermal ectoderm of bilaterian embryos. The dorsal–ventral (D/V) axis of vertebrate embryos is likely to be inverted with respect to that of invertebrates. BMP signalling represses expression of neural genes in dorsal regions of the central nervous system (CNS) in a threshold-dependent manner in the neural ectoderm of Drosophila melanogaster embryos. BMPs also act in a dose-dependent manner to pattern the CNS of vertebrates and annelid worms. Neural patterning mediated by threshold-dependent BMP repression might be ancestral to all metazoans. D. melanogaster and vertebrates have different ventral patterning systems: flies use a gradient of the Dorsal transcription factor, whereas a gradient of secreted Sonic hedgehog is used in vertebrates. Patterning systems that are present in ventral regions of the CNS act in concert with dorsally produced BMPs to refine and sharpen D/V patterning, and might have evolved independently in vertebrate and invertebrate lineages. Genes, such as Hox genes, that control the initial steps in establishing anterior–posterior (A/P) cell fates might define abstract positional codes that allow for rapid morphological diversification; by contrast, comparable genes acting along the D/V axis might define conserved cell types. To account for the conservation of both A/P and D/V regulators we propose a hypothesis, referred to as regulatory treadmilling, in which gene targets of A/P regulators turn over (or treadmill) more rapidly than those of D/V regulators.