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Curr Biol
2008 Aug 05;1815:1156-61. doi: 10.1016/j.cub.2008.06.074.
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Sponge genes provide new insight into the evolutionary origin of the neurogenic circuit.
Richards GS
,
Simionato E
,
Perron M
,
Adamska M
,
Vervoort M
,
Degnan BM
.
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The nerve cell is a eumetazoan (cnidarians and bilaterians) synapomorphy [1]; this cell type is absent in sponges, a more ancient phyletic lineage. Here, we demonstrate that despite lacking neurons, the sponge Amphimedon queenslandica expresses the Notch-Delta signaling system and a proneural basic helix loop helix (bHLH) gene in a manner that resembles the conserved molecular mechanisms of primary neurogenesis in bilaterians. During Amphimedon development, a field of subepithelial cells expresses the Notch receptor, its ligand Delta, and a sponge bHLH gene, AmqbHLH1. Cells that migrate out of this field express AmqDelta1 and give rise to putative sensory cells that populate the larval epithelium. Phylogenetic analysis suggests that AmqbHLH1 is descendent from a single ancestral bHLH gene that later duplicated to produce the atonal/neurogenin-related bHLH gene families, which include most bilaterian proneural genes [2]. By way of functional studies in Xenopus and Drosophila, we demonstrate that AmqbHLH1 has a strong proneural activity in both species with properties displayed by both neurogenin and atonal genes. From these results, we infer that the bilaterian neurogenic circuit, comprising proneural atonal-related bHLH genes coupled with Notch-Delta signaling, was functional in the very first metazoans and was used to generate an ancient sensory cell type.
Figure 1. Neurogenic Gene Expression in Amphimedon queenslandicaWhole-mount in situ hybridizations with probes for AmqNotch, AmqDelta1, and AmqbHLH1. (A), (E), (E′), (I), and (I′), top row, show cartoons of sponge development, anterior down. (F)–(H) and (J)–(L) show whole-mount in situ hybridizations. (B)–(D), (F′)–(H′), and (M)–(O) show sectioned in situ hybridizations. (F′)–(H′) and (I′)–(L′) show magnification of dotted circles in (F)–(H) and (I)–(L), respectively. In spot-stage embryos, all genes are expressed in the outer layer (B–D). As ring formation proceeds, the expression of all genes coalesces in the subepithelial layer (F–H) and is found in the large, round pre-migratory globular cells (indicated by arrowheads in (F′)–(H′); see also Figure S1). In late-ring-stage embryos, globular cells (indicated by arrowheads) migrating away from the subepithelial layer strongly express AmqDelta1 (K), whereas expression of AmqNotch (J) and AmqbHLH1 (L) remains more highly expressed in the subepithelium. The migration of globular cells originating in the subepithelial layer of ring-stage embryos (M) and their intercalation into the epithelial layer of larvae (P) is visualized by AmqDelta1 expression (M–P). Arrowheads indicate globular cells; dashed lines indicate subepithelial-epithelial layer boundary. The following abbreviations are used: ps, pigment spot; pr, pigment ring, icm, inner cell mass; sel, subepithelial layer; and el, epithelial layer.
Figure 2. AmqbHLH1 Has Neurogenin-like Proneural Activity in Xenopus and Atonal-like Proneural Activity in Drosophila(A–F) X-ngnr-1 or AmqbHLH1 mRNAs were injected, together with lacZ mRNAs, into one cell of two-cell-stage Xenopus embryos and analyzed by in situ hybridization. Probes are indicated on the left of the pictures. Injections of lacZ mRNA alone were used as controls. Dorsal views of neurula-stage embryos with the injected side on the right visualized by the light-blue staining that corresponds to lacZ expression are shown.(G–J) Dorsal views of N-Tubulin-labeled neurula-stage embryos that have been injected with X-ngnr-1 (left pictures) or AmqbHLH1 (right pictures) mRNAs in combination with X-NotchICD (G and H) or X-Notch ICD + X-MyT1 (I and J). The injected side is on the right.(K–L) Dorsal views of the central part (vein III-vein IV region) of the wing of wild-type (K) and Ptc-Gal4, UAS-AmqbHLH1 (L) flies. In wild-type, only three sensilla are found on vein III (indicated by arrows in [K]). AmqbHLH1 induces the formation of many sensory organs (indicated by arrows in [L]) as well as abnormal vein/intervein formation (see Figure S7 for more details).
Figure 3. Summary Diagram Mapping the Evolution of the bHLH Gene Family with the Evolution of Neurogenic Circuitry and Neural Morphology in the MetazoaChoanoflagellates diverged from the Metazoa prior to the emergence of Notch and Delta genes, and the expansion of the bHLH gene family in the metazoan lineage [33]. Subsequently, in the metazoan stem lineage, multicellularity was acquired, and we propose that this state also included differentiated sensory cells. An initial expansion of group A bHLH genes occurred before the poriferan divergence, but the major expansion of this family occurred later, in the Eumetazoa. There is no evidence of neurons in the Porifera; however, the Amphimedon gene AmqbHLH has proneural capability comparable to that of Atonal superfamily genes in bilaterians. We propose that proneural capability was possessed by the last common ancestor of the Atonal superfamily (the “ProtoAtonal” gene), and that this capability was inherited by AmqbHLH as well as by certain bilaterian descendents.
