A view to kill

Abstract

Genome and proteome data from Hydra magnipapillata have opened the way for the molecular analysis of an ancient nervous system, which includes stinging cells, an unusual neurosensory and neurosecretory cell type. They hold some surprises for the mechanisms and evolution of sensory transduction that could not have been anticipated from what has been learned from flies and vertebrates. Research in BMC Biology now implicates the ancient opsin-mediated transduction pathway in the neuronal control of stinging cell discharge.

Figure 1

Cellular organization of a battery cell complex in Hydra. (a) Schematic representation showing the location of the different cell types in a battery cell complex. (Adapted from Figure 1e of [6]). (b, c) Double staining of tentacle tissue of Hydra magnipapillata with the monoclonal antibodies (b) NVl (neuron-specific, green) and (c) H22 (nematocyte-specific, red) shows innervation of the nematocytes (arrowheads indicate (b) the NV1+ nerve cell body and (c) four large nematocysts in different battery cells). (Reproduced from Figure 1a, b of [6]). (d, e) Staining of tentacle tissue with (d) NV1 monoclonal antibodies and (e) the CnAsh probe, which detects the product of achaete-scute, a gene specifically expressed in nematocytes and the sensory neurons of battery cells. (Reproduced from Figure 5 of [8]). Scale bar: 25 μm (b, c); 5 μm (e, d).

Figure 2

Schematic representation of the hypothetical evolution of the cnidarian nematocyst. The secretome of early eukaryotes exhibited secretory vesicles that contained enzymes used for extracellular digestion. It evolved structural proteins with biomechanical properties that finally gave rise to a variety of protozoan extrusive organelles, which exhibit sophisticated discharge mechanisms. Some of these organelles are structurally similar to nematocysts, suggesting that nematocysts might have evolved from the extrusomes of a non-cnidarian precursor. Note that core components of the phototransduction cascade evolved in protozoans [9], as indicated by red vesicles and lettering. A nematocyte-sensory receptor complex evolved only in multicellular cnidarians/metazoans (Adapted from Figure 6 of [3]).