Fig. 1.

Features of a molluscan model organism, Aplysia californica. A Phylogenetic relationships of gastropods (Aplysia) to other prominent model organisms: insects (Drosophila), nematodes (Caenorhabditis elegans), sea urchins (Strongylocentrotus), ascidians (Ciona), vertebrates (represented by dog, Canis, and zebrafish, Danio), and cnidarians (hydrozoan Hydra). The evolutionary distance (indicated as relative branch length) from Canis or Danio to Aplysia is shorter than the distance from the vertebrates to Drosophila or C. elegans, suggesting that the amino acid replacement rate has been lower in the lineage leading to the gastropod Aplysia than in the lineages leading to Drosophila and C. elegans. B Nociceptive sensory neurons in the right ventrocaudal (VC) cluster revealed by in situ hybridization staining for a sensory neuron-specific neuropeptide, sensorin. A somatotopic map (‘Aplunculus’) of the ipsilateral body surface is represented in the cluster. Note that sensorin mRNA is found in neurites and axons throughout the pedal ganglion. C Expression of huntingtin occurs in the giant mucus release motor neuron, LPl1, and other neurons in the left pleural ganglion (in situ hybridization). Scale: 450 μm. D In situ hybridization shows choline acetyltransferase mRNA (a marker for cholinergic neurons) in diverse neurons, including LPl1, in the left pleural and pedal ganglia. Panels A, C and D are modified from Moroz et al. [2006]; panel B is modified from Walters et al. [2004].