The California sea hare, Aplysia californica, is the first mollusc to be sequenced. Its genome sequence will be useful in the study of invertebrate evolution, developmental biology, polyploidy, and toxicity, among other areas. But it will be best used in the study of the sea hare’s remarkable nervous system — a system that could not be designed better for neurobiological experimentation. Aplysia not only has a rather small number of central nervous system neurons (only 20000, instead of the 1012 of mammals), but those neurons are immense – ranging from 0.1–1 mm in diameter. They are the largest somatic cells in the animal kingdom; only eggs are larger. Aplysia neurons are so large that subcellular structures can be dissected out of them, DNA and antibodies can easily be injected into them, and cDNA libraries can be made out of individual cells. Also, researchers have attributed small groups of neurons to individual behaviors, making the biological study of learning, memory, and social behavior possible. And finally, the neurons can be cultured in vitro in networks, such that they make excellent models for the study of synaptogenesis, neural development, specialization, and degeneration.
The Broad Institute has sequenced to 11x coverage Aplysia californica from a line inbred at the Miami NIH Aplysia Center. We are now producing an all Illumina assembly from that same individual. We have also performed RNA-seq from many libraries derived from multiple tissues and developmental stages of the sea hare to aid in gene annotation. We hope that the genome sequence of Aplysia californica will not only serve as an essential phylogenetic node and an important outgroup for flies and nematodes, but will also teach us a great deal about the development, function, and deterioration of the human brain.