Gene sharing, lens crystallins and speculations on an eye/ear evolutionary relationship

The crystallins comprise 80-90% of the water-soluble proteins of the transparent, cellular, refractive eye lens and are responsible for its optical properties. Comparative studies have established that the crystallins are surprisingly diverse and often differ among species in a taxon-specific fashion. In general, the crystallins are derived from or identical to metabolic enzymes or stress (small heat shock) proteins that are expressed to a lesser extent in other tissues where they have non-refractive roles. We call the phenomenon of having the small heat shock protein or enzyme and lens crystallin encoded in the identical gene "gene sharing"; examples include small heat shock protein/αB-crystallin, α-enolase/τ-crystallin and argininosuccinate lyase/δ2-crystallin. Lens crystallins have evolved by gene sharing in vertebrates (all) and invertebrates (cephalopods, scallops, jellyfish). Similar cis-elements and transcription factors (including Pax6 among others) appear to unify lens expression of crystallin genes in vertebrates and mollusks (especially scallops). Instead of Pax6, cnidarians have a PaxB gene encoding a Pax2 DNA-binding paired domain and octapeptide, and a Pax6 homeodomain; PaxB appears important for ocellus (eye) development and crystallin gene expression in the cubomedusan jellyfish, Tridpedalia cystophora. Finally, we speculate on the basis of our current studies on Tripedalia that eyes and statocysts (associated with mechanoreceptors in many cnidarians and thus possibly ears in vertebrates) are evolutionarily related. Numerous examples indicate that gene sharing is widely used, consistent with changes in gene regulation being an evolutionary driving force for innovation of protein function.