Meeting Abstract

106.1  Sunday, Jan. 6  The molecular evolution of chiton ‘shell’ eyes SPEISER, DI*; OAKLEY, TH; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara dispeiser@gmail.com

Understanding the evolution of complex traits has been a major goal of biologists for generations. When studying the origin of a particular complex trait, it is useful to first ask about the separate evolutionary histories of its components. In the case of an image-forming eye, for example, these components likely include sets of genes involved with phototransduction, pigment synthesis, and organogenesis. Chitons (Phylum Mollusca; Class Polyplacophora) are a promising system in which to study the process of eye evolution. All chitons have “aesthetes”, small sensory tentacles that fill narrow channels in the dorsal shell plates. In certain chiton species, some of the aesthetes terminate in a ‘shell eye’ with a pigment layer, retina, and aragonite lens. From a phylogenetic perspective, these eyes are clearly a derived trait; further, from a historical standpoint, they may be the most recently evolved animal eyes – the chiton fossil record extends back to the Cambrian, but eyed chitons have only diversified within the last 25 million years. Using immunohistochemistry we find that the aesthetes, but not the eyes, of the chiton Acanthopleura granulata express an r-type opsin similar to those expressed by the cephalic eyes of many other invertebrates. Next, through transcriptome sequencing, we find that chiton aesthetes generally express a wide range of vision-related genes, including opsins and arrestins, pigment synthesis genes, and canonical “eye development” transcription factors such as Pax6. We hypothesize that both A. granulata’s aesthetes and eyes are light-sensitive, but that they operate via different phototransduction pathways and mediate different photo-behaviors. Further, our work suggests that the extra-ocular expression of vision-related genes may be a widespread trait in mollusks.