Meeting Abstract
48.1 Sunday, Jan. 5 10:00 The origin of screening pigments in the evolution of the ‘shell’ eyes of chitons (Mollusca: Polyplacophora) SPEISER, D.I.*; DEMARTINI, D.G.; OAKLEY, T.H.; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara; Univ. of California, Santa Barbara dispeiser@gmail.com
Image-forming eyes have evolved multiple times in Mollusca. We are studying these eyes to learn: 1) the functional consequences associated with the separate steps of eye evolution and 2) the changes in genotype associated with the transitions between these steps. We are working on chitons because they have unique sensory structures embedded in their shell plates (“aesthetes”) that vary from the ancestral condition of non-pigmented, light-sensitive cells (“photoreceptors”), to bundles of pigmented cells (“eyespots”), to what may be the most recently evolved eyes with lenses (“eyes”). The eyes and eyespots of animals use a variety of pigments – i.e. melanins, pterins, and ommochromes – to screen off-axis light from reaching photoreceptors. To study the evolution of the eyes of chitons, we are characterizing the screening pigments used by their eyes and eyespots, as well as the genes associated with the production of these pigments. Our findings – based on evidence from spectral analysis, high-performance liquid chromatography (HPLC), and mass spectroscopy – indicate that several chitons with eyes (Acanthopleura granulata and Squamopleura araucariana) and eyespots (Chiton virgilatus) use pheomelanin, a red-brown pigment that is soluble in NaOH, as their screening pigment. Chitons are thus the first mollusks known to use pheomelanin as a screening pigment. From transcriptome sequencing, we find that pigmented and non-pigmented aesthetes from chitons express a homolog of tyrosinase, the gene responsible for the production of melanin in other mollusks, as well as many other organisms. We hypothesize that changes in the expression level or enzymatic activity of tyrosinase may have contributed to the origin of screening pigments in chitons.
