Meeting Abstract
New genomic and transcriptomic resources for non-model study systems have revealed that light-sensitive cells are distributed more broadly across metazoan taxa and tissue types than suspected previously. Combined with a phylogenetic perspective on the distribution of light-detecting structures across animals, it appears that eyes in different metazoan lineages have evolved separately through the co-option of particular sub-sets of these non-visual photoreceptors. Here, I will discuss how we can use the diverse array of extracephalic visual systems found in mollusks – particularly those in chitons (Class Polyplacophora) and bivalves (Class Bivalvia) – to ask how and why eyes evolve from less complex light-sensitive structures. To learn how eyes evolve, I will describe how we identify the molecular components of eyes using a high-throughput, tree-based approach and then infer the separate evolutionary histories of these components by studying their functions and patterns of expression across taxa. To illustrate these methods, I will present evidence that the unique shell-eyes found in certain species of chiton are homologous to light-sensitive, non-visual organs found in eyeless relatives. By inferring ancestral character states for these homologous organs, we can construct a step-by-step account of how the eyes of chitons may have evolved. To address why eyes may have evolved in some lineages (and may not have evolved in others), I will discuss the functional benefits, costs, and constraints associated with transitions from non-visual light-sensitive organs to those that provide spatial information about light. For example, spatial vision may help chitons and bivalves distinguish approaching predators from uniform decreases in illumination, thereby reducing costly defensive responses to non-existent threats, but it may come at the cost of decreased sensitivity to small or rapid changes in light levels.
