Consequences of losing metamorphosis on the evolution of opsin and phototransduction gene expression in the eyes of Brook Salamanders.

Metamorphosis can provide an opportunity for animals with complex life cycles to transition between differing environments and still maintain optimal functionality across ontogeny. The structure and function of the amphibian visual system generally remodels to overcome differences in media (water, air) and fluctuations of available light. This in turn causes a shift in the selective pressures acting on the visual system at each life stage. Few comparative studies have evaluated how metamorphosis drives the evolution and expression of visual system genes across closely related species that occupy disparate habitats. Salamanders offer an excellent model to study visual system adaptations because of their diverse life cycles (biphasic and paedomorphic) and drastic habitat transitions (surface and cave-adapted). We sequenced whole-eye transcriptomes of larvae and adults across 13 lineages of Brook Salamanders to test whether there is a relationship between the expression of opsin and phototransduction genes, and lineages that differ in life cycle mode. Our study revealed that numerous differences in gene expression were lineage-specific. However, there was significant upregulation of rhodopsin and several rod-specific phototransduction genes in biphasic adults, whereas paedomorphic salamanders retained juvenilized expression patterns similar to larvae. The paedomorphic troglomorph species, Eurycea latitans, exhibited aberrant expression patterns of rhodopsin and some rod-related genes. This study has implications for how metamorphosis allows salamanders to adaptively decouple their visual system in order to optimize to disparate larval and adult environments, as well as the consequences of secondarily losing metamorphosis.