Adaptation of the eyes of grass puffer (Takifugu niphobles) to the riverine and marine environments

Abstract

Since fish inhabit heterogeneous light environments, fish vision is known as a prime example of adaptive evolution. Many species of marine fish migrate to rivers, but little is known about whether these species switch their vision when inhabiting rivers or whether they are adapted to the environment of both rivers and the sea. Grass puffer (Takifugu niphobles) is a marine fish species that frequently migrate to the rivers. In this study, we investigate grass puffers from both riverine and marine populations and analyzed the gene expression in the eyes. Phylogeny and population genetics of riverine and marine grass puffers indicated that riverine and marine grass puffers are from the same population. High-throughput RNA sequencing was used to compare expression levels of two marines and one riverine pufferfish species, as well as riverine and marine glass puffers. The results suggest that the grass puffers are estimated to be adapting their visual system to two environments, the river and the sea, rather than switching their visual system by changing the expression of vision-related genes. Four of the six genes with higher expression in riverine populations than in marine populations of grass puffer were heat shock proteins and related genes. The expression levels of these genes were higher in riverine grass puffer than in other marine and river pufferfish species, suggesting that the grass puffer individuals are adapting to the environmental difference that they face when they migrate to the river by increasing the expression levels of heat shock protein and related genes.