Evolution and functional specializations in nocturnal environments: insights from visual pigments in Chiroptera

Sensory systems act as a direct interface between organisms and their environment, constituting an ideal model to investigate how diverse evolutionary pressures shaped sensory adaptation. At the molecular level, vision is initiated through activation of visual pigments, light-sensitive complexes expressed in photoreceptor cells in the retina that have been shown to evolve in response to changes in ecology and light environment. In this thesis, I employ molecular evolutionary analyses and experimental characterization of visual pigments to study the molecular basis of visual system specializations in bats, one of the most striking and ecologically diverse mammalian radiations. In Chapter 2, I review recent work on the ecological and evolutionary pressures mediating sensory adaptation in vertebrates, including remarkable specializations of bats. In Chapter 3, I use comparative sequence analysis to test whether ecological factors known to influence bat visual ecology mediate shifts in evolutionary pressure in cone opsin genes, Lws and Sws1. I find significant evidence that long-term shifts in selection constraint in cone opsins occur in response to ecological factors underlying reliance on visual information and exposure to varying light environments. In Chapters 4 and 5, I focus my study on the evolution of the dim-light visual pigment rhodopsin (Rh1) in response to echolocation, a remarkable sensory adaptation of bats. In Chapter 4, I experimentally characterize rhodopsin of several bat species and find that changes in kinetic properties that influence dim-light visual performance are associated with differing echolocation abilities. In Chapter 5, I reconstruct and experimentally resurrect the ancestral rhodopsin pigment of Chiroptera and Scrotifera. I find that changes in rhodopsin kinetics occurred during the evolution of bats are likely associated with the origins of echolocation. This thesis combines in vitro and in silico approaches to investigate visual pigment evolution in Chiroptera. More broadly, this thesis also discusses the role of diverse ecological pressures in shaping visual gene evolution, the molecular underpinnings of visual pigment function and adaptation to light-limited environments as well as the complex interactions between sensory specializations.Ph.D.2020-11-16 00:00:0