Abstract
Animals inhabiting the darkness of caves are generally blind and de-pigmented, regardless of the phylum they belong to. Survival in this environment is an enormous challenge, the most obvious being to find food and mates without the help of vision, and the loss of eyes in cave animals is often accompanied by an enhancement of other sensory apparatuses. Here we review the recent literature describing developmental biology and molecular evolution studies in order to discuss the evolutionary mechanisms underlying adaptation to life in the dark. We conclude that both genetic drift (neutral hypothesis) and direct and indirect selection (selective hypothesis) occurred together during the loss of eyes in cave animals. We also identify some future directions of research to better understand adaptation to total darkness, for which integrative analyses relying on evo-devo approaches associated with thorough ecological and population genomic studies should shed some light.
Highlights
Animals inhabiting the darkness of caves are generally blind and de-pigmented, regardless of the phylum they belong to
Conclusions on molecular evolution data In the two species for which large-scale molecular evolution data are available, the results converge to show an overrepresentation of “eye genes” in those that are affected by loss of function or radical amino acid substitutions [90,96]
The lens seems to play a central role in eye degeneration [46,56], and lens crystallins seem targeted by substitutions in coding and in non-coding regions
Summary
And they have been much debated and opposed to each other, the neutral and selective hypotheses for the loss of eyes in cave animals are not mutually exclusive processes. Evidence from developmental biology and from molecular evolution studies suggests that both occurred together. Are genetic linkage and QTL clusters controlling the concerted evolution of multiple traits in cave animals [69,104]? Selection or drift, when surface-type ancestors were trapped into caves? Were some pre-existing and necessary mutations present at low frequencies in the surface populations, allowing undelayed selection and rapid adaptation for the great survival and reproductive challenges associated with the cave environment? These questions may get some answers from integrative analyses relying on multiple evo-devo approaches associated with thorough ecological and population genomic studies. Authors’ contributions SR and DC wrote the review. Both authors read and approved the final manuscript
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