BackgroundAnimals living in well-lit environments utilize optical stimuli for detecting visual information, regulating the homeostatic pacemaker, and controlling patterns of body pigmentation. In contrast, many subterranean animal species without optical stimuli have evolved regressed binocular eyes and body pigmentation. Interestingly, some fossorial and cave-dwelling animals with regressed eyes still respond to light. These light-dependent responses may be simply evolutionary residuals or they may be adaptive, where negative phototaxis provides avoidance of predator-rich surface environments. However, the relationship between these non-ocular light responses and the underlying light-sensing Opsin proteins has not been fully elucidated.MethodsTo highlight the potential functions of opsins in a blind subterranean animal, we used the Mexican cave tetra to investigate opsin gene expression in the eyes and several brain regions of both surface and cave-dwelling adults. We performed database surveys, expression analyses by quantitative reverse transcription PCR (RT-qPCR), and light-dependent locomotor activity analysis using pinealectomized fish, one of the high-opsin expressing organs of cavefish.ResultsBased on conservative criteria, we identified 33 opsin genes in the cavefish genome. Surveys of available RNAseq data found 26 of these expressed in the surface fish eye as compared to 24 expressed in cavefish extraocular tissues, 20 of which were expressed in the brain. RT-qPCR of 26 opsins in surface and cavefish eye and brain tissues showed the highest opsin-expressing tissue in cavefish was the pineal organ, which expressed exo-rhodopsin at 72.7% of the expression levels in surface fish pineal. However, a pinealectomy resulted in no change to the light-dependent locomotor activity in juvenile cavefish and surface fish. Therefore, we conclude that, after 20,000 or more years of evolution in darkness, cavefish light-dependent basal activity is regulated by a non-pineal extraocular organ.
A key goal of evolutionary genomics is to harness molecular data to draw inferences about selective forces that have acted on genomes. The field progresses in large part through the development and benchmarking of advanced molecular-evolution analysis methods. Here we evaluated the rigor and performance of a test of directional, cis-regulatory evolution across genes in pathways, using stem cells from Mus musculus subspecies as a model. We discovered a unique program of induction of ribosomal large-subunit genes in stem cells of the Southeast Asian mouse M. m. castaneus relative to its sister taxa, driven in part by cis-regulatory variation. As a complement, we used sequence analyses to find population- and comparative-genomic signatures of selection in M. m. castaneus, at the upstream regions of the ribosomal gene cohort. The unique high-expression program for these loci in M. m. castaneus was recapitulated in stem cell-derived neurons. We interpret our data under a model of changes in lineage-specific pressures across Mus musculus in stem cells with high translational capacity. Together, our findings underscore the rigor of integrating expression and sequence-based methods to generate hypotheses about evolutionary events from long ago.
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