Recruitment of Rod Photoreceptors from Short Wavelength Sensitive Cones during the Evolution of Nocturnal Vision in Mammals. Mus musculus

Vertebrate ancestors had only cone-like photoreceptors. The duplex retina evolved in jawless vertebrates with the advent of highly photosensitive rod-like photoreceptors. Despite cones being the arbiters of high-resolution color vision, rods emerged as the dominant photoreceptor in mammals during a nocturnal phase early in their evolution. We investigated the evolutionary and developmental origins of rods in two divergent vertebrate retinae. In mice, we discovered genetic and epigenetic vestiges of short wavelength cones in developing rods and cell lineage tracing validated the genesis of rods from S-cones. Curiously, rods did not derive from S-cones in zebrafish. Our study illuminates several questions regarding the evolution of duplex retina and supports the hypothesis that, in mammals, the S-cone lineage was recruited via the Maf-family transcription factor NRL to augment rod photoreceptors. We propose that this developmental mechanism allowed the adaptive exploitation of scotopic niches during the nocturnal bottleneck early in mammalian evolution. Overall design: Fixed, GFP positive cells from Nrlp-GFP mouse retinas at post-natal ages P2, P10, and P28 were isolated by flow sorting by FACSAria II (Becton Dickinson). Cells resuspended in shearing buffer were processed for sonication using Covaris S220 focused-ultrasonicator to obtain chromatin fragments with the average size of 150-200 bp. DNA chromatin solution was then incubated with anti-H3K4me3, anti-H3K27me3 or normal IgG. The immunoprecipitated DNA was then purified using Agencourt AMPure XP beads. Sequencing libraries were prepared using TruSeq DNA sample preparation kit (Illumina) with multiple adjustments, which include usage of 1/100 dilution of TruSeq adapters and gel-free DNA size selection using AMPure XP beads. Libraries were PCR-amplified for 18 cycles and each library was single-end sequenced in an independent lane of a GAIIx at a length of 76 bases. Fastq files were generated from reads passing chastity filter.