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Background  Divergence within  cis -regulatory sequences may contribute to the adaptive evolution of gene expression, but functional alleles in these regions are difficult to identify without abundant genomic resources. Among African cichlid fishes, the differential expression of seven opsin genes has produced adaptive differences in visual sensitivity. Quantitative genetic analysis suggests that  cis -regulatory alleles near the  SWS2 - LWS  opsins may contribute to this variation. Here, we sequence BACs containing the opsin genes of two cichlids,  Oreochromis niloticus  and  Metriaclima zebra . We use phylogenetic footprinting and shadowing to examine divergence in conserved non-coding elements, promoter sequences, and 3'-UTRs surrounding each opsin in search of candidate  cis -regulatory sequences that influence cichlid opsin expression.    Results  We identified 20 conserved non-coding elements surrounding the opsins of cichlids and other teleosts, including one known enhancer and a retinal microRNA. Most conserved elements contained computationally-predicted binding sites that correspond to transcription factors that function in vertebrate opsin expression;  O. niloticus  and  M. zebra  were significantly divergent in two of these. Similarly, we found a large number of relevant transcription factor binding sites within each opsin's proximal promoter, and identified five opsins that were considerably divergent in both expression and the number of transcription factor binding sites shared between  O. niloticus  and  M. zebra . We also found several microRNA target sites within the 3'-UTR of each opsin, including two 3'-UTRs that differ significantly between  O. niloticus  and  M. zebra . Finally, we examined interspecific divergence among 18 phenotypically diverse cichlids from Lake Malawi for one conserved non-coding element, two 3'-UTRs, and five opsin proximal promoters. We found that all regions were highly conserved with some evidence of CRX transcription factor binding site turnover. We also found three SNPs within two opsin promoters and one non-coding element that had weak association with cichlid opsin expression.    Conclusions  This study is the first to systematically search the opsins of cichlids for putative  cis- regulatory sequences. Although many putative regulatory regions are highly conserved across a large number of phenotypically diverse cichlids, we found at least nine divergent sequences that could contribute to opsin expression differences in  cis  and stand out as candidates for future functional analyses.

Divergence in cis-regulatory sequences surrounding the opsin gene arrays of African cichlid fishes