Ultraconservation identifies a small subset of extremely constrained developmental enhancers

While experimental studies have suggested that non-coding ultraconserved DNA elements are central nodes in the regulatory circuitry that specifies mammalian embryonic development, the possible functional relevance of their>200bp of perfect sequence conservation between human-mouse-rat remains obscure 1,2. Here we have compared the in vivo enhancer activity of a genome-wide set of 231 non-exonic sequences with ultraconserved cores to that of 206 sequences that are under equivalently severe human-rodent constraint (ultra-like), but lack perfect sequence conservation. In transgenic mouse assays, 50percent of the ultraconserved and 50percent of the ultra-like conserved elements reproducibly functioned as tissue-specific enhancers at embryonic day 11.5. In this in vivo assay, we observed that ultraconserved enhancers and constrained non-ultraconserved enhancers targeted expression to a similar spectrum of tissues with a particular enrichment in the developing central nervous system. A human genome-wide comparative screen uncovered ~;;2,600 non-coding elements that evolved under ultra-like human-rodent constraint and are similarly enriched near transcriptional regulators and developmental genes as the much smaller number of ultraconserved elements. These data indicate that ultraconserved elements possessing absolute human-rodent sequence conservation are not distinct from other non-coding elements that are under comparable purifying selection in mammals and suggest they are principal constituents of the cis-regulatory framework of mammalian development