A putative cis-regulatory element near Shox2 in the jerboa genome (Jaculus jaculus) is not sufficient to alter mouse...

Vertebrate limb bones grow at different rates within and between species to achieve a striking variety of proportions and functions. However, most coding mutations that alter bone growth in mice and humans affect the entire skeleton, pointing to cis-regulatory modularity as the primary mechanism to diversify proportion. In a search for causative loci, we previously identified 1,755 disproportionately differentially expressed genes associated with the extreme hindlimb elongation in bipedal jerboas, Jaculus jaculus, compared to laboratory mice. We then took a focused approach to identify putative cis-regulatory elements near select candidate genes in a highly fragmented draft Jaculus jaculus genome assembly. We identified an ATAC-seq peak near Shox2 that was reproducible in jerboa metatarsals and not in radius/ulna. Shox2 gained a novel expression domain in jerboa metatarsals and is both necessary and sufficient to modulate mouse limb bone length. This 139 bp putative enhancer in the jerboa genome centers on a ~1 kb deletion of sequence found broadly throughout placental mammals. Here, we show that replicating the deletion and replacement with the 139 bp jerboa sequence in mice causes no detected phenotype. A genome-wide analysis of our ATAC-seq data using a high-quality Jaculus jaculus genome assembly replicated the peak called at this location but also identified 7,445 peaks that are similarly reproducible in metatarsals and not in radius/ulna. Certainly not all of these peaks are causative, but it is likely that many small-effect mutations diversified proportion. It is therefore crucial to apply unbiased filters to genome-wide analyses and to not assume that the few historically favored genes will have the largest effect.