Genome-wide discovery of cis-regulatory elements in a large genome

Identifying non-coding regulatory elements in the genome poses a challenge in most organisms. Classical methods rely on trial and error to test the regulatory activities of DNA fragments using reporter constructs. In large eukaryotic genomes, where cis-regulatory elements can spread over long distances, separated by large stretches of non-functional DNA, this trial and error approach is particularly challenging. Here, we generate two types of resources that can be used to narrow the search for such cis-regulatory elements in the 3.6 Gbp genome of Parhyale hawaiensis (comparable in size to the human genome). First, we use bulk ATACseq to uncover genome-wide patterns of chromatin accessibility in embryonic and adult tissues of Parhyale (whole embryos and legs), and single-nucleus ATACseq to identify regions of open chromatin in diverse cell types recovered from adult legs, including epidermal, neuronal, muscle and blood cells. Second, by sequencing the genomes of three congeneric species of Parhyale hawaiensis - P. darvishi, P. aquilina and P. plumicornis - we identify islands of sequence conservation across the genome, corresponding to DNA elements that are functionally constrained during evolution. We present an approach by which low-coverage (10-15x) short-read genome sequencing, without genome assembly, is sufficient to provide reliable maps of sequence conservation. This approach cuts the cost and labour required to generate these maps, making the identification of cis-regulatory elements more widely accessible. We demonstrate the utility of these resources by identifying cis-regulatory elements that drive robust expression of fluorescent reporters ubiquitously and in specific cell types.