A chromosome-level genome assembly of the highly heterozygous sea urchin Echinometra sp. EZ reveals adaptation in the regulatory regions of stress response genes

<p><em>Echinometra</em> is the most widespread genus of sea urchin and has been the focus of a wide range of studies in ecology, speciation, and reproduction. However, available genetic data for this genus are generally limited to a few select loci. Here, we present a chromosome-level genome assembly based on 10x Genomics, PacBio, and Hi-C sequencing for <em>Echinometra</em> sp. EZ from the Persian/Arabian Gulf. The genome is assembled into 210 scaffolds totaling 817.8 Mb with an N50 of 39.5 Mb. From this assembly we determined that the <em>E</em>. sp. EZ genome consists of 2n = 42 chromosomes. BUSCO analysis showed that 95.3% of BUSCO genes were complete. ab initio and transcript-informed gene modeling and annotation identified 29,<span>405</span> genes, including a conserved Hox cluster. <em>E.</em> sp. EZ can be found in high-temperature and high-salinity environments, and we therefore compared gene families and transcription factors associated with environmental stress response ("defensome") with other echinoid species with similar high-quality genomic resources. While the number of defensome genes was broadly similar for all species, we identified strong signatures of positive selection in non-coding elements near genes involved in environmental response pathways as well as losses of transcriptions factors important for environmental response. These data provide key insights into the biology of <em>E</em>. sp. EZ as well as the diversification of <em>Echinometra</em> more widely and will serve as a useful tool for the community to explore questions in this taxonomic group and beyond.</p>