The origins and evolution of chromosomes, dosage compensation, and mechanisms underlying venom regulation in snakes.

Here we use a chromosome-level genome assembly of a prairie rattlesnake (Crotalus viridis), together with Hi-C, RNA-seq, and whole-genome resequencing data, to study key features of genome biology and evolution in reptiles. We identify the rattlesnake Z Chromosome, including the recombining pseudoautosomal region, and find evidence for partial dosage compensation driven by an evolutionary accumulation of a female-biased up-regulation mechanism. Comparative analyses with other amniotes provide new insight into the origins, structure, and function of reptile microchromosomes, which we demonstrate have markedly different structure and function compared to macrochromosomes. Snake microchromosomes are also enriched for venom genes, which we show have evolved through multiple tandem duplication events in multiple gene families. By overlaying chromatin structure information and gene expression data, we find evidence for venom gene-specific chromatin contact domains and identify how chromatin structure guides precise expression of multiple venom gene families. Further, we find evidence for venom gland-specific transcription factor activity and characterize a complement of mechanisms underlying venom production and regulation. Our findings reveal novel and fundamental features of reptile genome biology, provide insight into the regulation of snake venom, and broadly highlight the biological insight enabled by chromosome-level genome assemblies.

Insight into the roles of selection in speciation from genomic patterns of divergence and introgression in secondary contact in venomous rattlesnakes.

Investigating secondary contact of historically isolated lineages can provide insight into how selection and drift influence genomic divergence and admixture. Here, we studied the genomic landscape of divergence and introgression following secondary contact between lineages of the Western Diamondback Rattlesnake (Crotalus atrox) to determine whether genomic regions under selection in allopatry also contribute to reproductive isolation during introgression. We used thousands of nuclear loci to study genomic differentiation between two lineages that have experienced recent secondary contact following isolation, and incorporated sampling from a zone of secondary contact to identify loci that are resistant to gene flow in hybrids. Comparisons of patterns of divergence and introgression revealed a positive relationship between allelic differentiation and resistance to introgression across the genome, and greater-than-expected overlap between genes linked to lineage-specific divergence and loci that resist introgression. Genes linked to putatively selected markers were related to prominent aspects of rattlesnake biology that differ between populations of Western Diamondback rattlesnakes (i.e., venom and reproductive phenotypes). We also found evidence for selection against introgression of genes that may contribute to cytonuclear incompatibility, consistent with previously observed biased patterns of nuclear and mitochondrial alleles suggestive of partial reproductive isolation due to cytonuclear incompatibilities. Our results provide a genome-scale perspective on the relationships between divergence and introgression in secondary contact that is relevant for understanding the roles of selection in maintaining partial isolation of lineages, causing admixing lineages to not completely homogenize.