Fig. 2.

Gene losses in two major toxin-gene families partly facilitate the evolution of a simple venom phenotype in the Tiger Rattlesnake (C. tigris). Genomic position for the ${\mathrm{P}\mathrm{L}\mathrm{A}}_2$ (Upper) and SVMP (Lower) toxin tandem arrays across several rattlesnake species. Toxin genes are shown as black boxes, and toxin-gene nomenclature is taken from Dowell et al. (22, 23). Flanking nontoxin genes (e.g., OTUD3) are shown as white boxes. The Tiger Rattlesnake genome has large deletions in both toxin tandem arrays, although deletions do not completely explain the simplest venom phenotype; for example, five functional SVMP genes are retained in the genome, but only two are expressed. PII, type II SVMP; PIII, type III SVMP.