The evolution of black widow venom

Black widow venom contains α-latrotoxin, infamous for causing intense pain. Combining 33kb of Latrodectus hesperus genomic DNA with RNA-Seq, we characterized the α-latrotoxin gene and discovered a paralog, 4.5kb downstream. Both paralogs exhibit venom gland specific transcription, and may be regulated post-transcriptionally via musashi-like proteins. A 4kb intron interrupts the α-latrotoxin coding sequence, while a 10kb intron in the 3′ UTR of the paralog may cause non-sense-mediated decay. Phylogenetic analysis confirms these divergent latrotoxins diversified through recent tandem gene duplications. Thus, latrotoxin genes have more complex structures, regulatory controls, and sequence diversity than previously proposed.

Evolutionary relationships of latrotoxin paralogs indicating the relatively recent duplication of tandem latrotoxins. Shown is a midpoint rooted Bayesian phylogenetic tree of latrotoxin proteins. Numbers at nodes indicate clade posterior probabilities (PP). The node marked by an asterisk has a PP of 1.00. Sequences from L. tredecimguttatus are in red and L. hesperus in blue; sequences from L. geometricus and S. grossa are in orange or green, respectively. Arrows indicate the tandem sequences assembled by Cufflinks from this study. All other L. hesperus and L. tredecimguttatus sequences were from de novo assembly of RNA-Seq reads. Shaded beige boxes label clades that contain a functionally characterized paralog. The larger gray shaded clade contains the two genomic insert latrotoxins and more closely related sequences, including all α-latrotoxins. Branches tested for sites under positive selection are indicated by the number 1 in purple text.