The genetics of venom ontogeny in the eastern diamondback rattlesnake (Crotalus adamanteus)

Abstract

The same selective forces that give rise to rapid inter- and intraspecific divergence in snake venoms can also favor differences in venoms across life-history stages. Ontogenetic changes in venom composition are well known and widespread in snakes but have not been investigated to the level of unambiguously identifying the specific loci involved. The eastern diamondback rattlesnake was previously shown to undergo an ontogenetic shift in venom composition at sexual maturity, and this shift accounted for more venom variation than geography. To characterize the genetics underlying the ontogenetic venom compositional change in C. adamanteus, we sequenced adult/juvenile pairs of venom-gland transcriptomes from five populations previously shown to have different adult venom compositions. We identified a total of 59 putative toxin transcripts for C. adamanteus, and 12 of these were involved in the ontogenetic change. Three toxins were downregulated, and nine were upregulated in adults relative to juveniles. Adults and juveniles expressed similar total levels of snake-venom metalloproteinases but differed substantially in their featured paralogs, and adults expressed higher levels of Bradykinin-potentiating and C-type natriuretic peptides, nerve growth factor, and specific paralogs of phospholipases A2 and snake venom serine proteinases. Juvenile venom was more toxic to mice, indicating that the expression differences resulted in a phenotypically, and therefore potentially ecologically, significant difference in venom function. We also showed that adult and juvenile venom-gland transcriptomes for a species with known ontogenetic venom variation were equally effective at individually providing a full characterization of the venom genes of a species but that any particular individual was likely to lack several toxins in their transcriptome. A full characterization of a species’ venom-gene complement therefore requires sequencing more than one individual, although the ages of the individuals are unimportant.

More yellow more toxic? Sex rather than alkaloid content is correlated with yellow coloration in the fire salamander

“…Intraspecific toxin variation has been subject to diverse studies on trophically venomous species that have demonstrated how a venom's composition and bioactivity may differ between populations (e.g. Amazonas et al, 2018), sexes (Rash, King & Hodgson, 2000), and ontogenetic stages (Rokyta et al, 2017). Animal venoms primarily utilized in prey subjugation often evolve under strong positive selection and allow for evolutionary tinkering, resulting in higher compound variability.…”

Comparative venom-gland transcriptomics and venom proteomics of four Sidewinder Rattlesnake (Crotalus cerastes) lineages reveal little differential expression despite individual variation

“…We isolated RNA via a standard TRIzol extraction following Rokyta et al . 20 , 44 , 45 . Briefly, venom glands were finely diced and placed in TRIzol solution (Invitrogen).…”

“…Extender assemblies used 1000 merged reads as seeds, extending the seeds based on exact overlaps of 120 base pairs. This method has efficiently assembled toxin sequences in a variety of snake taxa, including other Crotalus species 19 , 20 , 44 , 45 , 48 – 50 . De novo NGen assemblies were performed with default settings in NGen v. 14.…”

“…To determine if consensus transcripts were potentially absent in any individuals, we aligned the merged reads of each individual to the consensus transcriptome following Rokyta et al . 20 and calculated coverage of each site. Briefly, alignments were carried out using BWA MEM 52 and reads with more than three mismatches—via gaps or nucleotide differences—were removed.…”

“…Most studies of venom variation in rattlesnakes focus on populations of single species, where geographic variation is commonly detected and associated with variable diets 14 , 15 , local environments 16 , or interactions with coevolving prey 16 , 17 . These patterns are largely attributed to differential gene expression 17 – 20 . However, few intraspecific studies have explored the role of differential gene expression in driving venom variation where there is phylogenetic structure within a species across its range.…”