|The rear fang of the African Twig snake, Thelotornis capensis.|
Snake venoms have been subjected to increasingly sensitive analyses for well over 100 years, but most research has been restricted to front-fanged snakes, which actually represent a relatively small proportion of extant species of advanced snakes. Because rear-fanged snakes are a diverse and distinct radiation of the advanced snakes, understanding venom composition among “colubrids” is critical to understanding the evolution of venom among snakes. In a new paper Junqueira-de-Azevedo and colleagues (2016) review the state of knowledge concerning rear-fanged snake venom composition, emphasizing toxins for which protein or transcript sequences are available. The authors have also added new transcriptome-based data on venoms of three species of rear-fanged snakes. Based on this compilation, it is apparent that several components, including cysteine-rich secretory proteins (CRiSPs), C-type lectins (CTLs), CTLs-like proteins and snake venom metalloproteinases (SVMPs), are broadly distributed among “colubrid” venoms, while others, notably three-finger toxins (3FTxs), appear nearly restricted to the Colubridae (sensu stricto). Some putative new toxins, such as snake venom matrix metalloproteinases, are in fact present in several colubrid venoms, while others are only transcribed, at lower levels. This work provides insights into the evolution of these toxin classes, but because only a small number of species have been explored, generalizations are still rather limited. It is likely that new venom protein families await discovery, particularly among those species with highly specialized diets. The authors conclude that it is now abundantly clear that the venoms produced among the colubrid rear-fanged snakes are homologous with the much better characterized venoms of the front-fanged snakes. As trophic adaptations that facilitate feeding, venoms vary in composition with several important factors, including phylogeny, and so it is to be expected that among the diverse colubrid lineages, novel compounds, and new functional variants of better-known venom proteins, will be encountered. Much progress toward understanding rear-fanged snake venom composition has been made in the last decade, but, as indicated above, we have barely begun to explore the diversity of advanced snakes that comprise the colubrids. Transcriptomic and genomic approaches will greatly facilitate this work, but it must be remembered that functional assays should also accompany analysis of any venom, because the common recurring motif in venom biochemistry is to make the most of a stable molecular scaffold, perhaps best exemplified by the varied pharmacologies of the three-finger toxin superfamily. These small, structurally conservative peptides have very similar crystal structures but affect systems as diverse as neurotransmission, the blood clot cascade, ion channel function, and salamander limb regeneration and courtship. As Dr. Jay Fox once said, in venoms “we find only what we are looking for”, and, to find truly novel toxins that will likely be present in some colubrid venoms, we will have to look beyond the “normal” families of venom proteins.
Junqueira-de-Azevedo, I. L., Campos, P. F., Ching, A. T., & Mackessy, S. P. (2016). Colubrid venom composition: an-omics perspective. Toxins, 8(8), 230.