Wednesday, August 10, 2016

Colubroid venom composition- an overview

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.

Viper diversification examined

In a forthcoming paper in MPE Alencar et al (2016) look at the diversification of vipers. The cosmopolitan family contains about 329 venomous species showing a striking heterogeneity in species richness among lineages. While the subfamily Azemiopinae comprises only two species, 70% of all viper species are arranged in the subfamily Crotalinae or the “pit vipers”. The radiation of the pit vipers was marked by the evolution of the heat-sensing pits, which has been suggested to be a key innovation for the successful diversification of the group. Also, only crotalines were able to successfully colonize the New World. The authors present the most complete molecular phylogeny for the family to date that includes sequences from nuclear and mitochondrial genes representing 79% of all living vipers. They also investigated the time of divergence between lineages, using six fossils to calibrate the tree, and explore the hypotheses that suggest crotalines have undergone an explosive radiation. The phylogenetic analyses retrieved high support values for the monophyly of the family Viperidae, subfamilies Viperinae and Crotalinae, and 22 out of 27 genera, as well as well-supported intergeneric relationships throughout the family. The study found strongly supported sister clade to the New World pit vipers that comprises Gloydius, Ovophis, Protobothrops and Trimeresurus gracilis. Time of divergence estimates suggested that vipers started to radiate around the late Paleocene to middle Eocene with subfamilies most likely dating back to the Eocene. The invasion of the New World may have taken place sometime close to the Oligocene/Miocene boundary. Diversification analyses suggested a shift in speciation rates during the radiation of a sub-clade of pit vipers where speciation rates rapidly increased but slowed down toward the present. Thus, the evolution of the loreal pits alone does not seem to explain their explosive speciation rates. The auithor suggest that climatic and geological changes in Asia and the invasion of the New World may have also contributed to the speciation shift found in vipers.

Alencar, L. R., Quental, T. B., Grazziotin, F. G., Alfaro, M. L., Martins, M., Venzon, M., & Zaher, H. (2016). Diversification in vipers: Phylogenetic relationships, time of divergence and shifts in speciation rates. Molecular Phylogenetics and Evolution.

An endangered homalopsid snake, Enhydris jagorii

Jagor's Water Snake, Enhydris jagorii. Photo D. R. Karns

Jagor's water snake (Enhydris jagorii) is a freshwater snake endemic to the Chao Phraya-Ta Chin basin of Thailand. Habitat change and destruction are the main threats to this snake, where a large area of the wetland has been rapidly transformed into urban and agricultural areas. Moreover, uncontrolled fishing seriously threatens the what is most likely the last  remaining population of this snake. Pongcharoen et al. (2016) conducted a field study in the Bung Ka Loh wetland from October, 2010 to August, 2014 and collected 108 specimens of this species. Analysis of the stomach contents revealed that it is piscivorous, with cyprinids being the dominant prey. Prey items were usually less than 10% of the snake body mass and multiple prey items were occasionally found. No significant difference in diet was noted between the sexes. In addition, predation on this snake by Cylindrophis ruffus was first recorded in this study. The smallest gravid female collected had a snout-vent length of 34.0 cm. The clutch size and mass ranged from 1 to 28 embryos and 3.1–123.0 g, respectively, and both of these quantities increased significantly with increased female size. Reproduction was possibly seasonal and occurred in the rainy season. A preliminary study of other wetlands in the central plain of Thailand failed to detect the existence of this species. Accordingly, the conservation status of this species should be changed from Data Deficient to Critically Endangered.

Pongcharoen, C., Voris, H. K., Seelanan, T., Pradatsundarasar, A. O., & Thirakhupt, K. (2016). Diet, female reproduction and conservation of Jagor’s water snake, Enhydris jagorii in Bung Ka Loh wetland, Uttaradit province, Thailand. Agriculture and Natural Resources. Available online 25 June 2016