A new study by researchers at The University of Texas at Arlington challenges the practice of classifying a hepatic species as “venomous,” if the snake’s oral glands secrete some of 20 gene families considered “venom toxins” — a challenge to the evolution model that could have additional implications in this field of study. In support of their contention, the scientists are also developing a new snake venom model, with their work, published in the journal Molecular Biology and Evolution, based on painstaking analysis that compares related gene groups or gene families identified in different parts of Burmese python (Python molars bivittatus) tissue.
The UT Arlington team is led by assistant professor of biology Todd Castoe and includes researchers from Colorado and the United Kingdom. The team has discovered that similar levels of these purportedly toxic gene families are found in python oral glands as well as in python brain, liver, stomach and a number of other python organic tissue, and contend that these findings demonstrate a great deal about venom gene functions prior to their evolution into venoms, as well as showing that just gene expression related to nominal venom toxins in snakes’ oral glands doesn’t constitute sufficient information to determine categorically that the species is venomous.
“Research on venom is widespread because of its obvious importance to treating and understanding snakebite, as well as the potential of venoms to be used as drugs, but, up until now, everything was focused in the venom gland, where venom is produced before it is injected,” Dr. Castoe notes in a UT Arlington release. “There was no examination of what’s happening in other parts of the snake’s body. This is the first study to have used the genome to look at the rest of that picture.”
Dr. Castoe observes that more knowledge about venom evolution could be helpful to scientists in developing better anti-venom medicines and contribute to better understanding of human gene evolution, noting that that as genetic analysis capabilities become more sophisticated, scientists are revealing more evidence supporting a long-held theory that highly toxic venom proteins evolved from non-toxic genes, which have ordinary functions in the body, such as cellular functions or digestive.
“These results demonstrate that genes or transcripts which were previously interpreted as toxin genes are instead most likely housekeeping genes, involved in the more mundane maintenance of normal metabolism of many tissues,” says study co-author Stephen Mackessy, a biology professor at the University of Northern Colorado.
Research in Dr. Mackessy’s lab is focused on venomous snake biology and biochemistry related to their venoms. Of particular interest to his team is the interface between the animals’ evolution/ecology and the biochemistry/pharmacology of their venoms. Specific areas of investigation include analyses of rear-fanged snake venoms (metalloproteinases, PLA2s and three-finger toxins), ontogenetic and regional variation in venom composition, and prey-specific toxicity of venoms and isolated toxins. Recently the lab has been evaluating venoms as sources of novel compounds for potential drug discovery and design. The Mackessy Lab’s results also suggest that instead of a single ancient origin, venom and venom-delivery systems most likely evolved independently in several distinct reptile lineages.
UT Arlington’s Dr. Castoe was lead author of a 2013 study mapping the Burmese python genome. Pythons are not considered a venomous species even though they have some of the genes that have evolved into highly toxic venoms in other species — the distinction being that in highly venomous snakes like rattlesnakes or cobras, these venom gene families have expanded to make divers copies of these shared genes, with some of these copies having evolved into highly toxic venom protein producing genes.
Graduate student and member of Dr. Castoe’s lab team Jacobo Reyes-Velasco is the new paper’s lead author. In addition to Drs. Castoe and Mackessy, other co-authors include: Daren Card, Audra Andrew, Kyle Shaney, Richard Adams and Drew Schield, all from the UT Arlington Department of Biology; and Nicholas Casewell, B.Sc., Ph.D., of the Liverpool School of Tropical Medicine in the U.K.
Dr. Casewell’s areas of scientific interest include reconstructing the evolutionary history of venom systems and their toxic components, in order to understand the molecular basis behind these adaptations and pursuant variations in composition of venoms. Dr. Casewell is also investigating parallel evolutionary development of venom components found in different animal lineages; the relationship between the genome, transcriptomes and proteomes of venomous animals and how this relates to venom production; how snake venom variations impact antivenom therapy; and testing the immunological cross-reactivity, safety, stability and efficacy of snake antivenoms and development of novel methods for their manufacture.
Dr. Casewell’s research focus is toward understanding mechanisms causing variations in venom (toxin) composition, which can be medically-important in undermining antivenom efficacy. This research involves utilizing various omic technologies (genomics, transcriptomics, proteomics) to (i) investigate the evolutionary history of venom in different animals, (ii) detect venom variation at different taxonomic levels and (iii) investigate the processes that alter the transcription and translation of toxin gene loci. These investigations have led to the publication of the first snake genome sequences. Dr. Casewell’s research also includes application of novel techniques to improve the specificity and efficacy of snake antivenom therapies and assessment of how venom variation affects functional activity and pathologies induced by venoms.
The UT Arlington led study paper, entitled “Expression of Venom Gene Homologs in Diverse Python Tissues Suggests a New Model for the Evolution of Snake Venom” (Mol Biol Evol. 2014 Oct 21. pii: msu294. [Epub ahead of print]), is coauthored by J. Reyes-Velasco, D.C. Card, A.L. Andrew, K.J. Shaney R.H. Adams, and D.R. Schield, and T.A. Castoe of the Department of Biology, University of Texas at Arlington; N,R. Casewell of the Alistair Reid Venom Research Unit, Liverpool School of Tropical Medicine, Liverpool, United Kingdom; and S.P. Mackessy of the School of Biological Sciences, University of Northern Colorado.
The coauthors note that while snake venom gene evolution has been intensively studied for several decades now, most previous studies lacked the context of complete snake genomes and of gene expression across diverse categories of snake tissue.
The coauthors propose an evolutionary model for snake venoms in which venom genes are preferentially recruited from genes with particular expression profile characteristics, thereby facilitating a nearly neutral transition toward specialized venom system expression.
The research team studied 24 gene families associated with venom that are shared by pythons, cobras, rattlesnakes and Gila monsters, noting that traditionally venom evolution has been viewed as a core venom system developed at a particular point in snake and lizard evolution, and referred to as the Toxicofera. They propose that that evolution of highly venomous snakes (caenophidian snakes) came afterward, and note that little explanation has been provided as to why evolution would focus on just 24 genes in development of highly toxic venom-encoding genes, from the 25,000 or so available.
“We believe that this work will provide an important baseline for future studies by venom researchers to better understand the processes that resulted in the mixture of toxic molecules that we observe in venom, and to define which molecules are of greatest importance for killing prey and causing pathology in human snakebite victims,” Dr. Casewell says.
“When they looked at the python, the team found several common characteristics among the venom-related gene families that differed from other genes. Compared with other python gene families, venom gene families are expressed at lower levels overall, expressed at moderate-high levels in fewer tissues and show among the highest variation in expression level across tissues,” Dr. Castoe adds.
“Evolution seems to have chosen what genes to evolve into venoms based on where they were expressed (or turned on), and at what levels they were expressed.”
Based on data compiled by the team, the paper presents a model proposing three steps for venom evolution. Initially, potentially venomous genes arrive in the oral gland by default, due to their expression in low but consistent ways throughout the body. Then, the process of natural selection on this expression in the oral gland being beneficial, mouth tissues begin expressing those genes in higher levels than in other parts of the body. Ultimately, as venoms evolve to become more highly toxic, the associated genes’ expression of in other organs is decreased in order to limit potentially harmful effects that would result from secreting such toxins into other body tissues.
The research team calls its new model the “Stepwise Intermediate Nearly Neutral Evolutionary Recruitment” or “SINNER” model, maintaining that differing venom levels in snakes and other animals could be traceable to variability in where different species, or different genes within a species, rest along the SINNER model continuum.
Dr. Castoe says in the release that the next research step would be examination of highly venomous snake genomes to test whether SINNER model holds up. In the meantime, he and the team hope their findings concerning presence of venom-related genes in other parts of python tissue will change conventional criteria under which species become labeled venomous.
“What is a venom and what species are venomous will take a lot more evidence to convince people now,” Dr. Castoe observes. “It provides a brand new perspective on what we should think of when we look at those oral glands.”
University of Texas at Arlington
Molecular Biology and Evolution
Liverpool School of Tropical Medicine
University of Northern Colorado
University of Texas at Arlington
Liverpool School of Tropical Medicine
University of Northern Colorado