Posted on 02/09/2006 11:00:54 AM PST by GreenFreeper
There is a general consensus that most of today’s nonvenomous snakes are descendants of venomous snakes that lost their venomous capabilities secondarily. This implies that the evolutionary history of venomous snakes and their venom apparatus should be older than the current evidence from the fossil record. We compared some of the oldest-known fossil snake fangs from the Lower Miocene of Germany with those of modern viperids and elapids and found their morphology to be indistinguishable from the modern forms. The primary function of recent elapid and viperid snake fangs is to facilitate the extremely rapid, stablike application of highly toxic venoms. Our findings therefore indicate that the other components of the venom delivery system of Early Miocene vipers and elapids were also highly developed, and that these snakes used their venom in the same way as their modern relatives. Thus, the fossil record supports the view that snakes used their venoms to rapidly subdue prey long before the mid-Tertiary onset of the global environmental changes that seem to have supported the successful radiation of venomous snakes.
A copy of this article can be downloaded gratis by visiting the CNAH PDF Library at
http://www.cnah.org/cnah_pdf.asp
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Among extant reptiles only two lineages are known to have evolved venom delivery systems, the advanced snakes and helodermatid lizards (Gila Monster and Beaded Lizard)1. Evolution of the venom system is thought to underlie the impressive radiation of the advanced snakes (2,500 of 3,000 snake species)2–5. In contrast, the lizard venom system is thought to be restricted to just two species and to have evolved independently from the snake venom system1. Here we report the presence of venom toxins in two additional lizard lineages (Monitor Lizards and Iguania) and show that all lineages possessing toxin-secreting oral glands form a clade, demonstrating a single early origin of the venom system in lizards and snakes. Construction of gland complementary-DNA libraries and phylogenetic analysis of transcripts revealed that nine toxin types are shared between lizards and snakes. Toxinological analyses of venom components from the Lace Monitor Varanus varius showed potent effects on blood pressure and clotting ability, bioactivities associated with a rapid loss of consciousness and extensive bleeding in prey. The iguanian lizard Pogona barbata retains characteristics of the ancestral venom system, namely serial, lobular non-compound venom-secreting glands on both the upper and lower jaws, whereas the advanced snakes and anguimorph lizards (including Monitor Lizards, Gila Monster and Beaded Lizard) have more derived venom systems characterized by the loss of the mandibular (lower) or maxillary (upper) glands. Demonstration that the snakes, iguanians and anguimorphs form a single clade provides overwhelming support for a single, early origin of the venom system in lizards and snakes. These results provide new insights into the evolution of the venom system in squamate reptiles and open new avenues for biomedical research and drug design using hitherto unexplored venom proteins.
Archive?
"Evolutionary question for the day: How would it be advantageous for an iguanian species to lose its ability to deliver venom?"
The iguanan lizards are primarily vegetarian. Plants don't get away, so they needn't be poisoned. It's a good example of adaptation. No venom needed; eventually none is produced.
From talktothevet.com:
"A fairly recent discovery has provided a probable explanation for the premature deaths of young iguanas despite apparently adequate diets. In the wild, young iguanas obtain needed intestinal bacteria and protozoa by eating the feces of adult iguanas. The microorganisms acquired this way are essential for digestion of plant material. Baby iguanas are not born with these microbes, so young iguanas reared in captivity never acquire them.
All newly acquired, domestically raised (not wild-caught) iguanas should receive fresh feces from a healthy, parasite-free adult iguana (preferably wild-caught). One dose should inoculate the iguana for life. Some veterinary hospitals are prepared to assist new iguana owners with this process."
Further on that- the inability to produce venom not an advantage but neither is it a disadvantage, ergo offspring without venom stand an equal chance of survival. The same phenomenon is occurring with humans; diabetes has become far more common since the discovery of insulin because diabetics now live long enough to have children of their own.
> the inability to produce venom not an advantage but neither is it a disadvantage, ergo offspring without venom stand an equal chance of survival.
True. However, additional, needless systems can disappear through fairly minor mutations... the genes for 'em can be accidentally switched off, or otherwise negated via mutation. And since there's no need for those systems, the mutant's offspring will ahve an equal chance as a non-mutant offspring. So, an unused system stands a good chance of eventually simply slipping away.
And I'm not sure that unused venom is not a disadvantage to the organism. A minor injury or infection could cause the venom to leak into the oganisms blood system, killing it.
All newly acquired, domestically raised (not wild-caught) iguanas should receive fresh feces from a healthy, parasite-free adult iguana (preferably wild-caught). One dose should inoculate the iguana for life. Some veterinary hospitals are prepared to assist new iguana owners with this process."
I wonder if this is some evolutionary hold-over from switching to a plant based diet?
"In the wild, young iguanas obtain needed intestinal bacteria and protozoa by eating the feces of adult iguanas. "
Ick! But I understand it.
It requires expendature of energy to make toxin. The animal not making it would be at a competitive adavantage if it were not needed.
There are more diabetics now for at least two reasons.\
1. There are more type I diabetics because we are keeping them alive longer with treatment. If they die, you don't see them.
2. There are more type II diabetics because we are getting fat and not excercising.
There are urban legends about people with C. dificil colon infections being treated with fecal milkshakes. Not necesarrily with their knowledge or consent.
There are urban legends about people with C. dificil colon infections being treated with fecal milkshakes. Not necesarrily with their knowledge or consent.
More info than I needed. Really.
Always glad to share. ;)
Could be that the venom making is extremly taxing on the lizard.
Got in late on the fun...but besides all the facts others described. Venom is very costly to produce, not necessary, and given a stressful environment could be the difference between life and death.
"Hey, mister, you done with that?"
http://www.sciencedaily.com/releases/2006/02/060202075525.htm
It's a case of evolutionary detective work. Biology researchers at Lewis & Clark College and the University of Arizona have found evidence for an ancient transfer of a toxin between ancestors of two very dissimilar organisms--spiders and a bacterium. But the mystery remains as how the toxin passed between the two organisms. Their research is published this month in the journal Bioinformatics, 22(3): 264-268, in an article titled "Lateral gene transfer of a dermonecrotic toxin between spiders and bacteria." "We are piecing together an historical puzzle with evidence from living descendants of an ancient ancestor," said Greta Binford, assistant professor of biology at Lewis & Clark. Her coresearcher on the project is Matthew Cordes, assistant professor of biochemistry and molecular biophysics at the University of Arizona. The toxin is uniquely found in the venom cocktail of brown or violin spiders, including the brown recluse, and in some Corynebacteria. The toxin from the spider's venom can kill flesh at the bite site; the bacterium causes various illnesses in farm animals. "Our research was inspired by the fact that we have a group of spiders with a unique toxin, and that toxin also happens to exist outside the animal kingdom in this particular bacterium," she added. "A pattern like this raises the possibility of lateral gene transfer as a explanation." Lateral gene transfer refers to the movement of genes between the genomes of unrelated organisms. This contrasts with vertical transfer of genes from parent to offspring. Cordes and Binford found a common structural motif at the end of both toxic proteins that is not found in any other proteins. Evidence for common ancestry (homology) of the toxins had previously been noted, but this uniquely shared structural bit is best explained by these toxins being more closely related to each other than they are to any other known protein. "That one structural detail--which resembles a plug or cork at the end of a barrel-shaped enzyme--is evidence that the spider and bacterium share a relatively recent common ancestor," Cordes said. "Aside from being an example of lateral transfer between very distantly related organisms, this study is an unusual example of using structural motifs in proteins to answer questions about common ancestry when gene sequences are too different to be clear about these relationships." "We're still left with the question of whether this venom enzyme hopped species from the spider to the bacteria, or the other way around. Either way, the presence of this medically-relevant toxin in one of these groups of organisms is likely the result of transfer from the other lineage," Binford said. "Understanding the importance of this structural motif in the toxic activity may help with developing treatments that minimize the effects of bites of brown recluse and their relatives. If this motif is central to protein function, treatments designed for the spider bites may also work for treating problems caused by the corynebacterial toxin," she added.
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