Wednesday, April 15, 2009
Venemous Evolution
On pages 153-154, Caroll talks about four unrelated animals -- a sea anemone, a scorpion, a marine cone shell snail, and a black mamba snake -- who all independently evolved to use venom. The protein sequences within the venoms are all structurally different, yet they have an equivalent effect on the animal's prey. Explain what factors contributed to their evolution and how essentially the same adaptation could arise in these unrelated species and give examples of other adaptations that have similarly evolved in different organisms. Also, given the venoms' different structures, address how these distinctly different genes can effectively have the same purpose and why they may be so different.
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Carroll cites the sea anemone, scorpion, marine cone shell snail, and black mamba snake as having all evolved independently to use venom. In all of these organisms, venom is injected into a predator for defense as well as into prey for hunting. The venom is an analogous structure in all of these organisms meaning that it arose due to convergent evolution. Carroll discusses convergent evolution and states that “when similar forces converge, similar results emerge” (154). Although these organisms all live in very distinct ecosystems, they occupy a similar ecological niche and because of their niche, they have evolved a common trait. Venoms contain a variety of peptide toxins, proteases, which hydrolyze protein peptide bonds, and nucleases, which hydrolyze the phosphodiester bonds of DNA. While the sequences of the venom in these organisms are different, each sequence codes for its own set of proteases and nucleases that serve similar functions. Also, gene redundancy can allow for the same amino acid to be produced by a variety of codon sequences. Venom generally stops the victims nervous functioning due to blocking of various ion channels, depending on the specific type of venom. While the gene sequences may be different, venom in different animals can still harm nervous system function. For example, the k conotoxin from the cone snail is different than the charbytoxin chemical structure in the yellow scorpion, but they both target the potassium ion channel.
ReplyDeleteCarroll uses the example of Antarctic and Arctic fish both having antifreeze proteins to further develop convergent evolution. These two species live on opposite sides of the planet, however, they occupy similar ecological niches and due to the freezing cold temperatures in both locations, convergent evolution has given both species a set of antifreeze proteins. The convergence in the sequence of Antarctic and Arctic antifreeze peptides is due to natural selection; their performance in preventing ice formation in both species was a favorable trait given their cold conditions. While the sequences serve the same function, they have varying internal spacer sequences. However, both of the sequences are simple and repeat, allowing them to interact with the repeating structure of ice crystals.
http://books.google.com/books?id=vgHXTId8rnYC&printsec=frontcover#PPA1006,M1
http://en.wikipedia.org/wiki/Venom
http://genome.cshlp.org/content/early/2008/05/07/gr.7149808.full.pdf+html
The venoms in each of these unrelated animals are made up of proteins and in each animal the proteins are potassium channel blockers found in the venom. Each of these animals belong to a different phylum, cnidarian (sea anemone), arthropod (scorpion), mollusk (marine cone shell snail), and vertebrate (black mamba snake). All these animals have a deadly venom in common that have different molecular solutions for binding to a blocking the potassium channels. These channels that we are currently learning about are very important in sending electrical signals between the neurons and muscles. By blocking the potassium channels in their prey the prey loose nerve and muscle control and they loose the functions of muscles and nerve. All the venoms in these animals follow these functions but they all evolved independently.
ReplyDeleteFactors that may have contributed to this evolution are the need to kill their prey quicker. As evolution continued, body muscle increased and the need for a quick kill was necessary. The body of the snake was less quick at moving when the body mass increased. Therefore, the venom was created to help the snake obtain its food. Scorpions needed fast acting venom to paralyze their prey so they could eat it slowly. Sea anemone needed venom for protection from predators and to paralyze their prey. Snails also needed protection from their predators.
Carroll said “when similar forces converge, similar results emerge”. The similar needs of each animal have caused repetition to occur in evolution and venoms with the same functions emerged. Potassium channel blocking venoms accomplish this task and therefore, have evolved throughout many animals.
The venoms have very similar protein structures, the length of them differ and the amino acids coded for different. This is studied heavily because they accomplish the same task. This can happen because many proteins can have the same functions. Natural selection may cause the venom to be passed down when the venom is successful in helping the animal survive and reproduce. These animals probably went through many different types of venom that were not as effective.
http://www.nature.com/nature/journal/v439/n7076/abs/nature04328.html
http://www.abc.net.au/science/news/ancient/AncientRepublish_927762.htm
The use of venom by sea anemone, scorpion, marine cone shell snail, and black mamba snake show the need of protection and hunting for each of the animals. Venom is a very powerful tool that poisons the object they shoot at and let’s is the predator or prey to capture it or escape and protect itself from being eaten. The venom arose from convergent evolution, making all the different venoms analogues structures. The fact that it is an analogous structure means that it has the similar or same function, but the structure is different, as Carroll explained in the book. This arose due to convergent evolution, which is the evolution of a similar structure because it proves to be effective and is a selective advantage to that organism. Although these organisms all live in very distinct ecosystems, they occupy a similar ecological niche. That is why when similar stresses and conditions arise on the different organisms they will develop a similar answer to their problem, and it has been venom. The reason they all are in the same niche is because they are in the middle of a food chain, so they need venom for protection and hunting. Venom is a variety of toxins and harmful things that work as a poison. Animal venoms are a mixture of about 20-30 molecules, mostly protein and peptides, in a water mixture. The poisonous species primarily have alkaloids, which are small molecules that have very strong biological effects. One of the functions of the venom is to immobilize it so it can escape or attack even better for the kill. To do this the molecules of the venom attack Acetylcholine receptors, potassium channels, and calcium channels. They do this because many nerve endings have these receptors and it would inhibit the movement of muscles and other parts to escape further damage to the animal.
ReplyDeleteAnother example of this convergent evolution where analogous structures are created is good examples with birds and bats. They both come from different ancestors whose previous state did not have wings. However, for their hunting of prey adapting wings is very useful. Since birds are closer related to humans than bats, you can tell by their bone structure is similar to ours. In their wing they have the two bone structure like humans have in their arms. Also you can tell bird’s digits are much smaller than bats, and they use their feathers and other aviation adaptations to help them maneuver in the air. A bat on the other hand has wings, but with extended digits and there is a large flap of skin between each of the elongated digits. Even though their functions are created to fly and therefore evolved from the same thing. Their use of flight and optimal conditions of flight a vastly different. Bats are much better at slower and higher maneuverability in the air, because their large area of their wings allow for this to happen. However, a bird’s wing is more designed for high speed flight and much less maneuverability. A time in evolutionary history demanded that they both learn how to fly, and this gave them analogous structures that are still different because their use is much different. The wings for a bat are made to maneuver and be able to access food quickly, while a bird uses their wings to get away and be safe.
http://www.sciam.com/article.cfm?id=bats-wing-strokes-unlike-a-birds
http://en.wikipedia.org/wiki/Convergent_evolution
http://www.fathom.com/course/10701017/session3.html
There can be many factors that contributed to the evolution of venom. An article explains venomous animals as being “characterized by the possession of a venom apparatus composed of a specialised gland, which produces a toxic secretion coupled with an inoculation device. Venomous apparatuses present high diversity in their anatomy as well as in their embryological origin. Passively toxic animals (amphibians) have only venom glands. Venoms are toxic only by inoculation (parenteral route), whereas the toxins of poisonous animals are noxious by ingestion (enteral route). “ There are venomous species in all phyla and the general reason for that would be that it is a way to protect themselves from attack. These genes may be so different for a few different reasons; first off, they did evolve separately.
ReplyDeleteHere, the theme of evolution is prevalent. These animals needed some sort of protection, so they evolved this ability to produce toxins that can kill or paralyze other animals. It is important for these animals to be able to do so because their body type may not be a selective advantage. Snakes, for example, do not have legs. Therefore they cannot kick their prey. The do, on the other hand, have teeth that have a channel that produces venom. Many snakes, such as rattle snakes, can produce this venom which will paralyze or kill their enemy.
Likewise, there are also other species that have venom and can be extremely dangerous. Scorpions have posed a big problem to people as well. According to an article, “With the expansion of human communities in Western Brazil, the geographic range of T. serrulatus has increased considerably. Accordingly, it poses an exceptional health problem in Brazil, due not only to its innate prolificacy and rapid expansion into urban areas, but also to its great toxicity. This species possesses the most lethal venom of all the South American scorpions” We always think of snakes as being the only species to have venom, but as Carroll mentions, snakes are not the only ones with the ability to produce venom and kill their enemies.
The purpose for the evolution of venom is for an otherwise helpless species to have a way to protect themselves against bigger and stronger enemies. All a snake needs to have is quick reflexes, which allows it to bite quickly. As long as that happens, the snake can protect itself without having a big and strong body to back itself up.
According to a journal article, “Venom neurotoxins. Venom is defined as a mixture of substances which are produced in specialized glandular tissues in the body of the venomous animal and injected, with the aid of a stinging-piercing apparatus, into the body of its prey in order to paralyze it. The majority of the venomous animals (such as snakes, spiders, scorpions, venomous snails, various coelenterates) are slow, and even static predators which feed on freshly killed prey of mobile and relatively vigorous animals. The locomotory inferiority of the venomous predator is largely compensated by the neurotoxic components of his venom, the neurotoxins, which are able to induce a rapid paralysis of the prey at a very low range of concentrations (10-9-10 -12 M).” This proves that these animals do not need to be big and powerful to be able to overpower their prey. Therefore, venom is a very selective advantage, and it generally helps keep the species who have venom, alive.
http://en.wikipedia.org/wiki/Snake_venom
http://www.scielo.br/scielo.php?pid=S1678-91992008000100003&script=sci_arttext http://www.springerlink.com/content/915k4649w515n3qk/
As we are now learning in class, Calcium channels play a key role in the neurological pathways in our brain. They are absolutely necessary for most messages to be released from the presynaptic neuron, as they are the factor that “pushes” the neurotransmitter-containing vesicle toward the synapse. By inhibiting the channels that are responsible for this calcium release, these poisons effectively disrupt the function of the neurons in our body, thus slowing or even shutting down our nervous system. All four animals have the common theme that they use their neurotoxins to slow down or even immobilize their prey.
ReplyDeleteI would first like to look at the Black Mamba. The history of venom in snakes has been a controversial debate in the world of medicine. It was always thought that snakes in different regions evolved venom producing glands separately, or through independent convergence. However, recent research done by Dr. Bryan Fry, biologist from the University of Melbourne, has refuted this opinion based on the false assumptions that relatives of venomous snakes were non-venomous. Dr. Fry has found, in accordance with the Genome Project, that snakes previously thought to be non-venomous, such as the Gartner snake, are actually nominally venomous. This has led to his theory that all snakes have inherited their venom producing glands from one common, venomous ancestor. This ancestor probably only produced very small amounts of venom, like the Gartner, with the purpose of slowing down small creatures like mice and other lizards. This inhibiting of prey’s ability to flee was a crucial development in snake evolution for it expanded the snake’s possible niches. Previously, snakes were only able to constrict and strangle their prey. This tactic would require the snakes of old (some survive today such as the anaconda and other constrictors) to be big muscled and slow moving. However, in what realm would chance serve to evolve venom producing glands in snakes? Dr. Fry has found the answer. He has discovered that venom glands originally were modified saliva glands, only capable of rapidly breaking down food. This became a selective advantage as snakes that had turned this venomous saliva into a “lethal weapon” had a higher chance of killing prey and sustaining life. This resulted from a process called accidental duplication, in which a gene encoding a protein from another organ got copied into the region of the saliva glands. Some of these “accidental” proteins actually had the chance of being harmful in snake victims, and over time, the ones that aided in predation were selected to stay. “Natural selection then favors mutations that make these proteins more lethal”. Thus we have the Black Mamba, a snake that has developed proteins capable of killing a victim in 30 minutes (Zimmer). One such protein is the 60 amino-acid long polypeptide known as Calciseptine. This protein blocks the Calcium channels in cardiovascular tissue. It has two effects: one is the relaxation of the smooth muscle and the other is its ability to inhibit aortic contractions. This, coupled with the numerous other neurotoxins in Mamba venom has given this snake a weapon unparalleled by any other in the snake family. It allows the snake to strike once and then wait only a short period of time for its victim to collapse(2). This is ideal for the Mamba, because in its native areas on the Savannahs, it can not afford the energy to chase its prey for any great distance. Thus, developing a neurotoxin capable of quickly paralyzing its prey is a selective advantage for this venomous snake.
The immobile Sea Anemone has developed these calcium channel blocking toxins independently of the mamba. However, its similar need is what necessitated the similar toxin. Anemones are cnidarians that are rooted to the ocean floor, thus they are unable to move in search of food. As a result, they have developed a toxin very similar to that of the mamba, which slows down the muscles of its prey. However, the sea anemone also contains toxins which shut down the nervous system of its prey by blocking Potassium and Sodium channels which are crucial in synaptic junctions (3). These toxins are administered by cells called cnidocytes, which function as a defense as well as a means to capture prey. Cnidocytes contain nematocysts, capsule-like organelles which shoot out of the tentacles. Each nematocyst contains a small vesicle filled with the toxins and an inner filament and an external sensory hair. When the hair is touched, it mechanically triggers the cell explosion, a barbed, hook-like structure which attaches to the prey, and injects a dose of poison in the flesh of the aggressor or prey. After the toxins have done their job and paralyzed the prey, the anemone’s tentacles are then able to bring the food into its gastrovascular cavity where it can be digested.
The scorpion has also developed venom capable of blocking calcium channels. However, besides serving the purpose of slowing down prey, some scorpions use their venom in mating. Scorpion venom is very potent, containing toxins capable of blocking just about every ion channel in the brain besides calcium channels (3). Scorpion venom is produced in glands in the vesicles of the hypodermic aculeus, a barb which is located on the tip of the stinger (metasoma). Scorpians are believed to have evolved from a similar aquatic arthropod millions of years ago. Fossil records show aquatic arthropods with very similar metasoma structures, and DNA records have almost proved that his is where the scorpion lineage originated. Scorpions, with the evolution of their barbed stingers, were now adequately supplied with a weapon with which to hunt a more diverse group of prey. This is probably what aided in their migration onto land. Now, scorpions cover many areas of the globe, and, equipped with their paralyzing tail, have been very successful in surviving. They have also been successful in mating, as they perform a dance and release various chemicals from the hypodermic aculeus which attract mates. It’s a win lose situation because the stinger also provides the females with an adequate means of killing their next victim, the sexually spent male scorpion (5).
Carroll’s last example of calcium channel inhibiting toxin was from the marine cone shell snail. In the dynamic marine environment in which cone shells reside, it has been necessary for these gastropods to develop an effective mechanism for immobilizing their otherwise speedy prey. The solution to the snail's lack of adequate mobility has been the development of a highly potent venom, which it uses to paralyze its prey. The cone shell detects prey in its environment using a "siphon" which is covered with chemoreceptors. It then extends its proboscis in the direction of its prey. The venom is produced in a long tubular duct that is often several times the length of the snail itself and at one end is attached to a muscular bulb which is thought to contract to provide the necessary force of venom injection through the 'tooth'. Hollow spear-like radular teeth, which are made in the 'radular sac' and filled with venom, are transported through the 'buccal cavity' to the tip of the proboscis where they are retained by radular muscle. Upon contact with the prey, the proboscis impales the harpoon like tooth into any exposed tissue and injects the venom through this. The harpoon is attached to the gastropod via a 'thread' so that the prey is actually tethered to the snail (although the organism is often paralyzed within one or two seconds, leaving little opportunity to escape). Once the prey is paralyzed, the gastropod retracts the cord by which the prey is attached and engulfs the prey through it's the radular opening of it's proboscis and into its distensible stomach where it is digested. The cone shell can reload further teeth from the radular sac for multiple envenomation by retracting the proboscis into the radular sac and grasping another tooth with the radular muscle. All these mechanisms have turned a relatively harmless creature into a lean, mean, fish eating machine that does not have to worry about prey escaping after its venom has struck (6).
These four examples not only provide a compelling insight into the phenomena of independent convergence, but they also enlist fundamental evidence in the quest to prove the theory of evolution.
1. Zimmer, Carl. "Open Wide: Decoding The Secrets of Venom. " New York Times [New York, N.Y.] 5 Apr. 2005, Late Edition (East Coast): F.1. New York Times. ProQuest. 5 Apr. 2009 http://www.proquest.com/
2. Monteiro R. Q., Foguel D., Castro H. C., Zingali R. B. Subunit dissociation, unfolding, and inactivation of Calciseptine from snake venom. Biochemistry. 2003;42:509–515. [PubMed]
3. Diochot, Sam. “Sea Anemone Peptides with a Specific Blocking Activity against the Fast Inactivating Potassium Channel Kv3.4J”. Biol. Chem. 1998 273:6744-6749 [PubMed]
4. Bora Inceoglu, Jozsef Lango, Jie Jing, Lili Chen, Fuat Doymaz, Isaac N. Pessah and Bruce D. Hammock. “One Scorpion, Two Venoms: Prevenom of Parabuthus transvaalicus Acts as an Alternative Type of Venom with Distinct Mechanism of Action”. Proceedings of the National Academy of Sciences of the United States of America, Vol. 100, No. 3 (Feb. 4, 2003), pp. 922-927. National Academy of Sciences. http://www.jstor.org/stable/3138261
5. http://www.thaibugs.com/Articles/scorpion_facts.htm
6. “Poisonous Plants and Animals: Cone Shell”. Thinkquest. 2000. http://library.thinkquest.org/C007974/2_1con.htm