From extensive to specialized ── Evolution of rattlesnake venom

Time and tide wait for no man? Modern medicine has created a panacea against cancer and avoided the genetic test of fortune telling. Know in advance that sea elves and fascinating creatures born in luminous capsules are indestructible. Rattlesnake venom is mainly distributed in arid areas from Canada to South America. Its tail has a ring that can hiss to warn predators who invade its territory. When they encounter danger or prey attack, they will try their best to inject their own bleeding venom, and those who have been kissed by snakes will gradually become weak because of bleeding. In addition, some types of venom are also neurotoxic, which can cause additional symptoms such as drooping eyelids, numbness of limbs and even dyspnea [Note 3]. In the past, in order to prepare an antidote or extract valuable ingredients from rattlesnake varieties, researchers spent a lot of energy collecting and analyzing venom. Recently, with the progress of gene technology, they hope to further clarify the evolution of toxicity.

Add from ancestors or unify? Because there are many kinds of protein that constitute the toxicity of snake venom, even different individuals of the same species have certain differences, so the evolution process of snake venom protein and related genes has always been a problem that academic circles hope to solve. From the perspective of genes, we must look at it from two aspects: first, what changes have taken place in the genetic code during the evolution of species; On the other hand, individuals choose which genes to express in the process of gene transcription and translation [Note 4]. In the aspect of venom protein gene difference, it is speculated that the gene will undergo random mutation during replication, and the good mutation will be retained through natural selection [Note 5]; However, the research published by Dowell et al in 20 16 does not support this statement. Because the PLA2 toxins of different species of rattlesnakes may evolve into different toxicity, such as neurotoxicity, muscle toxicity or blood toxicity, Dowell and others chose this protein as the target to sequence the PLA2 genes of the small shield rattlesnake, the western diamondback rattlesnake and the eastern diamondback rattlesnake. After comparing the results with the data of other teams, they think that rattlesnakes probably have an ancestor with all toxin genes, and their respective genotypes were formed by deleting different genes, and no new genes were produced in the process. The research team also found that some rattlesnakes without neurotoxin actually retain the corresponding genes, but they are not expressed in the venom [Note 6].

Why did the rattlesnake abandon its weapons? Dowell et al. pointed out that the ancestors of rattlesnakes with all toxin genes existed about 22 million years ago, and the three varieties selected in the study branched out independently after gene deletion from 4 million to 7 million years ago. Because there is no actual sample, we can't know whether rattlesnake venom was really powerful 22 million years ago, but the gradual decrease of venom genes in today's species may be more helpful to adapt to the changing environment. It's not just rattlesnakes. After analyzing the PLA2 gene of other snakes in the past, Lynch also found that the useless or unnecessary part of PLA2 gene does disappear gradually with the change of prey attributes [Note 7].

Instead of knocking out useless genes, why not keep all the genes for a rainy day? Scientists speculate that it may be related to energy consumption. The evidence is that the metabolic rate of poisonous snakes will increase significantly after they secrete toxins, which means that synthesizing toxins is a quite energy-consuming activity [Note 8]; In addition, many rattlesnakes adjust the amount of venom secretion according to the size of their prey [Note 9], which shows that the amount of venom is limited like a bullet and must be cherished. Therefore, when the eating habits of rattlesnakes gradually change and focus on specific prey, if we can stop making unnecessary toxin components, we can retain more energy.

On the other hand, because enemies and prey will gradually produce antitoxicity, poisonous snakes have to secrete more venom to achieve the effect; At this time, rattlesnake varieties that can concentrate resources to produce a large number of specific toxins will definitely break through the anti-toxicity of their prey and survive. Similar situations are not uncommon in nature. For example, after Antarctic ice fish developed a metabolic system independent of red blood cells, only heme A gene was left in the body, and heme B gene was deleted [note10]; Cave fish abandon opsin gene when they live in the dark [note11]; And with the development of trichromatic vision in higher primates, olfactory genes gradually evolved into nonfunctional pseudogenes [note 12]. From this point of view, although preserving gene diversity can flexibly cope with various environmental changes, it is not necessarily a bad thing to let some genes degenerate based on specific considerations; Even this gain and loss is the reason why nature can keep balance for a long time.

References: 1.changmc Isoplasma1998; 91:1582-9.2. Hatir et al., Toxicity Review1999; 29:1-19.3. biochemistry of airdsd et al.1985; 24:7054-8.4. Proceedings of the National Academy of Sciences 2014; 111:9205-10.5. Malhotra A et al. Toxicon 2015; 107:344-58.6.Dowell NL et al., Contemporary Biology 20 16. doi: 10. 10 16/j . cub . 20 16.07 . 038。 [Epub before printing. Lynch VJ BMC Evol Biol 20077:2.8. Copea, Maryland, McCue 2006:8 18-25. 9.Galán JA et al., Toxicon, 2004; 43: 791-9.10. Cocca e et al. Proceedings of the National Academy of Sciences1995; 92:1817-21.1.Yang et al. 14:1.12. gilady et al. Biology of the Public Library of Science, 2004; 2:E5。

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