Pseudogenes: Why cats, hyenas and seals don’t love sugar

The molecular basis for taste is relatively straightforward. On your tongue, you have numerous taste buds that harbor cells with little proteins hanging out on the top. These proteins have the capacity to bind a number molecules on your tongue, and thereby transmit information regarding nutritional content.

Screen-Shot-2013-09-14-at-2.52.32-PM
Mammalian taste receptor proteins and molecules that activate them.

One of these proteins is TAS1R2, which, along with the protein TAS1R3, binds sweet tasting molecules. This sweet receptor is what allows you to savor the delicious sucrose (table sugar), fructose (fruit sugar) and even artificial sweeteners such as saccharin and aspartame. If you love cake, ice cream, and cookies, be grateful that you have a functional sweet receptor!

Not all animals are fortunate enough to enjoy these treats, however, and evolution is likely to be blamed. Many species are particularly adapted to eating foods that are nearly devoid of sugars, and therefore are not expected to benefit from maintenance of the gene encoding TAS1R2. Indeed, scientists [1] have found that a number of carnivorous mammals, including cats, hyenas, seals, the banded linsang, fossa and Asian small-clawed otter have a pseudogenized (nonfunctional) version of the TAS1R2 gene.

F1.large
Examples of TAS1R2 pseudogenes in various carnivores

This helps explain why cats behaviorally seem uninterested in sugar. The scientists also confirmed that the Asian small-clawed otter also is not drawn to sweets, consistent with its TAS1R2 pseudogene. By contrast, they found that the spectacled bear, which has an intact TAS1R2 gene and is a known devourer of sweet things like fruits and honey, prefers sugary solutions over water.

spectacled-bear

These data suggest that the ancestral carnivores did eat sweets on occasion, but certain species avoided sugary foods for so long that sweet receptors were no longer necessary. Eventually mutations rendered the TAS1R2 gene nonfunctional in different carnivore lineages, rendering these species impervious to the effects of sweets.

Questions for Creationists

Why did God create some carnivores with a nonfunctional version of the sweet taste receptor gene? Would it not have made more sense for Him to create them without the gene altogether? Is it just a coincidence that He also created other animals with specialized feeding strategies, such as giant pandas and whales, to lack certain taste receptors?

References

1. Jiang, P., Josue, J., Li, X., Glaser, D., Li, W., Brand, J. G., … & Beauchamp, G. K. (2012). Major taste loss in carnivorous mammals. Proceedings of the National Academy of Sciences109(13), 4956-4961.

Photo Credit

Taste receptors, hyena, harbor seal, banded linsang,  fossa, otter, pseudogene figure, spectacled bear

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11 thoughts on “Pseudogenes: Why cats, hyenas and seals don’t love sugar

  1. In my molecular methods class two years ago we hypothesized that bowhead whale and belugas – two organisms for which I can DNA of in my freezer – would have pseudogenized copies of the sweet and umami taste receptor genes. Groups of students designed there own primers and tested them. None of the 6 groups were able to ampllfy the sweet taste receptor but several were able to amplify the umami taste receptor. We sequenced those products and found that the Bowhead has multiple stop codons and a large – more than 10bp -deletion in the gene. I haven’t looked in the Bowhead genome sequence which is now available but I expect that we weren’t successful because it is either completely missing or has so many mutations our primers wouldn’t have worked.
    Just as you say above, my students hypothesized that the whales didn’t need to taste sweet or umami and therefore didn’t need those genes but they also hypothesized that if whales had land animal ancestor that at one point their genomes would have had functional version of those genes. The results they obtained made perfect sense within the context of common ancestry.

    1. Ah, very cool class assignment! I’d love to be able to incorporate scientific discovery into a classroom someday.

      Indeed, Jiang et al. also looked at the sweet receptor in the bottlenose dolphin and confirmed what you all expected. A later study demonstrated this more broadly in whales, showing they’ve lost pretty much all of their tasting abilities!:

      https://academic.oup.com/gbe/article/6/6/1254/580618/Massive-Losses-of-Taste-Receptor-Genes-in-Toothed

  2. TFBW

    Why did God create some carnivores with a nonfunctional version of the sweet taste receptor gene?

    Why do you assume it was created nonfunctional?

  3. TFBW

    That would be a charitable interpretation of what I said, yes. Alternatively, “why do you phrase the question so that it contains the assumption that it was created non-functional,” if you’re a fan of precision over brevity.

    1. Because in my experience, creationists typically don’t think of genetic changes on this sort of scale as being possible. Therefore, the assumption is usually that the Creator designed the organism with this already in their genetic code.

  4. TFBW

    I don’t suppose you have an actual citation to back up your “typical creationist”, do you? If I accused you of attacking a straw man, would you have anything more than personal anecdotes with which to defend your position?

    1. Perhaps this is wholly unintentional, but your tone isn’t particularly friendly in some of these comments. They read as if you think I write with insidious intentions. Again, perhaps this is an unintended consequence of your writing style, but if I am correctly detecting a negative attitude, please understand that the point of this blog is not to demean creationism nor argue with people. The purpose is to simply present evidence for evolution, and discuss what it might mean for some of the common models of creationism. Many people are not aware of the data that I present, and are only familiar of things like Archaeopteryx, pepper moths and other textbook examples of evidence for evolution. These examples have been rationalized to death by leaders in the creationist community, so some creationists might go about their lives thinking that all of the alleged evidence for evolution has been explained away. Of course, it doesn’t mean I think that what I present here will convince all creationists that evolution is real, but hopefully it at least helps them become aware that there’s more evidence out there and/or gets them thinking about what they believe about the Creator.

      The questions I post at the bottom actually are not rhetorical, but are meant to either get creationists to think about the data I present and what it implies for their view of creationism, and/or for them to directly respond with how they view the data, ask questions, etc. I’m not trying to entrap anyone, set up straw men, argue people into submission, any of that. I realize that beliefs are diverse and everyone is trying to make sense of what they see in the world and reconcile that with their worldview, so I’m not as much of a fan of argumentation as I am of discussion. I’m presenting data for one particular worldview, and if someone can take in all of these data and still believe that a Creator created every organism as is 6000-10000 years ago, I’d truly be fascinated to hear their thoughts.

      Anyway, to answer your question: the first thing to keep in mind is that creationism can be diverse and contradictory, even within the same organization. There isn’t exactly a unified model of Creationism to the same extent that we have for evolutionary theory. Also, I just pulled up a few examples here rather than thoroughly mining all of the creationist literature. Maybe I’ll make time for this later, but someone who is far more up on this literature is Joel Duff and could probably answer your question more thoroughly: https://thenaturalhistorian.com

      To start, it is well known that Creationists typically accept evolution but disbelieve in macroevolution (i.e., large evolutionary changes over long periods of time), at least in part because most Creationists believe in a young earth. The problem is how to define “large changes”. In this article (https://www.icr.org/article/1156/285) the author refers to macroevolution as invertebrates turning into fish and hoofed mammals turning into whales, which are clearly consistent with this definition. So where does the loss of some taste perception, as evidenced by taste receptor pseudogenes, fall on the spectrum between micro and macroevolution? What about other pseudogenes that show evidence of loss of some aspects of vision in mammals that live underground? Or tooth pseudogenes in animals that completely lack teeth, such as anteaters, birds and turtles? (some of the examples I’ve highlighted here: https://evolutionforskeptics.wordpress.com/category/pseudogenes-2/)

      To make this distinction all the more cloudy, creationists frequently contend that mutations are never (or almost never) beneficial, which seemingly forms the basis for why macroevolution is implausible. From the above article: “Genetic mutations produce new genetic material, but do these lead to macroevolution? No truly useful mutations have ever been observed.”

      So do all beneficial mutations equate to macroevolution? Could the Creator have created these organisms with functional genes and these genes accumulated loss of function mutations and not have been “useful”? Is it not useful, if you’re a carnivore, to avoid wasting space on your tongue and metabolic energy by eliminating the capacity to taste sweets when you never encounter sweets in your diet? Is it not useful, if you’re a baleen whale that feeds using baleen plates instead of teeth, to get rid of your teeth since they crowd the mouth and aren’t helping with the way you now feed?

      I assume that this is in partly why creationists often believe that pseudogenes are not actually defunct copies of formerly functional genes, but instead suggest that they all must perform functions and were bestowed by the Creator (http://creation.com/pseudogenes-are-they-non-functional): “it is evident that these genetic elements, which are copiously spread in the genomes of different organisms, have been created with purpose”

      A final topic I’ll briefly mention is that a new type of creationism has arisen that assumes evolution is essentially true, except that it happened extremely rapidly and only within ‘kinds’ (frequently equivalent to taxonomic families). They assume that genetic diversity is due to created heterozygosity (i.e., genetic variation within the progenitor kinds): https://answersingenesis.org/natural-selection/speciation/on-the-origin-of-eukaryotic-species-genotypic-and-phenotypic-diversity/

      The above article is way way too long for casual reading, but you can see that Jeanson summarizes it in the conclusion. This idea that kinds diversified and evolved over time implies that the Creator created many organisms with pseudogenes since some genes are nonfunctional within entire taxonomic families/’kinds’ (e.g., taste pseudogenes in multiple whale families, vision pseudogenes in subterranean mammal families).

  5. TFBW

    I’m sorry if my tone comes across as harsh, but it’s common practice in evolutionists-against-creation blogger circles to mock and ridicule. Your blog doesn’t have a scoffing tone, but on the other hand it doesn’t go to any efforts to interpret the arguments of its opponents in the most charitable light. While this arguably makes it a cut above most of the alternatives, it’s still frustratingly obtuse in its treatment of creation, like the ignorant creationist who thinks he’s presented a real hum-dinger of a question when he asks an evolutionist, “if people evolved from monkeys, why are monkeys still around?”

    Your question, with its implicit assumption of creation-with-nonfunction, comes across as obtuse in this way because the following points are uncontroversial in creationist circles, and I think you ought to be aware of it.

    1. Creation was initially perfect.
    2. Sin and corruption entered the world, causing decay.

    Given these two points, why would anyone bat an eyelid at the presence of evidently broken genes? Assuming the identification is correct, it’s just more evidence of genetic decay. And why would you assume a broken gene was created that way?

    If you were aware of these common beliefs, then you had a charitable interpretation available to you, and you chose to pass it over, which is mean. If you weren’t aware of them, then you’re somewhat ill-informed about creationism, and you’re just exhibiting that ignorance with your questions (but doubling down when I suggest that it might be ill-founded). I’d prefer to argue with someone who is not operating out of ignorance or a lack of charity, and this analysis doesn’t bode well for those criteria. That’s a shame, because I thought this blog looked somewhat promising.

    You might not think of your questions as rhetorical, but they’re deterring actual engagement as far as I’m concerned. If that’s not what you’re aiming for, then take it under advisement.

    Would you like me to leave now?

    1. Hi TFBW:

      “I’m sorry if my tone comes across as harsh, but it’s common practice in evolutionists-against-creation blogger circles to mock and ridicule.”

      I have zero intention to mock or ridicule anyone, and I do not want anyone to do that in the comments either.

      “Your blog doesn’t have a scoffing tone, but on the other hand it doesn’t go to any efforts to interpret the arguments of its opponents in the most charitable light. While this arguably makes it a cut above most of the alternatives, it’s still frustratingly obtuse in its treatment of creation, like the ignorant creationist who thinks he’s presented a real hum-dinger of a question when he asks an evolutionist, “if people evolved from monkeys, why are monkeys still around?””

      It’s important to keep in mind that most creationists probably have not thought through many of these issues as much as you have. Most that I have encountered are creationist by association, i.e., their church taught them that creationism is true, evolution is demonstrably false, and to reject creationism is to reject God, Jesus and everything else that goes along with being a Christian (my impression is that a similar thing occurs in muslim circles). This blog is mostly meant to reach out to people like this, not as much to people like you. In other words, the intention is to make it more accessible to casual creationists, and therefore I believe that most of what I present here (including the questions, as simplistic as they are) is relatable to what they believe. I could of course be wrong about that, but I’m not above adapting what I present to accommodate whatever is the majority belief.

      Given that, I’m happy to discuss ideas, questions, rebuttals, etc. with anyone from any background, but your implied tone makes it difficult to want to continue talking with you.

      “Given these two points, why would anyone bat an eyelid at the presence of evidently broken genes? Assuming the identification is correct, it’s just more evidence of genetic decay.”

      Because the broken genes are nonrandom and instead appear to be associated with their particular lifestyle (=adaptations). If decay did appear randomly, genetically or otherwise, then I think that would be an interesting piece evidence for the creationist model. But decay of sweet taste receptors in species that don’t eat sweets (with an assortment of anatomical features that show they specialize on meat), the decay of tooth genes in species that do not have teeth or hardly use their teeth (relying on other anatomical traits to ingest food) or the decay of vision genes in species that are adapted to living in the deep ocean or underground is not random.

      This doesn’t mean that these species couldn’t have been created with functional genes that lost them over time, but it then raises all sorts of questions about the speed of mutations, fixation of mutation and anatomical evolution.

      “And why would you assume a broken gene was created that way?”

      You asked and I pointed you to creationist literature that suggests that pseudogenes were created. That could be one of many interpretations out there (it’s the only one I’ve seen), but Creation Ministries International is certainly a mainstream organization, and at least some of their members currently believe or have previously believed this.

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