Toothless, baleen whales have remnants of tooth genes

Blue, gray and humpback whales, are among the largest animals ever to have existed on earth. To reach and maintain such an immense size, they need to consume copious amounts of prey. Yet, unlike most vertebrates, they completely lack teeth. Instead they have a plates of protein known as baleen. Baleen acts as a sieve to filter out tiny crustaceans from the water column, allowing whales to gulp down thousands of prey in a single meal.

Baleen
Baleen

While this feeding apparatus is unique to these massive whales, there is evidence that the ancestors of baleen whales had teeth. For one, other whales, such as dolphins and sperm whales, have teeth, and the earliest whales in the fossil record all had teeth. Second, the earliest baleen whale fossils included species that appeared to have teeth and baleen side-by-side (e.g., Aetiocetus). Third, when baby baleen whales are in the womb, they form tooth buds, which are then reabsorbed as they develop. This evidence might seem pretty convincing by itself, but is there anything else to suggest that such toothless whales formerly had pearly whites?

Sure enough, when researchers have looked at the genomes of baleen whales, they have found remnants of tooth genes [1–4]. These genes, which are critical for the development of healthy teeth, are full of mutations that keep them from functioning properly.

Below you can see alignments of gene sequences for the ENAM gene, a gene involved in forming the hard outer cap of enamel in teeth, and the C4orf26 gene, a gene that appears critical for tooth development, in various baleen whale species (e.g., Eschrichtius, Eubalaena, Balaenoptera, Megaptera).

You can see highlighted examples of inactivating mutations in these two genes, with some examples of shared mutations, suggestive of gene function loss in a common ancestor. One particular gene involved in enamel maturation, MMP20, has the same knock-out mutation in all baleen whales [3], a rare SINE retrotransposon insertion, providing evidence of a loss of teeth after baleen whales split from toothed whales.

Together, DNA, the fossil record and development provide compelling evidence that the largest whales evolved from ancestors with teeth, before eventually replacing them with their unique baleen filtering apparatus.

Questions for Creationists

Why would the Creator create nonfunctional tooth genes in whales that completely lack teeth? Why would the Creator put the exact same inactivating mutations in multiple species? Is it just a coincidence that baby baleen whales have tooth buds that are reabsorbed, molecular phylogenetics suggests whales descended from tooth ancestors, and the fossil record appears to show a transition from toothed ancestors to baleen-only species? Is it a coincidence that other toothless vertebrates have dental pseudogenes?

References

1. Deméré, T. A., McGowen, M. R., Berta, A., & Gatesy, J. (2008). Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales. Systematic Biology57(1), 15-37.

2. Meredith, R. W., Gatesy, J., Murphy, W. J., Ryder, O. A., & Springer, M. S. (2009). Molecular decay of the tooth gene enamelin (ENAM) mirrors the loss of enamel in the fossil record of placental mammals. PLoS genetics5(9), e1000634.

3. Meredith, R. W., Gatesy, J., Cheng, J., & Springer, M. S. (2010). Pseudogenization of the tooth gene enamelysin (MMP20) in the common ancestor of extant baleen whales. Proceedings of the Royal Society B: Biological Sciences, rspb20101280.

4. Springer, M. S., Starrett, J., Morin, P. A., Lanzetti, A., Hayashi, C., & Gatesy, J. (2016). Inactivation of C4orf26 in toothless placental mammals. Molecular phylogenetics and evolution, 95, 34-45.

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2 thoughts on “Toothless, baleen whales have remnants of tooth genes

    1. Excellent question! There are multiple ways to get at this, and for obvious reasons, tooth genes have been extremely well characterized. One way is to find where these genes are expressed (i.e., turned on) and when, and examining developing mice is an excellent way to find out. We know that these various genes are turned on during tooth development, and we know enough details that we have a very good understanding of what each tooth gene is doing. For example, some of them produce a protein scaffold that allows for tooth crystals to form, and others break down the protein scaffold once the crystals are in place.

      But this doesn’t answer your question for the potential of multifunctionality. Indeed, many genes have multiple functions and losing them could potentially have disastrous consequences. In fact, for some tooth genes that we know are involved in other functions (e.g., bone function), toothless mammals still retain them as fully functional. Again this is where mouse models come in handy. Various studies have shown that if you knock out these tooth genes in mice, their teeth do not develop properly but their bodies are otherwise healthy. Additionally, these same tooth genes are associated with tooth diseases in humans (e.g., amelogenesis imperfecta), and besides dental issues, which can be quite severe, the diseased humans are otherwise asymptomatic.

      On top of that, researchers have discovered that not only have these genes accumulated deleterious mutations in the toothless baleen whales, which I describe here, but they’re also nonfunctional in toothless birds, toothless turtles, toothless anteaters and toothless pangolins. The enamel specific genes are also nonfunctional in enamelless species such as sloths, armadillos, narwhals, pygmy sperm whales, and aardvarks. I describe some examples here: https://evolutionforskeptics.wordpress.com/2014/11/08/pseudogenes-enamelless-mammals-and-toothless-turtles-and-birds-have-enamel-gene-remnants/

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