How baleen whales lost a gene and their teeth

ResearchBlogging.orgWhen a blue whale opens its mouth, tonnes of water surge in. The whale then forces the water back out with its tongue, in such a way that it flows through the baleen combs in the front of its mouth. These baleen plates can filter up to half a million calories worth of plankton, krill and small fish out of the water. That’s almost 1,000 hamburgers in one gulp. No wonder whales get so big.

The earliest ancestors of the blue whale fed themselves differently. They still had teeth, and no baleen. Some modern whales still have teeth (the Odontoceti – literally ‘toothed whales’), but they form a separate group from the baleen whales (Mysticeti) who have replaced all their teeth with baleen. This transition to toothlessness is documented by multiple fossil whales. Each of these fossils provides a snapshot of what must have been gradual change.

Several early baleen whales such as Janjucetus and Mammalodon still had fully developed, enamel-covered teeth. Eomysticetus had already exchanged its teeth for baleen plates. But it is Aetiocetus, that captures this evolutionary change in its entirety. This was a whale that still had teeth, but that also carried baleen, as small modifications in its skull reveal.

Whale embryos also contains hints that their distant ancestors once bore teeth. They still grow tooth buds that disappear before the young whale is born. Charles Darwin must have had the toothed ancestors of whales on his mind when he wrote the following sentence in the Origin of Species:

What can be more curious than the presence of teeth in foetal whales, which when grown up have not a tooth in their heads; or the teeth, which never cut through the gums, in the upper jaws of unborn calves?
~Charles Darwin, On the origin of species.

Reconstruction of Aeitocetus that lived 25 million years ago. This wale had both teeth and baleen.

Aside from the evidence from fossils and whale embryos, the loss of enamel-capped teeth also left traces in the genomes of modern whales. All reptiles and mammals have genes that produce proteins that mineralize the enamel of teeth. Since baleen whales have no teeth as adults, they have no need for these proteins. Over time such unnecessary genes tend to acquire mutations that impair the protein. This is exactly what happened in baleen whales. In all species of baleen whale, up to three tooth genes turned into pseudogenes (remnants of genes that can no longer produce a functional protein, but are still recognizable as former genes).

But there is something strange about how these mutations are distributed: every species of whale has a different set of mutations. Humpback whales have a mutated enamelin gene, for example. Blue whales carry a different mutation in enamelin. And sei whales have a mutation in a different tooth gene altogether.

Such an uneven distribution of mutations can mean a couple of things. One explanation could be that all the baleen whales lost their enamel independently from each other, due to different mutations in each lineage. Another possibility is that an hitherto unknown mutation that can be found in all baleen whales is responsible for the loss of enamel. Enamel-covered teeth would have been only lost once by the common ancestor of baleen whales. The fossil evidence supports this scenario: the distribution of toothed baleen whales is not nearly as patchy as the distribution of tooth gene mutations.

Scientists from the University of California suspected a gene called MMP20 might contain the mutation that had been overlooked so far. This gene seemed to be a good candidate, because the MMP20 protein is involved in processing tooth proteins such as enamelin and ameloblastin. A mutation in MMP20 could affect multiple enamel proteins downstream. Moreover, humans and mice that have a defective MMP20 gene develop bad and brittle enamel (amelogenesis imperfecta).

The family tree of whales, including extinct relatives. Baleen whales (top) and some pygmy sperm whales (bottom) have mutations in their tooth genes. Every orange symbol denotes a mutation; different letters represent different genes.

The team initially screened four different species of baleen whales for mutations in MMP20. They hit the jackpot right away. In all four whales, a stretch of DNA (a SINE) had inserted itself right inside MMP20, splitting the gene in two. When they extended their search to other species, they found that whale after whale had the same DNA insertion inside MMP20. This ubiquity gives a clear message: it is this insertion that rung the death knell for the whale’s teeth.

But the researchers discovered that some pygmy sperm whales (Kogia), that belong to the branch of toothed whales, also carry mutations in their MMP20 genes. These pygmy sperm whales are also known to have enamel-less teeth. But whereas baleen whales first lost MMP20 before the other tooth genes mutated, these sperm whales seem to have lost the tooth protein enamelin first, with MMP20 now having mutated secondarily in some individuals.

So here are two lineages of whales, caught in the act of evolving on different, but similar paths. Evolution is sometimes criticized for not being amenable to experimental scrutiny in the lab, but the pygmy sperm whales prove these critics wrong. As the authors note, “mammalian diversity presents a unique laboratory, complete with replicated experiments.” Life herself presents us with a multitude of ingenious experiments. It is up to us to interpret them. Personally, I couldn’t imagine a more exciting science.

Reconstruction of Aetiocetus by the great paleoartist Carl Buell. Check out his website here. Image used with his permission.
Whale phylogeny from reference, whales also drawn by Carl Buell.

Meredith RW, Gatesy J, Cheng J, & Springer MS (2011). Pseudogenization of the tooth gene enamelysin (MMP20) in the common ancestor of extant baleen whales. Proceedings. Biological sciences / The Royal Society, 278 (1708), 993-1002 PMID: 20861053

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