DNA changes turned whales into giants – 01/24/2023 – Science

Brazilian researchers managed to map some of the DNA changes that turned whales into the biggest animals of all time. Part of this recipe for gigantism has to do with bits of genetic material associated with growth, of course, but other important transformations are more unsuspected, such as the ability to control the onset of cancer and the deactivation of tooth formation.

A new study on the topic has just been published in the specialized journal Scientific Reports, signed by a team from the Laboratory of Evolutionary Genomics at Unicamp. According to the work’s coordinator, Mariana Freitas Nery, the group’s starting point was to analyze genes (roughly speaking, regions of DNA that contain instructions for the production of proteins) already known to be associated with the growth process in other species of mammals.

“They are species that are phylogenetically close to cetaceans”, that is, they belong to neighboring branches of whales in the evolutionary family tree of mammals, explains the researcher.

Despite the very different appearance they have acquired over the last tens of millions of years, whales are part of the large group of animals with an even number of hooves, which includes today’s pigs and cows.

“As the growth patterns of these animals are studied a lot because of the commercial interest, we already have an idea of ​​the genes that could play a role in the development of whales as well”, she says. “But what we saw is that the same genes important for the size of these animals did not always play the same role with whales. Evolution is very creative.”

For the analysis, the scientists defined a length of at least 10 meters as a “cutoff point” for gigantism in current cetaceans. Among the many current species that surpass that mark is the largest animal of all time, the blue whale (Balaenoptera musculus), which reaches about 30 m, the fin whale (B. physalus), up to 25 m, and the humpback whale (Megaptera novaeangliae), which can reach 19 meters.

All these species belong to a subgroup in which there was a loss of teeth and the formation of filtering fins in the mouth —filaments that help to retain small prey, such as the small crustaceans known as krill, when water passes through the animals’ open mouths. .

There are also giants in another subgroup of cetaceans that have kept their teeth—the great example is the sperm whale (Physeter catodon).

In the study, Nery and his colleagues compared genes from 19 species of cetaceans, among which seven were classified as “giants”. The idea, explains the researcher from Unicamp, was to identify whether alterations in these genes were being favored by natural selection over millions of years of evolution. That is, whether a change in DNA corresponded to greater success in reproduction and survival of these species over time.

One of the ways to try to estimate this is surprisingly simple. The chemical DNA “letters” of the genes contain the recipe for the body’s protein production. But not always changing a “letter” of DNA alters the protein corresponding to the gene. Some alterations leave the protein unchanged and are therefore called synonymous substitutions. It is, in fact, like a synonym of human languages ​​—”car” and “automobile” have, in general, the same meaning in Portuguese.

On the other hand, there are also non-synonymous substitutions, when changing a “letter” of DNA produces a protein that is very different from the usual one — like changing “car” to “corro” in Portuguese.

The method used in the new study investigates precisely the ratio of non-synonymous versus synonymous substitutions in the evolution of whales. The idea is that if a given gene has undergone more non-synonymous substitutions, it is a sign that the protein corresponding to it is undergoing significant changes over time. That is, new versions of this protein would probably be favored by natural selection.

Using this approach, the researchers identified changes in a few genes that seem to have a tailored role in transforming whales into the behemoths they are today. This is the case of NCAPG, a gene associated with weight gain, feeding efficiency and growth during puberty in cattle, or GHSR, whose activation can stimulate the release of growth hormones. But other elements of this story are far less obvious.

In the lineage of “toothless” whales, for example, the team identified that a gene known as EGF became a pseudogene —that is, in practice, it stopped working as a recipe for proteins. The point is that it is important precisely for the formation of teeth, which indicates that, in this subgroup, one of the keys to gigantism was precisely the loss of these structures.

“The evolution of a large size has everything to do with food”, explains Nery. “The loss of this gene early on in the group’s ancestor was important for the evolution of filter feeding through fins. This type of feeding allows the ingestion of a very large amount of plankton and zooplankton, which are very rich in energy. Gigantism in cetaceans probably also started at a time when the seas were very rich in nutrients.”

Finally, another gene identified by the study, IGFBP7, is important in the processes that regulate the growth, multiplication and aging of cells, helping to prevent, for example, the formation of tumors. Cancer, by the way, in theory would be a danger for animals that acquire enormous dimensions, because growing up requires a lot of cell multiplication, and it is precisely in this process that copying errors appear in the DNA that trigger the formation of tumors.

Whales, however, seem to handle this quite well. “And it’s not just because whales don’t smoke”, jokes the researcher. Changes in IGFBP7 over the course of the group’s evolution may have helped with this, according to the study.

Nery recalls that the genes identified in the study are only part of the story, since the evolutionary transformations that produced the current whales were complex and multifaceted. According to her, the team is also investigating the so-called regulatory regions of DNA, which do not correspond to genes themselves (they are not the basis for the production of proteins), but can influence the activation and deactivation pattern of genes.