Genetic miscommunication causes gigantic growth in whales

One has teeth, the other a butt. They eat up to 6,000 kg of food per day. With a length of thirty meters, one of them is the largest animal on earth. Although whales capture the imagination of many people due to their ‘gigantism’, it was long unknown why these beasts suffer from gigantic growth. Researchers from State University of Campinas in Brazil published a solution last week Nature for this mystery: four specific genes caused whales to grow into giants.

“When we look at the evolution of whales, it’s a little strange that they are so big,” said Mariana Freitas Nery, a marine biologist and co-author of the publication. “Their ancestors were relatively small, so the question is where the huge growth spurt came from.” In addition, gigantic growth is thought to bring evolutionary disadvantages. For example, large animals need more food. Associated competition results in smaller populations, which reduces the diversity of genes. Among other things, this affects the ability of whales to adapt to changes in their habitats and the availability of food. “In addition, larger animals are more susceptible to diseases such as cancer,” says Nery. “Large animals have many cells and often get extremely old, so the risk of cancer is higher than in small animals.”

The food quality is higher

Despite hypotheses contradicting the evolution of sea giants, there are currently around two million whales swimming in our seas and oceans – although not all of them are huge. A few possible explanations have already been explored. “Animals in the sea have more space and the quality of the food is higher than on land,” says Nery. However, she believes that this only explains part of the gigantic growth in whales. After all, the disadvantages of their gigantic appearance weigh heavily. Therefore, the Brazilian researchers looked at the gene composition. “There has been a lot of research into growth genes in cattle because bigger animals produce more meat,” says Nery. “Also, cows are quite similar to whales in terms of gene composition, so we took those growth genes into our research.” She also studied common growth genes found in many animals.

Nery and her colleagues examined nine growth genes that were present in all 86 whales.

As a whale grows, the risk of cancer decreases

Mariana Freitas Nery Marine biologist

Nery and her colleagues were the first to find a so-called stop codon on the RNA chain in the epidermal growth factor (EGF) gene. This RNA chain consists of several amino acids and acts as a molecular messenger from DNA to cells that make proteins for growth. A stop codon in that chain causes part of the message to be omitted, causing a gene to partially lose its function. “The specific stop codon we found caused the disappearance of growth retardation in whales,” says Nery. “We found by chance that this gene also caused tooth loss and the development of baleen bones.” These baleens filter the water in which krill and other microorganisms swim. This development allowed baleen whales to catch a lot of food, and it partly explains the enormous body sizes.

Hungry feeling

The sperm whale is the only toothed whale that suffers from giant growth, but the stop codon was not found here. The GHSR gene plays a big role here, explains Nery. This gene stimulates the release of growth hormones and promotes a feeling of hunger. In all giant whales, including baleen whales, this gene was more active than in mammals that do not suffer from giant growth.

Another active growth gene that the researchers found in giant whales was the IGFBP2 gene. It is almost three times as active here as in other mammals. This gene promotes cell growth and regulates cell division. “But the discovery that really jumped out at us was the increase in the productivity of the IGFBP7 gene as the whale’s BMI increases,” says Nery. After all, this gene not only promotes growth, but it also suppresses cancer. “So as a whale grows, the risk of cancer decreases,” says Nery. “This completely contradicts previous hypotheses that large animals are more susceptible to cancer.”

That growth genes that suppress cancer in whales also promote extra growth is the key to the ocean giant’s evolution. “These genes have been very important in terms of natural selection,” emphasizes Nery. “Combined with the large supply of food and the formation of baleen whales, whales can afford their large and heavy bodies.”

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