Genetic sequencing of sea turtles may help to understand disease that threatens the species – 08/05/2023 – Environment

As if pollution, incidental fishing and hunting were not enough, sea turtles suffer from a type of cancer that limits their survival. In recent decades, some of the threats caused by man have been alleviated with conservation actions. Now, a new horizon opens up to understand, and who knows how to find treatments for the disease.

This is because an international consortium of scientists carried out the most complete sequencing to date of the genome of two of the seven species that live in the sea, the leatherback turtle (Dermochelys coriacea) and the green turtle (Chelonia mydas), increasing the understanding of the immune system and the evolution of these animals.

The results of the first analyzes were published in the PNAS journal by a group that includes Brazilian researchers supported by FAPESP.

“Turtles are vulnerable to this tumor, called fibropapillomatosis, caused by infection with a herpes virus specific to these animals. Although most studies point to cases in green turtles, it has been found in other species”, explains Elisa Ramos, doctoral student at the Institute of Biology at the State University of Campinas (IB-Unicamp) and one of the authors of the study.

“A [tartaruga] green, however, seems to have more genes associated with the immune system in some specific chromosomes, which may give clues to how it fights against the virus”, completes Ramos, who carried out part of the analyzes during an internship at the Leibniz Institute for Zoo and Wildlife Research, in Germany, with a grant from FAPESP.

As a more detailed study of this region of the genome is being developed, the researchers hope to have confirmation of what exactly is happening with these genes in the green turtle soon.

The work is part of research carried out by the Vertebrate Genomes Project, which aims to sequence the genomes of all vertebrate animals on Earth. Currently, the other five species of sea turtle are having their genetic code mapped. With the new data, it will be possible to advance even further in the understanding of defense against diseases and other evolutionary characteristics of these reptiles.

“Our analyzes identified differences in the number of genes related to immunity between the two species and also allowed us to identify the location of the so-called Major Histocompatibility Complex [MHC, na sigla em inglês]which contains imperative genes for the response to pathogens”, says Blair Bentley, who is doing a postdoctoral internship at the University of Massachusetts, in the United States, and is the first author of the work.

According to the researcher, the genomes bring information that can be used to investigate this and other diseases, in addition to providing directions for future treatments and conservation actions.

the repentant

“Turtles have a slow evolution rate and are vulnerable to extinction. Therefore, it is important to understand which genes allowed them to succeed in the marine environment. This is reflected both in mutations and in the number of gene copies”, says Mariana Freitas Nery, professor at IB-Unicamp and co-author of the study.

Nery coordinates a project supported by FAPESP that investigates the genome of species whose ancestors, after abandoning the aquatic environment and adapting to the terrestrial one, returned to live in the water. “We joke that they are the regretful ones,” she says (read more here and here).

Turtles diverged from land-based ancestors that returned to the sea around 100 million years ago. The split between the leatherback and the green turtle, in turn, occurred about 60 million years ago. Even so, the slow evolution of the group meant that most of the genome of the two is still shared.

Among the differences found are the genes related to the olfactory sensors. Although they live in the sea, turtles breathe air and have a terrestrial ancestor. Therefore, they have both sensors that detect molecules in the air and others that perceive those that are dissolved in water, something essential for migration and reproduction, in addition to identifying prey, individuals of the same species and predators.

“As the green turtle is closer to the coast, it has more contact with pollution and other adverse conditions than the leatherback, which lives most of its life in deep waters. Furthermore, while the former has a varied diet, the the second makes great migrations to feed on jellyfish”, clarifies Ramos.

The researchers fear that the slow evolution of turtles makes them unsuited to rapid changes in the environment, such as those caused by climate change.

“In the leatherback turtle, for example, we show a low diversity in the functional regions of the genome, which suggests that populations may not have the capacity to adapt to the rapid increase in temperature caused by human action”, concludes Bentley.

The article Divergent sensory and immune gene evolution in sea turtles with contrasting demographic and life histories can be read here.