A new paper shows that a gene responsible for helping mammals break down harmful chemicals is faulty in several marine species, leaving them more vulnerable to the effects of pollution from pesticide, according to ScienceNews. Called PON1, the gene gives instructions for producing a protein that allows the body to deal with fatty acids from food, and in the modern world, has also functioned to help break down substances in organophosphates, a widely-used class of pesticides. They often make their way into waterways as part of agricultural runoff, potentially hurting wildlife, and in terrestrial mammals, the gene helps to destroy byproducts in blood plasma.

The chemical can damage the central nervous system, causing paralysis and brain damage, and the results of the study suggest that whales, dolphins, manatees, seals, and sea lions are particularly vulnerable.

In 53 species of land mammals, the gene was found intact, while six mammal species had PON1 genes that mutated in ways that rendered it non-functional. They found that the mutations had occurred between 21 and 64 million years ago, and suggested that it was related to changes in the diet and behavior of these species as they adapted to marine life.

Researchers do not know for sure why the functioning gene was lost over time, and whether it corresponded to another adaptation to the new marine environment, or whether it simply became unnecessary.

According the paper’s lead author, Wynn Meyer:

“It’s difficult to tell the difference between something that was lost because losing it was beneficial, or something that was lost because having it around was no longer necessary. Those are subtle differences, and especially when a gene has lost its function, it’s really hard to tell those two things apart.”

The paper was published in the August 10th issue of the journal Science.

According to the study’s senior author, Nathan Clarke of the University of Pittsburgh:

“The fact that it’s so absolutely conserved in terrestrial species, and is completely lost from most, almost all marine lineages—it’s really striking isn’t it? It seems like this gene has a story to tell.”

The researchers also tested how quickly organophosphate broke down in blood samples from the different species, finding that the level of toxins fell over time in terrestrial mammals, but remained nearly the same in the marine species.

Next, the team wants to investigate blood samples from marine mammals from coastal areas with agricultural runoff, to see whether the blood levels of the chemicals correspond with levels in their environment.

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