A ‘ghost’ gene leaves sea mammals vulnerable to some toxic chemicals | Science News for Students

A ‘ghost’ gene leaves sea mammals vulnerable to some toxic chemicals

Evolution left marine mammals unable to produce a pesticide-busting protein
Aug 28, 2018 — 6:45 am EST
an underwater photo of two manatees

The ancestors of manatees and other marine mammals shed certain genes millions of years ago. One of those genes protects against certain harmful pesticides.

Robert K. Bonde/USGS–Gainesville

Creatures deal with toxic chemicals in their environments in many ways. One strategy: Destroy them with chemical-busting proteins. Many mammals have a gene that helps them break down certain toxic chemicals. But sea-dwelling mammals have a dud version of this gene, new research shows. And without it, manatees, dolphins and other marine mammals appear vulnerable to dangerous pesticides.

The gene is called PON1. It carries instructions for making a protein that interacts with fatty molecules in the blood. But that protein has taken on another role in recent decades. It helps break down harmful chemicals found in a class of common pesticides. These chemicals are called organophosphates (Ohr-GAN-oh-FOSS-faytz). Farmers spray them onto many crops to fend off hungry insects.

But those chemicals don’t always stay put, notes Wynn Meyer. She studies evolution and genetics at the University of Pittsburgh in Pennsylvania. Rains wash these pesticides off of farm fields and into rivers and streams, she says. From there, they can pollute waterways and coastal areas.

That risks poisoning wildlife. But PON1 may offer a defense. It might help break down chemicals that get into animals’ bodies. Meyer and her colleagues wanted to know whether differences in the PON1 gene affected how animals dealt with pesticides.

First, they looked at the genes of 53 species of land mammals. All had a working form of PON1. And in lab tests, blood from five of the species — including sheep, goats and mice — could break down two pesticides. When mixed with blood from one of these species, the amount of the toxic chemicals dropped over time. Something in the blood — possibly PON1 proteins — was breaking them down.

The same was not true for aquatic mammals. In five tested species, the PON1 gene contained lots of changes, known as mutations. Those changes had turned off the gene, Meyer says. And sure enough, the sea animals’ blood had little to no effect on the toxic chemicals. That suggests marine mammals can’t get rid of organophosphate pesticides.

Mice genetically altered to lack PON1 couldn’t break down the chemicals either.

Meyer and her colleagues reported their findings Aug. 10 in Science.

Story continues below graph.

a chart showing the rate of organophosphate chemical breakdown in animals
Chlorpyrifos oxon and diazoxon are two types of organophosphate pesticides. Blood from marine and semiaquatic mammals (blue) could not break these toxic chemicals down very well. Land mammals better handled the compounds.
W.K. Meyer et al/Science 2018

Defunct defenses

The researchers compared PON1 genes in more than 60 species of mammals. They estimate that the gene went defunct in sea-dwelling species about 64 million to 21 million years ago. That means it happened after marine mammals’ ancestors moved from land to the sea. Changes in diet or behavior related to that move might have made the gene unnecessary, the researchers suspect. 

A busted PON1 gene may not mean marine mammals are helpless against organophosphates, says Andrew Whitehead. He works at the University of California, Davis, and was not involved in the new work. But Whitehead does study how genes change in response to the environment. Marine mammals might have other ways to cope with the chemicals, he says. Yet the new study does seem to find “they aren’t stepping up to the plate.”

What happens to pesticides that don’t get broken down? Some can build up in an animal’s body. DDT is a different pesticide that lingers in the environment. It builds up in animals’ tissues once they swallow it. DDT-poisoned birds lay eggs with fragile shells that break before hatching. And in marine mammals, high levels of DDT damage the nervous system and can cause birth defects. That’s why the United States and dozens of other countries have banned use of DDT.

It’s unclear if organophosphates build up in marine mammals’ bodies in a similar way. But even if they don’t, they may still cause problems.

“Organophosphates don’t stick around as long in the environment as DDT,” Whitehead says. But the chemicals are still used on crops and to kill mosquitoes and other pests. So there’s always more entering the environment.

The researchers next want to study animals in coastal areas that have chemical runoff from farms, says study coauthor Nathan Clark. He is an evolutionary biologist also at the University of Pittsburgh. They plan to collect blood samples from dolphins and manatees in those waters. They will look for evidence that the animals have been exposed to the pesticides. If so, that will help scientists look for links between chemicals in the environment and effects in animals’ bodies, he says.

Power Words

(more about Power Words)

aquatic     An adjective that refers to water.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

birds     Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

coauthor     One of a group (two or more people) who together had prepared a written work, such as a book, report or research paper. Not all coauthors may have contributed equally.

colleague     Someone who works with another; a co-worker or team member.

crop     (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers. 

DDT     (short for dichlorodiphenyltrichloroethane) This toxic chemical was for a time widely used as an insect-killing agent. It proved so effective that Swiss chemist Paul Müller received the 1948 Nobel Prize (for physiology or medicine) just eight years after establishing the chemical’s incredible effectiveness in killing bugs. But many developed countries, including the United States, eventually banned its use for its poisoning of non-targeted wildlife, such as birds.

defense     (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

diet     The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

dolphins     A highly intelligent group of marine mammals that belong to the toothed-whale family. Members of this group include orcas (killer whales), pilot whales and bottlenose dolphins.

egg     The unfertilized reproductive cell made by females.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

evolution     (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).

evolutionary     An adjective that refers to changes that occur within a species over time as it adapts to its environment. Such evolutionary changes usually reflect genetic variation and natural selection, which leave a new type of organism better suited for its environment than its ancestors. The newer type is not necessarily more “advanced,” just better adapted to the conditions in which it developed.

evolutionary biologist     Someone who studies the adaptive processes that have led to the diversity of life on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species change to adapt, and the fossil record (to assess how various ancient species are related to each other and to modern-day relatives).

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

link     A connection between two people or things.

mammal     A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.

manatee     A plant-eating marine mammal that lives, in the western hemisphere, primarily in the Gulf of Mexico and Caribbean Sea

marine     Having to do with the ocean world or environment.

marine mammal     Any of many types of mammals that spend most of its life in the ocean environment. These include whales and dolphins, walruses and sea lions, seals and sea otters, manatees and dugongs — even polar bears.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

mutation     (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

nervous system     The network of nerve cells and fibers that transmits signals between parts of the body.

pesticide     A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pets or livestock; or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

protein     A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Among the better-known, stand-alone proteins are the hemoglobin (in blood) and the antibodies (also in blood) that attempt to fight infections. Medicines frequently work by latching onto proteins.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

runoff     The rainwater that runs off of land into rivers, lakes and the seas. As that water travels through soils, it picks up bits of dirt and chemicals that it will later deposit as pollutants in streams, lakes and seas.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

strategy     A thoughtful and clever plan for achieving some difficult or challenging goal.

tissue     Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.


Journal:​ W.K. Meyer et al. Ancient convergent losses of Paraoxonase I yield potential risks for modern marine mammals. Science. Vol. 361, August 10, 2018, p. 591. doi:10.1126/science.aap7714.