MSES Seminars: Toxic Survivors

Today we have an entry from guest blogger Lizzie Cooke.  Lizzie is a third-year master’s student in SPEA’s joint MPA/MSES program, with a concentration in Energy. This year, she is also the recipient of a Foreign Language and Area Studies (FLAS) fellowship from the IU Center for Latin American and Caribbean Studies through which she is taking courses in Haitian Creole. Lizzie created her blog, www.labreport.org, as a way to combine her scientific curiosity with her love of writing.

The weekly Environmental Science seminars play an important role in community building for SPEA’s Environmental Science program. The seminars include lectures by researchers within SPEA, as well as by researchers from across the country and even around the world. They expose students and faculty to the broad range of research within the field of environmental science and policy.

Genetic adaptation is a painstakingly slow process that is impossible to observe except on geologic time scales, right? That might be what you learned in your middle school science class, but scientists now know that genes are surprisingly changeable.

Researchers have found that Daphnia pulex, a miniature crustacean commonly known as the water flea, is capable of altering its genes in response to environmental stresses and passing those changes on to its offspring, resulting in genetic adaptation in a very short period of time.

Due to daphnia’s remarkable adaptability to environmental conditions, scientists have studied this microcrustacean for centuries, using it as an indicator of the health of lakes and streams. For example, daphnia are capable of increasing hemoglobin production in response to low oxygen levels, a response that turns their tiny transparent bodies red. They can also change their physical form to deter predators.

“They get these big spikes on their heads that say, ‘Don’t eat me!” says Joseph Shaw, a professor and research scientist with Indiana University’s Center for Genomics and Bioinformatics.

Daphnia are also capable of alternating between sexual and asexual reproduction, a trait which gave them their name. According to Greek mythology, the nymph Daphne begged the gods for help when Apollo pursued her. The gods granted her wish, transforming her into a laurel tree, thus preserving her virginity. Most daphnia are female and reproduce asexually. Male daphnia only develop under certain environmental conditions when sexual reproduction becomes preferable for survival.

In 2011, researchers released the results of a project to map daphnia’s genome. They found that daphnia have 31,000 genes, more than any other organism mapped so far. By comparison, the human genome is estimated to contain 23,000 genes. Pretty remarkable for such a tiny creature—the daphnia genome has even inspired a poem about why size matters.

“They have a very compact genome structurally,” says Shaw, explaining how daphnia can physically accommodate so many genes. Essentially, daphnia have much smaller gaps between their genes than other species. The large number of genes is due to daphnia’s very high rate of gene duplication.

Approximately one third of daphnia’s genes are previously undiscovered genes. These daphnia-specific genes are important in explaining daphnia’s adaptability to a wide range of environmental conditions. Daphnia can turn these genes on and off in response to predators, metal contamination, and other stresses.

For example, in a study on the response of daphnia to metal contamination, Shaw and his colleagues discovered that daphnia can vary their production of metallothionein, a protein that renders metals biologically unreactive.

The protein essentially “grabs the metals and won’t let them go,” explains Shaw. As a result, the metals can no longer interfere with the biological functions of the daphnia. The tiny crustaceans become “toxic survivors,” capable of living in water with high levels of metal contamination.

The researchers found that individual daphnia not only protect themselves but also pass on this metal tolerance to their offspring. The researchers made this discovery by collecting samples of daphnia from lakes near Sudbury, Ontario, an area that has been polluted by mining and smelting operations since the mid-1800s. They then compared these daphnia to samples collected in lakes near Dorset, Ontario, an area that is similar in geology to Sudbury but uncontaminated.

The researchers found that daphnia from the contaminated lakes had a greater tolerance for exposure to high levels of cadmium. Furthermore, after 20 generations of reproduction, the offspring of the Sudbury daphnia still had greater tolerance than the offspring of the Dorset daphnia. Thus, the exposure to metal contamination in the lakes resulted in a heritable tolerance to cadmium.

Imagine if your parents were exposed to arsenic and as a result, you (and your children and their children and so on) were born with an ability to drink arsenic-contaminated water without getting sick. That would be incredible in humans, yet it is a trick mastered by the unassuming little daphnia.

The researchers were able to link daphnia’s cadmium tolerance to metallothionein production by observing that the Sudbury daphnia had higher levels of metallothionein production than the Dorset daphnia even without exposure to cadmium. In other words, their genes for metallothionein production were “turned on” to a greater degree at all times. When the daphnia were exposed to cadmium, both sets of daphnia exhibited higher metallothionein levels, but since the Sudbury daphnia were starting from a higher baseline, they exhibited greater overall metallothionein production and therefore greater tolerance to cadmium.

The National Institutes of Health recognize daphnia as a model organism for biomedical research. Daphnia are particularly important in monitoring water quality because of their rapid response to toxins. Potential water contamination can be flagged through changes in the physical appearance or behavior of daphnia. In addition, daphnia have more genes in common with humans than any other invertebrate model organism, making them a useful tool for identifying threats to human health.

Daphnia could be used to test the toxicity of chemical compounds. Shaw notes that there is a large backlog of chemicals with unknown toxicity. Currently, there are between 80,000 and 100,000 known chemical compounds, with 4,000 new compounds developed each year, but many of these compounds remain untested for their impact on human and environmental health. Testing methods that exploit the adaptability of daphnia could prove cheaper and quicker than current testing methods.

Daphnia could even be used to track genetic responses to environmental conditions over time. Shaw and other daphnia researchers are working on a project to extract ice cores and examine historical changes in daphnia gene expression over the past 300 years. Just as climate scientists use ice cores to study past carbon dioxide levels in the Earth’s atmosphere, daphnia researchers hope to use ice cores to study biological changes over time. They could then use the past to forecast the future.

Genome giants. Toxic survivors. Why not add soothsayers to the list?

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