Sperm Evolution to Combat Pollution

Scientists find duplicated versions of a gene involved in regulating sperm traits helps tree sparrows adapt to pollution

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Many of us are generally aware of the pollution and environmental disruption humans cause. Some of this pollution has detrimental effects that disrupts reproductive processes in many organisms, including the feminization of males, masculinization of females, biased sex ratios, altered reproductive timing, disruption of gonad development, and many others. This phenomenon does not exclude humans, where pollutants can cause reduced semen quality and fertility. Although reproductive disruption is putting some organisms at risk of extinction, there are some that are adapting through the process of evolution.

One example of this comes from studies regarding tree sparrows (Passer montanus). These birds have experienced adaptive changes in sperm traits, such as changes to morphology and motility, in response to heavy metal pollution (a type of pollution sperm traits are sensitive to). Simply put, scientists found tree sparrows in polluted areas had longer and faster swimming sperm, which may benefit reproductive success in areas with pollution. In a more recent study published in the journal Molecular Ecology, scientists investigated how this sperm adaptation has occurred at the genetic level.

To begin, scientists captured male tree sparrows from two ecologically similar locations, with one location being polluted while the other was not. The polluted location was Baiyin, China, which is contaminated by heavy metals due to a long history of metal mining since 1956. The unpolluted site was Liujiaxia, China, which was chosen because it is a protected water source that is relatively unpolluted. 

Next, the scientists sequenced the birds’ genomes (entire collection of an organism’s genes) from both locations for comparison. Specifically, the scientists looked for genes related to sperm variation undergoing positive selection in the birds from the polluted location. Positive selection refers to a process by which new advantageous versions of a gene can arise in a population due to selective pressures. Paired with this technique, the researchers also compared gene expression levels in testicular tissue between the two groups to identify genes that were more ‘turned on’ in birds from the polluted location. One gene stood out by both these methods – PIM1 – which suggested that it may play an important role in sperm evolution in response to pollution.

So what is PIM1? The gene PIM1 encodes a kinase, which is a type of protein that affects the function and activity of other proteins (e.g. turns them on) by attaching a phosphate group to them. PIM1 is evolutionarily conserved in many other multicellular organisms, including humans. In most organisms it exists as a single copy, however, the scientists found that PIM1 has undergone several duplication events (or increase in the number of copies) in multiple bird species including the tree sparrow. 

Whenever a gene duplicates, the copies are free to change by acquiring mutations, which in rare circumstances, can create new functions that allow for adaptations. This may be how the PIM1 copies have contributed to the tree sparrow’s ability to rapidly adapt sperm traits in response to environmental pollution. In line with this hypothesis, many of the extra PIM1 copies showed tissue specific expression in the testis and the average expression of the PIM1 copies was higher in the birds from the polluted location, even though the copy number between the two groups was equal.

So, overall, the scientists identified a gene that likely plays an important role in the regulation of sperm traits in tree sparrows. This gene has expanded in copy number, which has contributed to the ability of tree sparrows to rapidly adapt sperm traits in response to heavy metal pollution. 

So what interesting questions still remain? Well, we now know that PIM1 expression is higher in birds from the polluted location, but what is the molecular mechanism behind why it is higher? What proteins are involved in regulating the expression? We also now know that there are many extra copies of PIM1 that are expressed specifically in the testis, but how are all these extra copies at the protein level actually changing the regulation of sperm traits?

Further illuminating the mechanisms and evolution surrounding PIM1 function may help us understand more broadly how some organisms can rapidly adapt in response to environmental pollution. With all the pollution out there not going away any time soon, understanding how organisms are adapting to it seems like an imperative mission.