Science & Technology

McGill researchers spotlight Polypterus fish

A team of researchers led by Dr. Hans Larsson from McGill University has made a key discovery in the process by which creatures evolved from aquatic to terrestrial animals. The study used the Polypterus fish as a means to investigate phenotypic evolutionary changes.

Larsson, Canada’s Research Chair in Vertebrate Palaeontology, started his research due to his interest in macroevolution, the study of large-scale evolutionary transitions. The project—which is the brain-child of Emily Standen, now an associate professor at the University of Ottawa, started with help from Trina Du—a McGill PhD student. Strong efforts by all three helped shape the results. 

According to Larsson, the decision to focus on tetrapod­s—four-legged land animals—was obvious.The evolutionary in-between—which Larsson fondly calls a ‘fishapod’ is an important part of the evolutionary process. 

“The fish-to-tetrapod transition has a pretty good fossil record in how these ‘fishapods’ started to become more land capable,” Larsson said. “We’ll never get those fish alive, but it’s important to look at something that’s as close to the family tree as these ‘fishapods.’” 

The researchers concluded that the best option for the study would be the Polypterus, a type of ray-finned fish and a close-relative to the tetrapod. Polypterus’ pectoral fins stick out stalks from the body, which can be used for ‘walking.’ Their pectoral and pelvic fins also resemble tetrapod limbs.

“[The Polypterus] has a lot of similarities to the primitive fishapod,” Larsson said. “They have fully developed lungs, which makes them very similar to tetrapods.” 

Cross-section scan of Polypterus. (Courtesy of Trina Du)
Cross-section scan of Polypterus. (Courtesy of Trina Du)

After having chosen the model for the experiment, the team then shifted their attention to the difficulty of raising the animals in a terrestrial environment. Initially, the researchers were unsure about the fish’s ability to live only on land for extended periods of time.

“We were scared it wouldn’t work at all,” Du said. “It was [such a bold experiment] that something bizarre and unexpected could happen, and [the experiment] wouldn’t work at all.”

Much to their amazement, it worked. The fish were not difficult to raise on land, and by the end of the study, they had changed in both behaviour and anatomy. They walked more efficiently, raised their heads higher, had fins closer to their bodies, and slipped less. 

The results of the study have raised numerous questions—primarily: If evolution occurs through gene inheritance, how would walking traits get passed down through the generations? 

“The phenotype is not as tightly connected to the genotype as previously thought,” Larsson said. “Extreme environmental stress reveals a cryptic change that is usually not seen in the rest of the population.” 

For example, if people were raised underwater, say in scuba gear for their entire lives, they would look a lot different from the people who were raised on land. Their skin, hair, and even facial features would morph to adapt to their surroundings. 

The genome may not be coding for certain traits that would become present in new environments; it’s just loose enough to allow for it. Essentially, our body reacts to stress in strange ways.

The importance of this study is its role in showing a precursor step to Darwinian evolution. If new morphologies and behaviours—which may not be encoded in the genome—are developed, and thus that animal becomes the most fit individual, there’s a higher likelihood that the small percentage of functioning genes coding for that trait will be selected for. As the generations progress, that trait will become more and more dominant until it has been fully adopted—hence, evolution.

The team plans to restart the entire experiment with a fresh batch of fish to continue the project.

“The next step is to breed the Polypterus and see how these traits might be passed through the generations,” Larsson said.

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