From Looks to DNA: How Arctic Charr Evolve Within a Single Lake

"Arctic charr is a remarkable model for studying how a species develops — first starting with differences in appearance and later transitioning into changes in the genome", says Khrystyna Kurta a researcher at the Swedish University of Agricultural Sciences (SLU) and the first author of the publication.

Together with colleagues from Sweden, Iceland, Norway, and the UK, she recently published a study examining Arctic charr from two Norwegian and two Icelandic lakes. Two to four forms of Arctic charr appear in these four lakes, each with its own unique history, structure, clarity, and depth, ranging from 4.5 meters to 165 meters. In this study, the researchers examined how differences between the forms of Arctic charr are reflected in their entire DNA across multiple environments, not just in a single lake.

"These lakes provide an opportunity to see how variation in ecology influences genetic change and, ultimately, the evolution of Arctic charr", says Khrystyna Kurta.

An interesting example of this is the Icelandic lake Thingvallavatn. There, a fish about 50 centimeters long lives near the bottom. Higher up, a small, dark fish of about 13 centimeters swims. Closer to the surface, there’s a silvery plankton-eater around 19 centimeters, and at the very top, a 41-centimeter-long predator. They are all Arctic charr.

"We conducted the first complete analysis of the entire genome of Arctic charr and created what is called a reference genome. This allowed us to see in detail how they differ", says Khrystyna Kurta.

The differences between the four lakes turned out to be significant. In Iceland’s Thingvallavatn lake, with its uniquely clear water, the morphs showed strong genetic differences. Here, the researchers discovered several large structural changes in the genome — so-called inversions — that seem to play a role in maintaining separation between the morphs. In another lake in Iceland, only partial differences were observed in specific parts of the genome. In both lakes, the results suggest partial reproductive isolation between charr forms.

In one Norwegian lake, the morphs showed no genetic differences at all — there, the fish's different appearances seem entirely due to the species’ ability to adapt to its environment. Finally, in the second Norwegian lake, the charr morphs were so genetically distinct that they appear to be completely reproductively isolated.

"This is very exciting. It shows how both time and environment are crucial for how the fish change. There is no predetermined outcome for how they will evolve — the location is decisive", says Khrystyna Kurta, adding:

"These conclusions are possible thanks to advanced DNA analysis. It clearly shows whether, and if so, where in the genome the morphs differ."

The results provide a glimpse into how evolutionary development can unfold. Initially, the fish adapt to the environment they live in. Over time, the differences between morphs grow larger. Eventually, the groups may become so different that they no longer spawn together — a step toward becoming separate species.