Kerstin Johannesson: Snails teach me about genetics and evolution

When she was fifteen, Kerstin Johannessons´ parents bought a summer house on Tjärnö, outside of Strömstad in northern Bohuslän. The house was next to a marine zoological field station belonging to the University of Gothenburg. Her existing interest in the ocean expanded, and despite her guidance counsellor’s efforts to lead her to a career in hairstyling, she ended up studying biology at the University of Gothenburg. Today, Johannesson is a professor of marine ecology – and she manages that very same field station.

“My interest in evolution began with a master’s project; I was going to describe two periwinkle species. But I realised pretty quickly that these snails still shared genes and were not different species.”

Wave snails and crab snails are ecotypes

In fact, the species were two locally adapted ecotypes, in other words, two variations of the same species that have separately developed hereditary characteristics that “customize” them to live in a certain environment. In this case, the wave snail ecotype, is adapted to areas with strong wave action, and the crab snail ecotype, is adapted to areas populated by crabs. The different habitats produce different challenges for the snails: the force of large waves in the first case, and hungry crabs with strong claws in the second.

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“Darwin already understood that species formation is a gradually occurring process, and an initial step is the establishment of hereditary differences and the formation of ecotypes. That’s why studying ecotypes is so interesting – they may be in the midst of a species formation process.”

Species formation has long been considered a consequence of large-scale geological processes that took place over very long periods, such as when mountain ranges form or when two oceans split apart. These processes can lead to species splitting into different populations that are physically isolated from one another. When the genetic makeup from two populations is no longer mixed together, they become increasingly dissimilar as time passes. After a few million years, the populations will have changed so much that they can no longer reproduce together: new species have formed.

Speciation is the mystery of mysteries

Johannesson’s research shows that this split can occur significantly faster, in only a few thousand years, and with no physical barriers separating the populations.

“We now have numerous examples that species can form without isolation. This raises questions about how reproductive barriers develop despite ongoing genetic exchange, something which should prevent the establishment of dissimilarities. Darwin called speciation the ‘mystery of mysteries’, and the fact that such an important process is still so unclear is very exciting.”

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New research findings often tend to trickle in, but now and then a researcher discovers something utterly unique. This was the case when Johannesson and a few of her colleagues found the first signs of inversions in the genes of the periwinkles. Inversions are a type of mutation in which pieces of a chromosome have turned 180 degrees, reversing the order of a DNA sequence. More and more research now suggests that inversions may be very significant for the division of species into locally adapted populations and for species formation

“Twenty years ago, we suspected an inversion in another species of periwinkle, but now we’ve found several of them and they are fascinating, because they turn so many traditional rules and patterns for how genetic adaptation occurs completely upside down. Today, you can find inversions in lots of species and they seem to be incredibly important. But when we were able to verify our first inversions, the ‘inversion wave’ that’s sweeping over the research world hadn’t even started yet.”

Found a new kind of bladder wrack

Another career highlight was when Johannesson and her colleagues first analysed the genetics of rockweed from the Gulf of Bothnia.

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“We could see immediately that what we had found were not ordinary sexual individuals. They all belonged to one huge vegetative clone which we now know spans from Öregrund to just south of Umeå, and even over to the Finnish side. I’m still fascinated by this super clone. It consists of millions of seaweed plants, but we don’t really know why it exists and we don’t think an equivalent can be found anywhere else.”

Kerstin Johannesson has long been interested in the biodiversity of the Baltic Sea, and how marine species like rockweed and cod managed to adapt to their special environments. She has led multiple research projects on the subject and written numerous articles on it as well.

Important to share the knowledge

In 2020, she was awarded the Synergy Award at the Faculty of Science for her efforts to educate and share findings beyond the University. This has been a matter of course throughout her research career, whether making TV programmes for kids, training professional fishers in marine biology, or advising the Swedish government.

“My research teaches me how nature works, and I can apply my expertise much further. And it doesn’t really matter that I’ve learned about genetics and evolution from snails. I can apply this information to any species.”

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Johannesson believes that research should contribute to a better marine environment as a matter of course. She emphasises that managing our oceans requires understanding the significance of genetic variation and genetic biodiversity within species.

“A colleague and I are currently working on a pilot project to start monitoring the genetic diversity of marine and limnic environments. It’s especially fun that Sweden seems to be one of the first countries in the world to have taken this kind of initiative.”

Exciting research field continues to attract

Johannesson has a long career behind her. She has been devoted to Tjärnö and the periwinkles the entire time. And while she has reached the age of retirement, she does not intend to stop conducting research.

“With the technology we have today, especially DNA sequencing and even advanced modelling, research goes very quickly. Because I started before sequencing was even a word, I’ve been able to follow how much easier it has become to work with the questions evolutionary biology has been struggling with for a long time. It’s a really exciting time to be part of this research field.”

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Text: Susanne Liljenström