”Genetic conflicts are extremely interesting”

In her research, Sophie Karrenberg has studied red campion and white campion (in the picture) to find answers to why these species have drifted apart.

In fact, Sophie Karrenberg should be on an alpine peak in Switzerland collecting samples of the world’s smallest tree, the dwarf willow. But the spring’s Covid-19 pandemic effectively put a stop to all long-distance excursions. Fortunately, there was someone in place who could help with fieldwork for Sophie in the form of a postdoc at the WSL Institute for Snow and Avalanche Research in Davos.

Seeds will soon be collected from dwarf willows and then sent by courier to Uppsala. After planting in the greenhouse at the Evolutionary Biology Centre, it will hopefully take only two years before it can be discovered whether the offspring of these seeds will also demonstrate the same remarkable pattern as the adult dwarf willow individuals do in the field: that the population is 75% female.

“The dwarf willow, and almost all other species of willow, are dioecious, which is rare in plants and means that it has separate male and female plants. But unlike us humans, it doesn’t have X and Y sex chromosomes. What lies behind this difference in the evolution of sex chromosomes is not yet understood,” says Sophie Karrenberg.

The reason for the sex ratio bias in the field is probably not that female plants outperform male plants, according to earlier studies in Sophie Karrenberg’s group.

“What we do know, however, is that XY chromosomes and the like cannot produce such skewed sex ratios; instead, such systems result in equal proportions of females and males.

It is likely that something much more complicated lies behind this phenomenon, possibly some kind of genetic conflict that can ultimately drive the formation of species.

“The question is also how long this process takes. Just imagine if there were very rapid development of such sex-determining mechanisms and then you crossed different species. You would probably then run into problems with reproduction. In which case there will be fewer offspring. Investigating the background to why this happens is something that is very motivating for me,” says Sophie Karrenberg.

She and her research team are working with no less than five hypotheses about the cause of this puzzling sex ratio bias in the dwarf willow. Could it actually lie in hitherto undiscovered ecological differences between male plants and female plants? Or is it hidden in mitochondria, which are only transferred only by the mother?  The third hypothesis is that it is caused by a more complex system for sex determination.

There may also be potential ‘cheating’ alleles or variants of a gene that may affect meiosis to their own advantage. The fifth hypothesis is that ‘killer’ combinations of alleles lead to the death of the seeds. If these seeds happen to be male, this results in fewer male plants.

“There are many willow species and it appears that there are differences in their sex determination. So we will be comparing the genetic sequence of several different willow species. We will then try to figure out the evolutionary history behind the sex determination and the skewed sex ratio of dwarf willows and other willow species.”

Genetic material from the dwarf willow will be sent to SciLifeLab in Uppsala to investigate the tree’s genome. There, DNA fragments will be processed in DNA sequencers to later determine which sites in the genome might lie behind sex determination in the species.

A doctoral student will be linked to the four-year project from the autumn, who will be sent to Switzerland next year to collect new samples and for field work. Sophie Karrenberg collaborates with researchers in Switzerland since earlier research projects on dwarf willows in the Swiss Alps, which made it natural to locate the ongoing collection of samples in Switzerland. Here in Sweden, she also works with Pär Ingvarsson, Professor in Plant Genomics and Plant Breeding at the Swedish University of Agricultural Sciences, who has sequenced the genome of poplars, the nearest relative of the willow family.

But Sophie Karrenberg also thinks about from time to time how she, as an evolutionary biologist, can contribute to matters of immediate practical relevance to today’s society.

“Something I would like to do research on is how to diagnose species, which is needed when you count them. For how can you determine biodiversity when you don’t know which species or subspecies to include or exclude? At present, there are red listings at different levels in the EU, where each country itself decides what to do with these unclear subspecies.”

A broad and deep curiosity has been a feature ever since her childhood. Sophie Karrenberg grew up in northern Germany, just south of Hamburg. Even in her teens, she knew she wanted to become a scientist. This led to biology studies at the University of Kiel and later a PhD in natural sciences at ETH Zürich in Switzerland.

After a postdoc position in the USA and a five year position as junior faculty in Switzerland, she got a senior lectureship position in Uppsala in 2010. She was promoted to professor in 2019. She recently also became the Head for the Plant Ecology and Evolution programme at the Department of Ecology and Genetics. Despite this full timetable, she has also developed introductory courses in statistics using the program R for Master’s students and PhD students.

“It’s an introductory course for statistical analyses where students get to learn commands and code writing. The statistics program R is used in almost all our courses and if you can’t use it effectively, you quickly fall behind.”

As a teacher, she sees her role as inspiring her students and providing initial support. The important thing is to help students get across the threshold and daring to go after knowledge on their own.

“In the beginning, I am with them of course. But learning to present what they want to communicate, to think logically and to solve problems – these kinds of discoveries they must make for themselves. To learn from learning is important and useful, regardless of what you do later in life.”