Parallel evolution in three-spined sticklebacks
What happens in the Eastern Pacific, stays in the Eastern Pacific

Date:
June 22, 2020
Source:
University of Helsinki
Summary:
A group of researchers used novel and powerful methods to
disentangle the patterns of parallel evolution of freshwater
three-spined sticklebacks at different geographic scales across
their distribution range. The group concludes that the conditions
under which striking genome-wide patterns of genetic parallelism
can occur may in fact be far from common - perhaps even exceptional.



FULL STORY ==========================================================================
A group of researchers from the University of Helsinki used novel and
powerful methods to disentangle the patterns of parallel evolution of freshwater three- spined sticklebacks at different geographic scales
across their distribution range. The group concludes that the conditions
under which striking genome-wide patterns of genetic parallelism can
occur may in fact be far from common - - perhaps even exceptional.


==========================================================================
The three-spined stickleback (Gasterosteus aculeatus), a thumb-sized
fish distributed across the Northern hemisphere, is a textbook model
species in evolutionary biology. With the retreat of ice sheets since
the last glacial maximum, ancestral marine populations have repeatedly colonised newly formed freshwater habitats. Across their distribution
range, sticklebacks in these novel freshwater environments exhibit
remarkable similarities in their morphology, physiology and behaviour,
a phenomenon known as "parallel evolution." "What is really remarkable
in our results is that the repeatability of evolution in response to
similar selection pressures in different oceans can be so different,"
says group leader Juha Merila", Professor at the Faculty of Biological
and Environmental Sciences, University of Helsinki.

The genetic underpinnings of such parallel evolution have fascinated
scientists for years, and they have discovered that the observed marine-freshwater differentiation is underlain by surprisingly parallel
changes also at the genetic level. However, most studies on this topic
have been based on either limited geographic sampling or focused only
on populations in the Eastern Pacific region.

"As scientists, we are often tempted to provide simple narratives to
extremely complex problems. What I liked the most about this project
is that we did the exact opposite: we show that the story behind the three-spined stickleback's spectacularly fast adaptation to novel habitats
may be more complex than previously thought. I think that deciphering
the role of demographic history in shaping evolutionary adaptation is a necessary step in solving the mystery," says co-author Paolo Momigliano, postdoctoral researcher at the Faculty of Biological and Environmental Sciences, University of Helsinki.

Genetic parallelism 10 times higher in the Eastern pacific With novel
and powerful methods, a group of researchers from the University of
Helsinki disentangled patterns of parallel evolution of freshwater
three-spined sticklebacks at different geographic scales across their distribution range.

They found that the extraordinary level of genetic parallelism observed
in the Eastern Pacific region is not observed in the rest of the species' range. In fact, they found approximately 10-fold higher levels of genetic parallelism in the Eastern Pacific compared to the rest of the world.

"I have been studying the worldwide population histories of the species
in my PhD. We found their ancestral populations are residing in the
Eastern Pacific.

We predicted that the region harbours the source of ancestral genetic variations for parallel evolution, and such genetic variation could
be lost during colonisation to the rest of the world, for instance in
the Atlantic.

These predictions were tested by both empirical and simulated data,"
explains first author Bohao Fang, PhD candidate from the Faculty of
Biological and Environmental Sciences, University of Helsinki.

What happens in the Eastern Pacific, stays in the Eastern Pacific Their simulations showed that this difference in the degree of parallelism
likely depends on the loss of standing genetic variation -- the raw
material upon which selection acts -- during the colonisation of the
Western Pacific and Atlantic Oceans from the Eastern Pacific Ocean.

This discrepancy could have been further accentuated by periods of strong isolation and secondary contact between marine and freshwater habitats
in the Eastern Pacific, consistent with the group's results and the
geological history of the area. This secondary contact likely happened
after the colonisation of the Atlantic Basin, resulting in much more
genetic variation available for local adaptation in the Eastern Pacific
-- variation that never had the chance to spread to the Atlantic. In
other words, the discrepancy in genetic patterns of parallel evolution
between the two oceans is a result of the complex demographic history of
the species, which involved range expansions and demographic bottlenecks.

"Our less assumption-burdened methods have been a key to quantifying
parallel evolution at different geographic scales for the type of data
that was available for this study. I thoroughly enjoy developing novel
methods to study adaptation and evolution, and the idea that parallel
evolution might be exceptional in the Eastern Pacific compared to
the rest of the world has intrigued me for a long time. It was a lucky coincidence that I became a part of the Ecological Genetics Research Unit
led by Juha Merila" where the samples to finally test this hypothesis
became available," concludes Petri Kemppainen, co-first author, method developer, and postdoctoral researcher at the Faculty of Biological and Environmental Sciences, University of Helsinki.


========================================================================== Story Source: Materials provided by University_of_Helsinki. Original
written by Bohao Fang and Marjaana Lindy. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Bohao Fang, Petri Kemppainen, Paolo Momigliano, Xueyun Feng, Juha
Meril�. On the causes of geographically heterogeneous
parallel evolution in sticklebacks. Nature Ecology & Evolution,
2020; DOI: 10.1038/s41559-020-1222-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200622132933.htm

--- up 21 weeks, 6 days, 2 hours, 34 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)