Vetmeduni: When Genes Deceive – Why Flies in the Lab Take Different Paths to the Same Goal
Researchers at the University of Veterinary Medicine, Vienna (Vetmeduni), show that the genomes of fruit flies adapt in completely different ways to different temperatures – even though all flies ultimately achieve the same fitness gains. The reason: a previously underestimated genetic effect called pleiotropy.
When populations adapt to new environmental conditions, researchers often assume: if the same genomic site changes in the same direction across many individuals, then that site must be responsible for adaptation to that specific environmental factor. A team led by Christian Schlötterer from the Unit of Population Genetics at Vetmeduni now shows in a new study that this seemingly obvious conclusion can be misleading.
Two temperature regimes, one surprising result
The researchers maintained populations of the fruit fly Drosophila simulans in the lab for more than 160 generations under two different temperature conditions: one group was kept at a constant 23 °C, the other at fluctuating temperatures between 18 and 28 °C - with the same average temperature as the first group. Both scenarios simulate typical challenges that animals face in the wild.
A look at the genomes revealed a clear pattern: 93 percent of the genetic loci that changed substantially during adaptation were found exclusively in one or the other temperature group. Everything thus pointed to tailored, temperature-specific adaptation.
However, measurements of the flies’ actual fitness told a different story: regardless of the temperature at which they had developed, both groups showed an almost identical increase in fitness compared to their ancestors - and this held true regardless of the temperature at which their fitness was measured.
Pleiotropy as the explanation
How can such different genetic patterns yield such similar phenotypic outcomes? Using computer simulations, the team showed that a phenomenon called pleiotropy likely solves the puzzle: a single gene can influence multiple traits at once. Because both fly groups faced the same fundamental selection pressure imposed by the new laboratory environment, yet differed in temperature, seemingly temperature-specific genetic changes could arise even though the primary driver of the fitness increase was shared.
“If you look only at the genetics, you might easily conclude that two completely different adaptive strategies evolved,” explains Christian Schlötterer. “But the fitness measurements show that both fly groups essentially solved the same problem - just with different genetic tools.”
Implications for research
The findings have implications far beyond fly research. Many studies of adaptation in animals, plants, or microorganisms - for example, in the context of climate change - rely on comparing genomic changes between populations from different habitats. This new study urges caution when interpreting such genetic differences: not every genomic pattern can be mapped one-to-one onto a single environmental factor when organisms are simultaneously exposed to multiple stressors.