Male common fruit fly (Drosophila Melanogaster) - about 2 mm long - sitting on a blade of grass with green foliage background
In a groundbreaking study, researchers from Stanford University have unveiled new insights into how fruit fly populations preserve genetic diversity in fluctuating environments, a crucial factor for survival against future threats. The study, published on September 15 in Nature Ecology and Evolution, provides the first direct evidence supporting the theory of “dominance reversal” in genetics.
The research highlights that genetic variants can shift between dominant and recessive roles depending on environmental conditions, offering fruit flies long-term resistance to pesticides. This finding challenges the traditional understanding of genetic dominance, where alleles are typically seen as strictly dominant or recessive.
Unraveling Genetic Mysteries
Populations of organisms often reside in rapidly changing environments, where factors such as droughts, food availability, and human-induced habitat changes pose significant challenges. For scientists, this raises a fundamental question: How do populations maintain the genetic diversity necessary to adapt to future challenges when natural selection tends to eliminate non-beneficial variants?
Researchers at Stanford tackled this question by observing the evolution of fruit fly populations in a controlled outdoor orchard, where they manipulated pesticide exposure over time. This approach, combined with mathematical modeling, allowed them to explore the dynamics of genetic dominance in a real-world setting.
Dominance Reversal: A Hidden Shield
The concept of dominance reversal suggests that a genetic variant can be dominant when advantageous, such as providing pesticide resistance, but become recessive when detrimental, such as in pesticide-free environments. Dmitri Petrov, senior author and professor of biology at Stanford, explains,
“It’s like the flies have a hidden shield. When they don’t need it, it’s not in their way. But it’s ready as soon as they are threatened.”
This mechanism could be widespread in nature, helping maintain genetic diversity for varying environmental challenges. Marianthi Karageorgi, lead author and research scientist in the Petrov Lab, notes,
“What we’re seeing could be a general mechanism for populations to hold on to genetic variants they might need for future environmental shifts.”
Experimental Evolution in Action
Since the 1950s, the idea of dominance reversal has been proposed as a means to maintain genetic variation in changing environments. However, until now, there was no empirical evidence to support this theory. The Stanford team’s findings are based on a combination of surveys, lab experiments, field experiments, and mathematical models.
Before conducting experiments, the researchers analyzed genetic surveys of flies from various environments, including organic farms. Using flies bred by Paul Schmidt at the University of Pennsylvania, they assessed how different genetic variations influenced fly fitness with and without pesticide exposure.
The experimental evolution took place in an outdoor orchard, where large fruit fly populations evolved under near-natural conditions. Some cages were exposed to pesticide pulses, mimicking seasonal insecticide use, while others remained untreated. The researchers tracked pesticide resistance and gene variant frequencies in real-time, revealing that resistance alleles rose sharply with pesticide use and declined once exposure ceased.
Mathematical Modeling Confirms Hypothesis
Mathematical modeling of allele frequencies in treated and untreated cages confirmed the presence of dominance reversal. Resistance alleles acted as dominant when beneficial in the presence of pesticides but became recessive when costly in their absence. This flexibility allows pesticide-resistant alleles to quickly provide resistance when needed while remaining hidden from natural selection when harmful.
Karageorgi describes the unexpected results from untreated cages,
“If there is a cost associated with resistance, why doesn’t resistance drop over time, and why don’t the resistance alleles drop?”
Implications for Evolutionary Biology
The study also examined the broader genomic effects of pesticide application. Evolutionary changes at one chromosomal location can trigger a ripple effect, known as a selective sweep, affecting other loci on the same chromosome. Karageorgi explains,
“When we applied pesticides, we didn’t just change allele frequencies at the resistance locus – we affected loci all across the chromosome, which danced to the pesticide pulse.”
This research raises foundational questions about how selective pressures impact genomic diversity over time. Petrov emphasizes the challenge of understanding these forces,
“This field is trying to understand what forces are involved in evolution, how you measure them, and how much of an effect they have. But often these forces are hidden from us.”
The findings suggest that dominance reversal could play a significant role in maintaining genetic diversity, potentially setting the levels of genetic diversity in natural populations. As researchers continue to explore these dynamics, the study opens new avenues for understanding the complexities of evolution and adaptation in a rapidly changing world.
