Two centuries ago, when entomologists like Carl Fredrik Fallén and later Johan Wilhelm Zetterstedt were busy collecting insects for Lund University’s entomological collections, they were perplexed by the mysterious buzzing emanating from a can of raisins. Fast forward to the present, and the unassuming fruit fly, scientifically known as Drosophila melanogaster, has become one of the most extensively studied creatures on Earth.
Researchers from the University of Wisconsin–Madison and Lund University embarked on a remarkable journey, extracting and scrutinizing DNA from fruit flies preserved in museum collections in Lund, Stockholm, and Copenhagen. These flies were gathered by naturalists in Europe as early as the early 19th century, and some as recently as the 1930s.
The early insect collectors considered any insect they could lay their hands on worth preserving. Fallén’s specimens even include some flies that seem to have taken a liking to his raisins. However, they couldn’t have fathomed the pivotal role Drosophila would play in scientific research.
“This species has been a cornerstone in basic biological science for well over a century,” states John Pool, a genetics professor at UW–Madison. “Researchers have turned to it to unravel fundamental principles of life, explore genetic variations in natural populations, and understand how different evolutionary forces shape diversity, and that’s just in my field.”
Consequently, the genetic makeup of fruit flies may have been analyzed and documented more frequently than that of any other creature. However, these new DNA samples, described in a study published in PLOS Biology, originate from the distant ancestors of the flies currently hovering around our fruit bowls.
“It’s not uncommon to obtain useful DNA from ancient specimens of our human ancestors or other animals,” Pool explains. “But the number of generations—approximately 3,000—that have transpired in fly populations since some of these specimens were alive is roughly equivalent to the number of generations since humans first emerged from Africa.”
Lund zoologist Marcus Stensmyr managed to recover genetic material from the museum flies by immersing them in a solution that ruptures cell membranes, liberating large molecules. Following this process, the flies were carefully washed, dried, and returned to the museum collection. Their DNA was then extracted from the solution and meticulously analyzed at UW–Madison.
Remarkably, the research unveiled that the fruit flies collected in Sweden during the early 1800s bore a greater genetic resemblance to their 21st-century counterparts than to the Swedish samples from the 1930s. This phenomenon can be attributed to the early flies’ position in Drosophila history, as some of the first settlers so far north of their original range in Southern Africa. They represented a small outpost where random mutations had a more pronounced impact on the population, resulting in what’s termed “genetic drift,” as the 1800s transitioned into the 1900s. The genetic distinctiveness of Swedish flies dwindled as they intermingled with the broader European gene pool.
The period between the 1930s and the present saw a significant surge in fruit shipping and increased human transportation, which likely facilitated longer-distance migration of Drosophila, according to Pool. This migration effect appears to have homogenized genetic variation between the 1930s and today.
In their extensive analysis of the fly samples spanning centuries, the researchers pinpointed a few genes that exhibited signs of evolutionary pressure. One of the primary objectives of the study was to identify the genes that played a pivotal role in helping the fly population adapt to new climates and environments.
A notable gene that emerged in the more recent timeframe was Cyp6g1, known for conferring resistance to the pesticide DDT, a development that coincided with the introduction of DDT in the 1940s.
Further back in time, genetic shifts in the 19th century flies revealed the significance of the Ahcy gene in adapting to cooler temperatures and shorter days, critical factors in the fly’s reproductive cycles, particularly in high-latitude regions like Sweden.
Yet another gene, ChKov1, was initially associated with insecticide resistance, but the analysis of DNA from museum flies collected in the 1800s showed that this gene evolved before relevant insecticides were even invented. It was suggested that ChKov1 might also confer resistance to a virus known as sigmavirus, which is believed to have appeared in flies about 200 years ago.
The results strongly support the hypothesis of viral resistance over insecticide resistance. This showcases the valuable insights that can be gained from analyzing temporal samples of genes under natural selection.
This study not only pays tribute to the efforts of pioneering scientists from the past but also highlights how modern technology can be harnessed to explore similar frontiers. It underscores the potential of millions of museum specimens worldwide to shed light on the evolutionary changes in various species, as noted by Pool.
Source: University of Wisconsin-Madison