When fruit flies encounter a temperature shock, the gene-regulating responses involved in their adaptation can be passed down to at least four subsequent generations, even after temperatures return to normal. Researchers who identified this pattern believe it likely extends to other animals as well. The study’s findings have been published in the journal Molecular Biology and Evolution.
Environmental shifts invariably benefit those members of a population possessing traits suited to the new conditions, and heat tolerance is no exception. These advantageous genes are then more likely to be inherited by succeeding offspring, representing a classic intersection of Darwinian selection and Mendelian inheritance. However, adaptation can proceed at an accelerated pace when epigenetics facilitates the activation of genes that are currently beneficial while repressing those that are no longer advantageous.
To explore how these general concepts manifest in practice under hotter climates, female Drosophila melanogaster fruit flies were collected in both Finland and Spain, with the intent of acclimating them to vastly differing thermal environments. The investigators subjected their specimens to laboratory heat shocks and subsequently examined the ramifications for their genetics, the viability rate of laid eggs, and the developmental duration of the progeny.
It is known that all living organisms produce molecules called heat shock proteins (Hsp) following exposure to extremely high temperatures, so an expected outcome was an increase in these molecular levels. Nevertheless, the genes governing this process are quite diverse, and the researchers aimed to gain a more granular insight into how the flies’ systems would react overall.
Genes that conferred better coping mechanisms for elevated temperatures were observed in both populations following the heat shock, yet the Spanish flies exhibited a more structured and effective response. The Finnish flies displayed potentially helpful genes, but their regulatory method was less efficient—akin to a person unfamiliar with extreme heat who reads advice like “drink water” but lacks the precision to know the exact required amount.
Eggs laid within two days post-shock showed reduced viability and matured more quickly; furthermore, in the dry conditions, eggs deposited later by the flies in that cohort also developed faster. The authors speculate this might be an evolutionarily honed mechanism facilitating swift movement before the return of intense heat.
Since the flies mature and reproduce rapidly, the research team could track the descendants of the heat-exposed individuals across multiple generations and contrast them with a control group. Each successive generation displayed fewer genetic divergences when compared to the control group than the one preceding it, as the heat exposure was not repeated, yet variations persisted concerning the expression of 23 specific genes.
These extended repercussions were more pronounced in the flies of Spanish descent, which continued to develop faster four generations later—presumably because this proved to be an effective antidote to the thermal stressors that the Finnish population had not previously contended with.
If a single shock event occurring several generations ago could instigate such disparity in distant descendants, then repeated occurrences linked to a warmer world might indeed trigger rapid evolutionary shifts. Eventually, those lineages experiencing more frequent shocks might find interbreeding with peers from more climatically stable regions challenging.
Given that outcomes varied even within the same species, this work cannot definitively predict how any specific creature, particularly a vertebrate with diverse ancestry, will adjust to an increasingly warm world. Nevertheless, lead author Dr. Evan Hadfield from the University of Liverpool suggests there are insights to be gained. By pinpointing the precise genes whose expression remained altered and the mechanisms behind some of these changes, the team has provided other researchers studying different animals with potential avenues for further investigation.
“The transgenerational effects we observed in gene expression and developmental timing reveal that stress can not only facilitate the selection of better-adapted flies but can also drive evolution,” Hadfield stated in a release. “Understanding why some gene variants might respond better to these transgenerational shifts than others could be crucial in flagging populations at risk as Earth’s climate inexorably changes.”
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