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Researchers conducted experimental hybridization between two Drosophila fly species with different heat tolerances, demonstrating that strong heat stress can drive adaptive introgression of beneficial genetic variants despite long-standing barriers to gene flow between species. Heat-stressed hybrid lines showed approximately double the introgression from the heat-tolerant species compared to control lines, along with improved male fertility under heat stress. While chromosomal inversions on autosomes partially permitted introgression under selection, the X chromosome with overlapping inversions maintained a strong barrier preventing gene flow.
Why it matters
This study reveals that introgression from closely related species can provide populations with rapid access to adaptive genetic variation during climate stress, potentially offering a mechanism for species to survive increasing temperatures and extreme heat events. The findings suggest that hybridization between species could serve as a natural buffer against climate-driven extinctions when populations lack sufficient time for de novo adaptation.
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by Noora Poikela, Rhonda R. Snook, Jonna Kulmuni, Michael G. Ritchie
As global temperatures rise and heat waves become more frequent, populations must adapt rapidly to avoid local extinctions. Adaptive introgression allows species to quickly acquire adaptive genetic material from close relatives. However, certain genomic regions, containing barrier loci, may resist introgression. We experimentally tested whether strong heat stress selection can overcome such barriers and facilitate the transfer of adaptive alleles to improve heat tolerance. We also examined whether chromosomal inversions, which can tightly link adaptive and barrier loci through reduced recombination, promote or prevent introgression. We conducted a hybridize, evolve, and re-sequence experiment by crossing the heat-sensitive D. flavomontana with the more heat-tolerant D. montana, exposing heat-selection lines to transient heat for three generations while maintaining control lines. We also mapped long-term barriers to gene flow between natural populations using a demographically explicit genome scan. The heat-selection lines exhibited approximately twice the introgression and higher male fertility under heat stress compared to control lines and the heat-sensitive parental D. flavomontana. This introgression was primarily located in colinear autosomal regions and correlated with improved heat tolerance. Some introgression occurred also in inverted autosomal regions in the heat-selection lines but not in the control lines. The X chromosome, with three overlapping inversions, resisted introgression in both heat-selection and control lines, thereby maintaining a strong species barrier. Finally, genetic barriers in the control lines significantly overlapped with long-term barriers, but this overlap was less pronounced in the heat-selection lines. Our study demonstrates that strong selection can lead to adaptive introgression despite long-standing genetic barriers. These findings highlight that introgression can serve as a rapid source of adaptive genetic material under stressful thermal conditions, potentially mitigating climate-induced population extinctions.