Image: journals.plos.org · rights & removal

Adaptive introgression between two Drosophila species enhances heat tolerance despite barriers to gene flow

Reporting by PLOS BiologyRead the original at journals.plos.org

Executive Summary

Strong heat stress selection, applied over three generations during a hybridization experiment between heat-sensitive *Drosophila flavomontana* and more heat-tolerant *Drosophila montana*, facilitated the transfer of adaptive alleles. The heat-selection lines exhibited approximately twice the male fertility compared to control lines and 1.5 times greater fertility than the heat-sensitive parental species. This enhanced fertility correlated significantly with increased introgression from *D. montana* in colinear autosomal regions, suggesting that introgression provided a source of adaptive genetic material under thermal stress. However, this adaptive introgression was spatially restricted: while it occurred in some inverted autosomal regions, the X chromosome, featuring three overlapping inversions, resisted introgression due to strong long-term barriers. Overall, heat selection appeared capable of mitigating contemporary genetic barriers, particularly in colinear autosomal regions, though significant long-term barriers persisted across many genomic regions.

Facts Only

*Drosophila flavomontana* females were crossed with *D. montana* males to initiate the HER experiment.
Males of *D. flavomontana* exhibited reduced fertility with increasing temperatures (50%–80% sterility at 31–32 °C), while *D. montana* males showed higher fertility at these temperatures.
The HER experiment involved backcrossing F1 females to *D. flavomontana* males, creating BC1 hybrids that were subjected to three generations of heat selection (31 °C, 31.5 °C, and 32 °C) or control conditions.
Heat-selection lines exhibited, on average, 2.2 times greater male fertility than control lines after exposure to 32 °C.
Hybrid indices (HIs) indicated that heat-selection lines had a mean HI of 5.6% compared to 2.9% in control lines.
Introgression was most prevalent in colinear autosomal regions, where the proportion of SNPs with no introgression (HI=0) was significantly lower in heat-selection lines than in control lines.
The X chromosome with three overlapping inversions resisted introgression in both treatments.
Long-term barriers to gene flow were mapped using demographic modeling and genome scans, identifying significant barriers across 55.0% of the genome, with strong barriers concentrated on autosomal inverted regions (85.2%) and X chromosomal inverted regions (73.8%).

Full Take

The finding that strong selection can overcome contemporary genetic barriers—demonstrated by increased adaptive introgression during heat stress—suggests that evolutionary rescue via introgression is a potent mechanism when environmental pressures are severe. The differential effect of selection across genomic architectures, specifically the stronger outcome in colinear regions versus inverted regions, implies that recombination constraints imposed by inversions fundamentally influence the feasibility of adaptive gene flow. The pattern where contemporary barriers overlap with long-term demographic barriers suggests that divergence and subsequent barriers to gene flow have established a historical context that is partially resistant to rapid evolutionary change, yet the acute pressure of selection can override this inertia in specific genomic contexts. The impermeability of the X chromosome suggests sex-linked constraints are critical in maintaining species integrity across these barriers. This forces a deeper consideration of how different modes of genetic organization—colinear versus inverted regions—mediate the interaction between historical divergence and current environmental selection pressures on adaptive evolution.

From the original · PLOS Biology

This is an uncorrected proof. Figures Abstract As global temperatures rise and heat waves become more frequent, populations must adapt rapidly to avoid local extinctions.

Read the full story at journals.plos.org

Sentinel — Human

Confidence

The study investigates long-term barriers to gene flow between *Drosophila melanogaster* and *Drosophila montana* by combining genomic divergence estimates (using gIMble) with empirical evidence from laboratory crosses and heat selection experiments. **Key Steps and Findings:** 1. **Estimating Divergence Time ($T$):** The study first estimates the time since divergence ($T$) using mutation rates ($\mu = 2.8 \times 10^{-9}$ per site per generation) and divergence equations, assuming one generation per year. 2. **Identifying Genomic Barriers (gIMble):** The gIMble method is used in