Parallel evolution of industrial melanism in the peppered moth: one locus, many alleles

This study reveals that industrial melanism in the peppered moth evolved independently in Britain and continental Europe via distinct structural variants at the same *ivory* locus, demonstrating that parallel adaptation can arise from multiple different mutations within a single gene.

The Gist

Imagine the peppered moth as a living canvas. For centuries, these moths were mostly pale and speckled, blending perfectly with the lichen-covered trees of Europe. But then, the Industrial Revolution rolled in like a thick, black fog. Factories belched soot, turning the trees charcoal gray and black. Suddenly, the pale moths stood out like sore thumbs, making them easy targets for hungry birds.

In this high-stakes game of hide-and-seek, a new "dark mode" mutation appeared. This is the story of industrial melanism—how moths evolved to turn black to survive.

For a long time, scientists thought this was a simple story: One bad apple, one mutation. In Britain, a single genetic "glitch" (a jumping gene called carb-TE) popped up in one moth near Manchester. It was so powerful that it spread like wildfire, turning the whole British population black. It was a "hard sweep," like a single champion running a race and winning by a huge margin.

But this new paper asks: Did the same thing happen in Europe?

The Great European Mystery

When the soot started covering trees in Germany, Poland, and the Netherlands, the moths there also turned black. But did they get their "black paint" from the British champion moth? Or did they paint themselves black independently?

The researchers acted like detectives, using old museum specimens and modern moths to solve the case. Here is what they found:

1. The "Copy-Paste" Myth is False
They checked the DNA of the European black moths and found they did not have the British mutation. It's as if the British moth tried to send a "Black Moth 1.0" software update to the continent, but the European moths rejected it. Instead, they wrote their own code.

2. Many Roads to the Same Destination
In Europe, the story is much more chaotic and fascinating. Instead of one single mutation taking over, the moths evolved blackness in multiple different ways.

• Think of it like a city trying to fix a pothole. In Britain, one construction crew showed up and fixed it perfectly. In Europe, five different crews showed up from different neighborhoods. Some used a patch, some used a new layer of asphalt, and some used a giant steel plate.
• The result? The road is fixed (the moths are black), but the repair jobs look different under the hood.

3. The "Sollichau" Deletion
The main "hero" in Central and Eastern Europe wasn't a jumping gene like in Britain. It was a deletion—a tiny piece of DNA that got cut out. The researchers named this mutation sollichau.

• Imagine the moth's DNA is a long instruction manual. In Britain, someone accidentally pasted a huge, sticky note (carb-TE) onto the page, which made the moth black. In Europe, someone took a pair of scissors and snipped out a specific sentence (sollichau), which also made the moth black.
• Surprisingly, both the "sticky note" and the "scissors" ended up doing the exact same thing: they turned up the volume on a gene called ivory, which tells the moth to go dark.

4. The "Off Switch" Accident
The most dramatic proof came from a single moth found in the wild. This moth had the sollichau deletion (which should make it black), but it was actually pale (typical).

• Why? Because this moth had a second, accidental mutation right next to it. It had lost the gene for a tiny "dimmer switch" (called mir-193) that usually turns the darkness on.
• It's like having a light switch that is stuck in the "ON" position (the deletion), but someone also ripped the lightbulb out of the socket (the missing dimmer gene). The result? No light. The moth stayed pale. This accident proved that the sollichau deletion really was the cause of the black color.

Why Does This Matter?

This paper teaches us a profound lesson about evolution: Nature is messy and creative.

• The "Hard Sweep" (Britain): When a population is smaller or isolated, one lucky mutation can take over everything. It's a solo victory.
• The "Soft Sweep" (Europe): In a huge, diverse population with many different habitats, nature tries many different solutions at once. If one mutation works, great. If another one pops up nearby that also works, nature uses that too.

The peppered moth didn't just evolve once; it evolved many times in different ways to solve the same problem. It shows that when a species is large and spread out, evolution doesn't need a single "chosen one." It can have a whole team of different heroes, all wearing the same black uniform, just made from different materials.

In short: The British moths found a single key to unlock the door to survival. The European moths found a whole bunch of different keys, and they all unlocked the same door.

Technical Summary

1. Problem Statement

The paper addresses a central question in evolutionary biology: does adaptation to rapid environmental change (specifically industrial pollution) occur via a single advantageous mutation (a "hard sweep") or via multiple independent mutations (a "soft sweep")?

• Context: The peppered moth (Biston betularia) is a textbook example of natural selection. In 19th-century Britain, a single melanic allele (carbonaria), caused by a transposable element (TE) insertion in the ivory gene region, swept through the population due to industrial soot.
• The Gap: While the British case is well-characterized as a single-origin hard sweep, the genetic basis and evolutionary dynamics of industrial melanism in continental Europe remain unclear. Historical records suggest the black morph appeared in multiple European locations independently, raising the question of whether this represents a single migration event or parallel evolution via distinct genetic origins.

2. Methodology

The authors employed a multi-faceted approach combining historical ecology, population genomics, and functional genetics:

• Historical & Ecological Analysis:
  • Compiled an expanded dataset of first records of melanic moths in continental Europe (1867–1963) and Britain.
  • Modeled the spread rate of the melanic form using diffusion approximation, comparing observed spread rates against pollution data (black carbon emissions and $SO_2$ concentrations).
• Genomic Sequencing & Pangenome Construction:
  • Sample Collection: Analyzed 637 modern wild-caught moths (2000–2014) and 137 museum specimens (1893–1981) from Western, Central, and Eastern Europe.
  • Long-Read Sequencing: Generated PacBio HiFi reads for 8 melanic individuals to assemble 16 complete haplotypes.
  • Pangenome Graph: Constructed a graph-based pangenome (using Minigraph-Cactus) incorporating these haplotypes and the British reference genome to capture structural variation (SVs) and SNPs.
  • Genotyping: Used a k-mer based inference method (PanGenie) to genotype 800 samples (including museum DNA) against the pangenome, identifying 1.9M loci.
• Linkage Mapping & Association Studies:
  • Performed linkage mapping using pedigrees derived from crosses between European melanic males and British typica females.
  • Conducted genome-wide association studies (GWAS) contrasting melanic morphs (D–G) against non-melanic forms (A–B).
• Functional Validation:
  • Expression Analysis: Measured expression levels of the long non-coding RNA ivory and its effector microRNA mir-193 in pupal wing discs using RT-qPCR.
  • Rescue Experiment: Investigated a specific "typica" individual that carried a melanic-associated deletion but displayed a light phenotype to understand the regulatory mechanism.

3. Key Contributions

• Discovery of Multiple Origins: Demonstrated that unlike the single-origin hard sweep in Britain, continental European industrial melanism is a soft sweep driven by multiple independent mutations.
• Identification of Novel Alleles: Identified several distinct structural variants (SVs) in the ivory/cortex region associated with melanism in Europe, including:
  • Sollichau: An 805 bp deletion, identified as the primary driver of melanism in Central and Eastern Europe.
  • Transposable Elements: Multiple independent TE insertions (LINE/L2, LTR/Pao, PiggyBAC) distinct from the British carb-TE.
• Mechanistic Insight: Established that contrasting structural variants (a large insertion in Britain vs. a deletion in Europe) converge on the same regulatory mechanism: elevated expression of ivory and mir-193.
• Genetic Rescue Case: Provided serendipitous functional proof of the sollichau mechanism by identifying a typica individual carrying the sollichau deletion but rescued by a linked deletion of mir-193, which cancels the melanizing effect.

4. Key Results

• Spread Dynamics: The melanic form spread significantly faster in continental Europe (approx. 8 km/year) than in Britain (3 km/year). The high rate and spatial distribution of early records in Europe strongly support multiple independent origins rather than a single migration from Britain.
• Genetic Architecture:
  • All melanic forms map to the same genomic region (ivory/cortex on chromosome 17), confirming a single locus of adaptation.
  • However, the specific alleles differ. The British carb-TE insertion is absent in European samples.
  • European melanism is polyallelic. The sollichau deletion is the dominant allele in Central/Eastern Europe, while other SVs (including TEs) contribute to variation in Western Europe.
• Expression Patterns:
  • Melanic individuals (both carb-TE and sollichau carriers) show significantly elevated expression of ivory and mir-193 in early pupal stages compared to typica.
  • The sollichau deletion (805 bp) and the carb-TE insertion (21 kb) have equivalent regulatory effects despite being opposite structural changes (deletion vs. insertion).
• The "Rescue" Individual: A wild-caught typica moth (G7-2013-13) was found to be heterozygous for the sollichau deletion but also heterozygous for a 1,675 bp deletion removing mir-193. Because mir-193 is the effector of melanism, its loss suppressed the melanizing effect of sollichau, resulting in a light phenotype. This confirms mir-193 as the functional target.

5. Significance

• Evolutionary Theory: The study challenges the notion that adaptation always follows a simple "hard sweep" model. It illustrates that in large populations with heterogeneous habitats (like continental Europe), multiple mutations can arise and spread simultaneously (soft sweep), even for the same adaptive trait.
• Convergent Evolution: It highlights how different structural variants (insertions vs. deletions) can target the same regulatory node (ivory/mir-193) to produce the same phenotype, demonstrating the flexibility of evolutionary paths.
• Methodological Advance: The successful application of pangenome graphs and long-read sequencing to museum specimens allows for the detection of complex structural variants that are often missed in standard short-read reference-based analyses.
• Ecological Context: The findings suggest that the effective population size ($N_e$) and the spatial scale of environmental change are critical determinants of whether adaptation proceeds via a single mutation or multiple parallel origins. The larger, more fragmented European population allowed for multiple independent mutational events, whereas the British population experienced a classic hard sweep.