Independent major-effect loci and limited trait associations characterise warning colour variation across sexes and life stages in the wood tiger moth (Arctia plantaginis)

Technical Summary: Genetic Architecture of Warning Colour Variation in Arctia plantaginis

Problem Statement
The maintenance of warning colouration polymorphism remains a central puzzle in evolutionary biology. Despite strong positive frequency-dependent selection, which theoretically favours the most common morph, significant variation in aposematic signals persists across populations. This variation is hypothesized to arise from spatially or temporally variable selection pressures or from genetic associations with other traits (pleiotropy or linkage). The wood tiger moth (Arctia plantaginis) presents a complex case study for this phenomenon, exhibiting distinct sexual dimorphism: males display two discrete colour morphs controlled by a single locus, while females show continuous variation in hindwing colour, and larvae exhibit variation in warning signal size. The study seeks to determine whether these diverse warning signals are maintained through linked genetic architectures (e.g., a "supergene" or co-adapted complex) or through independent genetic mechanisms, and to identify the specific loci governing these traits.

Methodology
The researchers employed a whole-genome sequencing approach on a large sample size of 657 individuals. These individuals were derived from a laboratory stock originating from a wild, polymorphic population of A. plantaginis. The study design focused on mapping the genetic basis of multiple phenotypic traits, including:

• Male colour morphs.
• Female hindwing colouration.
• Larval warning signal size.
• Life-history traits, chemical defence mechanisms, pheromone production, and male activity levels.

The analysis specifically tested for genetic associations between male colour loci and other traits, while simultaneously attempting to identify the genomic regions responsible for female and larval colour variation.

Key Contributions and Results
The study successfully mapped the genetic architecture underlying the species' diverse warning signals, revealing a pattern of largely independent loci rather than a unified genetic complex.

• Distinct Loci for Sex and Life Stage: The research identified separate major-effect loci for female hindwing colour and larval signal size. Crucially, these loci are distinct from the single locus known to control male colour morphs.
• Limited Trait Associations: Contrary to the hypothesis that warning colours are tightly linked to other fitness-related traits, the study found that none of the measured traits (life-history, chemical defence, pheromones, or activity) showed a significant association with the loci controlling warning colours.
• Specific Phenotypic Correlations:
  • Melanism and Activity: Hindwing melanism was the only trait found to be associated with a warning colour locus. It mapped to a nearby major-effect locus and was linked to reduced activity at higher temperatures.
  • Pheromones: Female colour was found to be phenotypically correlated with the amount of pheromone produced, though the genetic basis for this correlation was not established as a direct linkage to the colour locus itself in the context of a supergene.
• Rejection of the Supergene Hypothesis: The data provided no evidence that male colour is part of a "supergene" or a complex co-adapted phenotype involving other traits.

Significance and Claims
The paper concludes that the maintenance of colour polymorphisms in Arctia plantaginis is likely driven by variable selection pressures acting on independent genetic architectures, rather than by genetic associations or pleiotropic effects linking warning colours to other life-history traits. By demonstrating that the genetic bases for male, female, and larval warning signals are distinct and largely unlinked to other phenotypic traits, the study challenges the notion of a tightly integrated, co-adapted phenotype for aposematism in this species. Instead, the results suggest that the persistence of these polymorphisms is a product of selection acting on separate genetic components.