Molecular underpinnings of plasticity and supergene-mediated polymorphism in fire ant queens

Characterizing molecular underpinnings of plastic traits and balanced polymorphisms represent 2 important goals of evolutionary biology. Fire ant gynes (pre-reproductive queens) provide an ideal system to study potential links between these phenomena because they exhibit both supergene-mediated polymorphism and nutritional plasticity in weight and colony-founding behaviour. Gynes with the inversion supergene haplotype are lightweight and depend on existing workers to initiate reproduction. Gynes with only the ancestral, non-inverted gene arrangement accumulate more nutrient reserves as adults and, in a distinct colony-founding behaviour, initiate reproduction without help from workers. However, when such gynes overwinter in the natal nest they develop an environmentally induced lightweight phenotype and colony-founding behaviour, similar to gynes with the inversion haplotype that have not overwintered. To evaluate the extent of shared mechanisms between plasticity and balanced polymorphism in fire ant gyne traits, we assessed whether genes with expression variation linked to overwintering plasticity may be affected by the evolutionary divergence between supergene haplotypes. To do so, we first compared transcriptional profiles of brains and ovaries from overwintered and non-overwintered gynes to identify plasticity-associated genes. These genes were enriched for metabolic and behavioural functions. Next, we compared plasticity-associated genes to those differentially expressed by supergene genotype, revealing a significant overlap of the 2 sets in ovarian tissues. We also identified sequence substitutions between supergene variants of multiple plasticity-associated genes, consistent with a scenario in which an ancestrally plastic phenotype responsive to an environmental condition became increasingly genetically regulated.