Interspecific differences in second-site modifiers reshape dominance in a Mendelian trait by altering environmental plasticity

In a study of the Drosophila montium species subgroup, researchers discovered that dominance in a Mendelian trait can be influenced by environmental factors and second-site modifiers. Traditionally, dominance has been thought to be determined by the alleles at the focal locus, but this study challenges that notion. The trait being examined is female pigmentation, controlled by a single gene, POU domain motif 3 (pdm3).

However, the dominance relationships between pdm3 alleles vary among the three closely related species studied. By conducting reciprocal introgressions between these species, the researchers were able to compare females with identical pdm3 heterozygote combinations but in different genomic backgrounds and temperatures. They found that altering the genomic background significantly affected the phenotype of these heterozygotes, indicating the presence of second-site modifiers of dominance.

Moreover, the impact of these modifiers was found to depend on the specific pdm3 allele pair and the temperature at which the flies were reared. These findings suggest that dominance in this system is not a fixed characteristic of individual alleles but rather a dynamic trait influenced by the genotype, environment, and developmental processes.

The researchers conclude that second-site modifiers can reshape reaction norms, providing a potential mechanism through which the evolution of dominance may be linked to phenotypic plasticity.