A shared gene but distinct dynamics regulate mimicry polymorphisms in closely related species

Abstract

Sex-limited polymorphisms, such as mating strategies in male birds and mimicry in female butterflies, are widespread across the tree of life and are frequently adaptive. Considerable work has been done exploring the ecological pressures and evolutionary forces that generate and maintain genetic variation resulting in alternative sex-limited morphs, yet little is known about their molecular and developmental genetic basis. A powerful system to investigate this is Papilio butterflies: within the subgenus Menelaides, multiple closely related species have female-limited mimicry polymorphism, with females developing either derived mimetic or ancestral non-mimetic wing color patterns. While mimetic color patterns are different between species, each polymorphism is controlled by allelic variation of doublesex (dsx). Across several species, we found that the mimetic and non-mimetic females develop male-like color patterns when we knockdown dsx expression, establishing that dsx controls both sexual dimorphism and polymorphism. We also found that mimetic dsx alleles have unique spatiotemporal expression patterns between two species, Papilio lowii and Papilio alphenor. To uncover the downstream genes involved in the color pattern switch between both species, we used RNA-seq in P. lowii and compared the results to previous work in P. alphenor. While some canonical wing patterning genes are differentially expressed in females of both species, the temporal patterns of differential expression are notably different. Our results indicate that, despite the putative ancestral co-option and shared use of dsx among closely related species, the mimicry switch functions through distinct underlying mechanisms. AUTHOR SUMMARYUnderstanding how a largely shared genome can encode the potential to develop multiple morphs while simultaneously restricting this potential to one sex has long been of interest to evolutionary and developmental biologists. This phenomenon, called sex-limited polymorphism, is widespread, occurring in organisms like crustaceans, insects, fish, birds, and mammals. Recent empirical work has begun to identify the genes controlling the switch between phenotypes, but the differences between developmental programs leading to those phenotypes remain unclear. Here we use a classic example of sex-limited polymorphism - female mimicry in swallowtail butterflies - to compare how closely related species have evolved to use the same gene, doublesex, in the development of multiple female morphs. Using a combination of functional experiments, we show that despite the shared use of doublesex, the developmental genetics underlying sex-limited polymorphism have evolved to function quite differently between two species that last shared a common ancestor approximately 15 million years ago.