Intersexual 'Arms Race' and the Evolution of the Sphragis in Pteronymia Butterflies

Abstract

Sexual reproduction is often associated with intra- A nd intersexual conflict, especially in species where females mate multiple times. A strategy that has evolved in males to ensure offspring paternity is the ability to produce a complex, external mating plug called a sphragis. The sphragis has been found in 273 butterfly species; however, little is known about the sphragides of the butterflies in the nymphalid genus Pteronymia. In this study, we describe the sphragides of all sphragis-bearing species in Pteronymia, including the newly discovered sphragides of P. alissa (Hewitson), P. andreas (Weeks), P. ozia (Hewitson), and P. zerlina (Hewitson). Three additional species, P. fulvimargo Butler & Druce, P. oneida (Hewitson), and P. ticida (Hewitson), are found to bear an irregular sphragis-like structure. We use molecular and morphological data from a recent study to construct a phylogeny of species in the genus and examine the number of independent origins of the sphragis. Our ancestral state reconstruction using Bayesian inference suggests that the sphragis evolved three times in Pteronymia, whereas parsimony character optimization performed on a maximum likelihood tree suggests only one origin of this structure. Our data on ancestral state patterns, frequency of incomplete sphragides, and morphology of female external genitalia suggest that sphragis-bearing Pteronymia may be in active intersexual conflict, where females develop strategies to prevent male plugging.