PAPKE, R.S.*; RUTOWSKI, R.L.: Reinforcement in the butterfly, Colias eurytheme.

When selection against hybrids occurs between two closely related species, females are expected to respond by accurately choosing conspecific males, and male secondary sexual characteristics are expected to evolve to clearly reflect species identity. This process, by which pre-zygotic barriers to gene exchange are improved by selection, is known as reinforcement. We tested the hypothesis that reinforcement is occurring in a sympatric population of Colias eurytheme, the orange sulfur butterfly. Colias eurytheme hybridizes at low levels with a sister species, C. philodice across the zone of sympatry. Male C. eurytheme and C. philodice have some pheromones in common and differ in others. However, shared pheromones are found in greater quantities in C. eurytheme. Additionally, male C. eurytheme reflect ultraviolet (UV) light from their wings while male C. philodice do not. Female C. eurytheme are known to use both pheromones and UV to distinguish between the species. To test whether or not reinforcement is occurring in C. eurytheme, we performed mate choice trials in a sympatric population in Chandler, AZ and an allopatric population in Blythe, CA. Results show that sympatric C. eurytheme females mated with conspecific males that 1) have greater quantities of shared pheromones and 2) reflect UV light more brightly, compared to males they rejected. Females in the allopatric population showed no preference with respect to pheromones or UV. When looking at population differences, C. eurytheme males in sympatry had greater quantities of pheromones and had brighter UV reflectance compared to those in allopatry. These data support the hypothesis that reinforcement is occurring in the sympatric population of C. eurytheme.