Pheromonal Mechanisms of Reproductive Isolation in Xiphophorus and Their Hybrids

Abstract

Pheromones play an important role in conspecific mate preference across taxa. While the mechanisms underlying the pheromonal basis of reproductive isolation are well characterized in insects, we know far less about the mechanisms underlying the production and reception of chemical signals in vertebrates. In the genus Xiphophorus, conspecific mate recognition depends on female perception of male urine-borne pheromones. I focused on interspecific differences between the sympatric X. birchmanni and X. malinche, which form natural hybrid zones as a consequence of changes in water chemistry. First, I identify the organ of pheromone production and compounds comprising chemical signals. I localized pheromone production to the testis; testis extract elicited the same conspecific preference as signals generated by displaying males. I used solid phase extraction (SPE) in combination with high performance liquid chromatography (HPLC)/ mass spectrometry (MS) to characterize pheromone chemical composition. Analyzing HPLC/MS readouts for pure peaks with high relative intensity identified two compounds of interest, which were identified according to their fraction pattern and retention times and then individually assayed for their effect on female behavior. The ability to directly measure the pheromones with paired responses of female conspecific mate recognition gives insight into what specific components are important to female mate choice. Elucidating the chemical composition of Xiphophorus signals sheds light into how communication acts as a reproductive barrier between species and how its breakdown facilitates hybridization. Next, I characterize intraspecific variation in pheromone signals. Understanding the relationship between a quantifiable male pheromone profile and measurable female response provides unique insight into female mate choice. I examined the variation in male morphology in X. birchmanni, and used SPE to measure changes in pheromone structure in relation to distinct morphometric traits. Lastly, I evaluate the relationship of male pheromone phenotype to population substructure. If pheromones play a role in reproductive isolation, pheromone profiles should map on to male genotype morphology. Hybrid zones vary from highly structured, with distinct birchmanni-like and malinche-like subpopulations, to highly admixed hybrid swarms. I measured pheromone profiles for individual males, I show the relationship between male morphology, pheromone profile and population structure.