In this project, I investigated the genetic mechanisms affecting the evolution of the most common mode of sexual communication behavior: chemical communication through sex pheromone signals. In the animal kingdom, sexual signals are highly variable among species. Sexual signal divergence is also an important reproductive barrier between species, because males and females typically only mate when they are reciprocally attracted to each other’s signals. It is therefore important to understand how sexual signals evolve and how they contribute to genetic barriers that maintain species diversity. Moreover, sexual signals are an important target for sustainable pest management. In moths, the female emits a long-distance pheromone signal as she calls for male mates. The moth species studied here belongs to the global pest lineage of the Noctuid moths. Knowledge about pheromone signaling in this lineage is exploited to develop traps that catch males searching for females in agricultural crop fields. This prevents males to find and mate females, ultimately preventing the development of the crop-eating caterpillars.
Sex pheromones are species-specific blends of various chemical components. Like most signals, they thus consist of multiple traits and can evolve in multiple directions. Furthermore, the evolution of one such pheromone component is not independent of the evolution of other components, because they are often biochemically and genetically linked. The aim of this project was to identify the genetic basis of variation in one group of pheromone components, the acetate esters, and to unravel the consequences of selection for acetate ester levels on the other pheromone components. Acetate esters serve a dual function in nature: they help H. subflexa females attract the males of its own species, while deterring the males of the closely related generalist crop pest, H. virescens.
Together with other lab members, I first investigated the relationship between sex pheromone and fitness. We tracked sex pheromone calling behavior, composition of the pheromone and reproductive output of H. subflexa females throughout their entire life (2 – 3 weeks in the lab). We found that variation in the sex pheromone was predictive of her fitness (life span and reproductive output). This was true both for variation among females and for variation in the signal during the life time of individual females. We concluded that pheromone variation is dependent on fitness, which had not been shown in such detail before in moths. These findings show that changes in the signal do not only affect the potential to attract a mate, they also have fitness consequences, resulting in trade-offs between sex pheromone and life history evolution. Furthermore, stemming from the many genetic factors that contribute to fitness, it is likely that the correlation found in this study is indicative of a shared genetic basis between fitness and sex pheromone variation. This correlation can help maintain genetic variance in pheromone traits under selection and thus facilitate evolutionary responses.
Together with two other members of the host lab, I initiated selection lines to investigate how acetate levels of the sex pheromone would evolve in response to selection. I also asked what the consequences of that selection pressure was on the other pheromone components, as well as on the genetic variation for acetate esters and other pheromone components. For ten generations we selected for increasingly higher or lower levels of acetate esters. We tracked the evolution of acetate esters and other pheromone components. Because we had the complete pedigree of all individuals that were part of the selection lines, we could use the information about relatedness to quantify the amount of genetic variance underlying pheromone traits at different time points during the 10 generations. Data analysis is still in progress, but these data show that acetate ester evolution is mostly unconstrained by other components in the pheromone despite significant correlations among the components. This somewhat surprising result underlines the evolvability of these signals, which helps us understand their diversity in nature.
Currently, I am awaiting genomic sequence data to help identify the genetic loci that contribute to the selection response observed in our experimental evolution approach. I crossed the final generations of the high and low selection lines. Accumulated differences between the lines could now segregate freely. Tracking these segregation patterns in both the sexual signal and in genetic markers obtained by sequencing the moths’ genomes will help pinpoint the locations in the genome that contribute to differentiation between the lines. The outcomes of this quantitative trait locus (QTL) analysis are expected later this year. Combining information about genomic locations with the known location of genes for which the function has been described in related organisms, such as the silk worm Bombyx mori, will help me identify candidate loci for the observed evolutionary responses.
