Kathy Darragh, PhD student in the Department of Zoology, writes:
Due to the visual nature of humans, when we think of communication in nature, we tend to focus on things we can see. In many groups, however, other types of signals, such as chemicals, are the main form of communication. These chemical signals are harder to detect, and therefore to study, meaning they have received less attention than their visual counterparts. Both plants and insects are known to rely heavily of chemical signalling, to signal both among and between each other.
Interestingly, plants and insects often use the same compounds for communication. Insects are thought to use plant-like pheromones as they already have the ability to detect these compounds for plant-finding. The evolution of similarities between species is known as convergence. An interesting aspect of this evolutionary phenomenon is its genetic basis, in particular, whether different species evolve similarities through shared or different genetic and molecular mechanisms. In other words, do they produce the compounds in the same way, or have they independently evolved different ways to produce the same chemical compound. In fact, for a long time it was thought that insects sequestered compounds from plants rather than synthesising the compounds themselves.
One example of convergence between plants and animals is a compound known as β-ocimene, found both in floral scents, and in the genitals of male Heliconius butterflies. This molecule belongs to a large family of compounds known as terpenes, which are very common and diverse in plants but rare in animals. Indeed, it was thought that animals were unable to synthesise terpenes; but recently it has been shown that a few insect species can perform this trick.
In the butterfly Heliconius melpomene, β-ocimene acts an anti-aphrodisiac pheromone, transferred from males to females during mating to repel further courtship attempts from other competing males. Not all Heliconius butterflies, however, produce β-ocimene. Heliconius cydno, a species closely related to H. melpomene, does not produce β-ocimene. During my PhD, we used these two species to study the genetic basis of β-ocimene production.
The first step of the project began during the first field season of my PhD at the Smithsonian Tropical Research Institute (STRI) in Panama. Here we have access to insectaries in a small town on the Panama Canal called Gamboa. By collecting butterflies and host plants from the local area, it is possible to rear captive populations of butterflies. We started by crossing the two species of butterfly to create hybrids. We then analysed both the pheromones produced by these hybrids and gathered genetic information by sequencing their genomes (the complete set of DNA of an organism). This allowed us to carry out a linkage analysis, asking which sections of the genome are associated with pheromone production. Once we had identified potential regions of the genome involved in β-ocimene production, we searched these regions for candidate genes. Genes are the sections of the genome which encode protein products such as enzymes. Anna Orteu, another PhD student at the University of Cambridge, had the task of analysing candidate gene expression data. This sort of data can tell us which genes are “turned on” in certain tissue types of an organism, and therefore where they might be active. In this case, we wanted to determine which genes are expressed in the abdomen of male H. melpomene, where the pheromone is expressed. After narrowing down the candidates, we expressed the genes in bacterial cells using the laboratory facilities in the Department of Zoology. As bacteria use the same genetic “code” as insects, it is possible for them to make proteins using genes from another organism, in this case the Heliconius butterflies. By producing the proteins, we could then test to see if they were capable of producing β-ocimene when supplied with the correct chemical environment. Finally, with Daniella Black, an undergraduate student at the University of St. Andrews, we investigated the evolutionary history of the candidate genes.
We identified a novel gene, HMELOS, which produces β-ocimene in H. melpomene but not H. cydno. Interestingly, this gene is not related to the plant genes known to produce β-ocimene. The production of β-ocimene has independently evolved via different genes in plants and butterflies, demonstrating how different molecular mechanisms can underly the production of a specific chemical compound. The independent evolution of the same trait, in this case pheromone production, multiple time, provides a great system to help us understand evolution. If we can identify the genetic basis of the trait in the different systems and ask which genes are involved, and what types of genetic changes underly the evolution, we can begin to understand how predictable these changes could be.