Happy Tuesday, everyone! As promised, here we are with a second Biology-based post. This time it’s an evolutionary biology topic.
Before we get into the details, here’s a little bit of background into the molecules I’ll be discussing – hormones. Hormones are a wide-ranging set of molecules released internally by organisms. Molecules called receptors bind to hormones, eliciting physical responses. These reactions range from changes in mood and appetite to sexual arousal. Specific receptors exist for specific hormones – they’re very much like a set of unique locks and keys – so receptors can’t bind to any other type of molecule. Remember that fact – it’s going to be important! A pheromone is simply a hormone that is released externally in order to act upon receptors in another organism.
Many organisms that reproduce sexually rely on the release sex pheromones to attract a mate. Throughout nature, there exists a massive array of sex pheromones, most of which are unique to the species that produce them. It has long been thought that receptors prevent alternative, mutant, pheromones from being passed onto the next generation by discriminating against them and only binding to the ‘normal’ pheromone. If they don’t attract a mate then the producer’s genetic material can’t be passed to an offspring. However, if this is the case, as time moves on and new species evolve, how have pheromones evolved and diversified with them? A group combining American and German scientists have used a species of wasp to investigate this.
Males of the wasp species, Nasonia vitripennis, attract virgin females by releasing a sex pheromone consisting of 3 molecules. The sex pheromones of closely related wasp species consist of only 2 of these molecules. The research group used a 2-stage investigation to find out how the 3rd molecule – a rearranged version (or ‘isomer’) of one of the 2 ‘original’ molecules – evolved in N. vitripennis.
First, they presented virgin females from N. vitripennis and one of its close relatives with various ‘scents’. These included their natural pheromones (each of which also attract members of the other species) and various combinations of the individual molecules from which the pheromones are made. The group compared which scents attracted females of each species most effectively.
They found that both species were attracted to the 3-molecule pheromone, but only N. vitripennis favoured it over the 2-molecule version. However, the 3rd molecule of N. vitripennis’ pheromone was not enough to attract females on its own. This all suggests that N. vitripennis’ relative just didn’t recognise the 3rd molecule – it certainly didn’t discriminate against it. It’s possible that this is what first happened in N. vitripennis – the 3-molecule mutation appeared, but didn’t affect pheromone activity, and the females evolved a preference to it afterwards. This is as opposed to the pheromone evolving to suit mutated receptors, as has long been thought to be the case.
The second stage of the group’s work involved working out how N. vitripennis’ evolution came about at a genetic level. They started by comparing the genomes of the 2 species used above and identifying which genomic regions were always the same in wasps that produced 3-molecule pheromones. The genes in these regions would likely be involved in ensuring that the 3rd molecule was produced. They eventually narrowed the search down to 3 near-identical genes in 1 region, all with the potential to be involved – they all rearrange molecules to produce stereoisomers.
By knocking down combinations of these 3 genes, the group managed to reduce the number of N. vitripennis wasps producing 3-molecule sex pheromones and increase the number producing 2-molecule versions. This means that at least 1 of these 3 genes is responsible for rearranging one of the 2 ‘original’ pheromone molecules to produce the 3rd.
Taken together, these 2 stages are an interesting first step towards explaining how sex pheromones could have evolved into the widely ranging set seen in nature today. A very slightly different copy of part of the pheromone is created, which doesn’t interfere with the existing components but is close enough for receptors to evolve to accommodate it relatively simply. Obviously, this will not be the case in all species but it shows how genetically simple the changes can be, explaining why so many have occurred.
Based on: Niehuis et al. (2013). Behavioural and genetic analyses of Nasonia shed light on the evolution of sex pheromones. Nature 494 345-348.