You may not have chosen your Valentine this week based on their smell or other invisible-to-the-naked-eye molecules their body gives off (at least not consciously), but if you were a moth, airborne chemical signals would be one of the most important determinants of who got your affection. Scientists have long known that so-called pheromones play a role in the reproductive preferences of many creatures in the animal kingdom — most organisms mate with partners who emit pheromones similar to their own. Now, a team of researchers has shown how small genetic mutations altering how moths produce pheromones can change not only a single moth’s behavior, but the entire fate of a species. When the structure of a single protein in moth’s bodies is changed, Lassance et. al. conclude in a new PNAS Early Edition paper, it shifts the balance of what pheromones they produce. This can make them an unattractive mating partner to the rest of the species and, over time, lead to a new population of moths. Those with the new pheromone combination will mate amongst themselves, while those with the original pheromones will prefer each other. This leads to what evolutionary biologists call “reproductive isolation” — a barrier established preventing two populations within a species from mating with each other. It’s the first time a molecular basis for this pheromone-based reproductive isolation has been pinpointed.
The researchers, mostly based in the Lund University lab of Christer Löfstedt, knew that different species of the moth genus Ostrinia had different combinations of pheromones. Within each species, the females emit pheromones that can be sensed from afar. Males follow the pheromone trails to find females of their species. Having unique chemical signatures allows many species of moths to live in overlapping environments without confusing each other’s messages — imagine the chaos if your Valentine’s cards were all unlabeled. The Löfstedt lab, though, wanted to know how each moth had evolved the ability to make a different pheromone.
In 2010, the group had compared the genomes of two Ostrinia moths that produced different pheromones and found differences in a protein required to make the pheromones. In their new work, they analyzed whether this finding represented an isolated case or whether changes to the protein, dubbed pgFAR, could also explain more widespread pheromone differences between Ostrinia species. They looked at the pgFAR protein — as well as the gene that encodes it — in eight different moths this time, and in more depth. Each moth, they found, had different slight variations to the protein. And those differences, the scientists showed through a series of many experiments, are enough to fully explain the variations in each moth species’ pheromone mixture. Changes over time to pgFAR, they concluded (even a change to a single amino acid building block out of more than 400 in the whole protein) can change moths’ pheromones.
Taking the work a step further, the scientists analyzed whether mutations in the pgFAR gene accumulated more quickly than in other genes, which could partially explaining why so many versions of the gene had evolved in different species. In one distinct part of the protein, they found, there was evidence of this “positive selection”, giving more weight to the argument that changes to pgFAR — and therefore to the pheromones — have played a role in the evolution of the Ostrinia moths.
As previous work has established that pheromone changes can lead to the reproductive isolation of one population within a species, and eventual formation of a new species, the new work offers an answer to the genetic driver behind such speciation. The question that remains is how the receptors that males use to sense pheromones change in conjunction with pheromones — if a female moth produces a new pheromone but no males recognize it as desirable, she just might be spending Valentine’s Day alone.