The Genetics of Speciation

Herbivorous insects that switch to eating a different kind of plant are evolutionary pioneers, adapting their lifestyle and even their bodies to make a living on the terrain of unfamiliar plants. Over generations, their descendants may change enough to form a separate evolutionary unit, diverging from the parent population and becoming a species in their own right. Researchers based in the UK and Germany have investigated the early stages of this process. By studying races of peach aphids (Myzus persicae) which have recently become able to feed on tobacco plants, they uncovered the genetic changes that enabled the insects to switch to a new food.

Plants produce a variety of defensive chemicals to protect themselves from insects that try to eat them. Nicotine is one example. In addition to being an addictive stimulant to mammals, it's also a potent insecticide. Any insects with a hankering for tobacco leaves would have to find an efficient way to detoxify the nicotine in them, and that's precisely what some races of peach aphid have done. Earlier work by some of the researchers found higher levels of the gene CYP6CY3 in a race of aphids that could digest nicotine. In the current study, the team demonstrated that this gene is responsible for the aphids' nicotine resistance. Aphids with higher levels of CYP6CY3 are better at breaking down nicotine, and introducing the gene into fruit flies made them resistant to nicotine as well.

What leads to increased levels of CYP6CY3 in these aphids? The team used a technique called genome walking to discover that the resistant aphids had multiple copies of CYP6CY3 — up to 49-times as many as the nicotine-sensitive races. Even though the extra copies boost CYP6CY3 expression, they're not enough to account for the differences the team found. Having multiple copies helps, but in four of the five races the researchers examined this was further augmented by changes in the regulation of CYP6CY3. The regulatory region of CYP6CY3 includes a stretch of repeating AC bases which is about three times longer in resistant races. By using longer and shorter versions of this region to control a light-producing enzyme in mosquito cells, the researchers showed that the longer version led to higher expression levels.

Such a small genetic change alone isn't enough to create a new species. These peach aphids can now break down nicotine, which enables them to feed on tobacco plants, but it will take the accumulation of many changes in their genome and life history for them to become a distinct entity of their own. Of course, we have no idea if that will ever happen. Gene flow between the populations may keep them from completely diverging, or perhaps the tobacco-eating races will die out for some reason. We can't know what the future will bring for them, but we're lucky enough to live at a time when humans have become able to peek at the genetic changes that can set populations off on their own course and to understand, at a molecular level, the origin of species.

Reference
Bass, C. et al. Gene amplification and microsatellite polymorphism underlie a recent insect host shift. PNAS 110(48):19460-19465 (2013)

Image credit
The peach aphid image by Scott Bauer is from the USDA archive.