No man is an island. Similarly, no non-human animal can function alone in the world without interacting with other organisms, be they other animals, plants or bacteria. However, the degree to which animals interact with each other varies, and so the evolutionary pressures upon them from that interaction will vary. To explain, let’s think about a fictional predator: it is an omnivorous mammal with blue fur. It was named the yellow drax by a scientist who thought he was being funny, but a name that has confused scientists for years since then and lead to many heated arguments at the annual drax conference. Now, the drax eats two types of prey: a micken – a creature somewhat like a mouse, but with a head that looks oddly chicken-like. It lives in grass land, and often wonders why it was equipped with such a weird head. It also eats a blizzard, an animal somewhat like a lizard, only skinner and with the ability to give people who look at it an uncomfortable cold feeling. The drax will eat the odd blizzard, if they happen to blow past its path, but really they’re not too fussed about these skinny little creatures. No, most of the drax’s diet consists of mickens, and without this odd-looking energy source, the drax would surely die. For this reason, most of the adaptations the drax has are for catching the fast-running mickens: strong legs, light weight and a stream-lined body shape. The reason for this is because any mutations that arose in the genes of drax offspring that offered any physical advantages towards being able to catch mickens were strongly selected for: with the ability to catch more prey, those individuals lived longer, were larger and stronger, and thus went on to bear many more drax with the same genes and physical traits. On the other hand, any drax that had random mutations that offered advantages to catching blizzards, in-built earmuffs for example, might have been better able to catch these flighty reptiles, but as they are so low in calories anyway, it never did much good and so the genes weren’t passed on. Because the drax is so reliant on the micken there is strong selection on it to be better at catching it. Conversely, as the micken’s main predator is the drax, any individuals that have mutations that make them better at escaping the drax are more likely to survive, reproduce, and pass on their superior genes. This intimate interaction between these two creatures leads them becoming locked in an arms race: over millions of years of evolution the drax became faster, lighter and more stream-lined (until it’s face eventually came to resemble a kayak). In turn, the micken also became faster, better at darting around and better able to smell an approaching drax. Of course, there is an upper limit to how fast or light an animal can be, at which point any other traits that give one animal an edge over the other will be selected for.
This fictional evolutionary arms race is present in many real-life organisms. Selection can act on many different capabilities of the animal: physical, cognitive and sensory. However, although in many cases it might seem easy enough to guess which traits of an animal may have been selected for in an arms race, it can be harder to prove, and even harder to find the underlying genes for those traits.
One new study, however, has done just this. Briscoe et al. used Heliconius melpomene butterflies to look at their evolutionary interactions with a plant, the passion flower Passiflora. These two organisms have a close relationship, as female butterflies lay their eggs on the passion flower, where they hatch into caterpillars that then live on the plants, eating away at them until they weave their cocoons. The adults also feed on the plants, eating their nectar and pollen. We might expect that there would be strong selection on the plant to evolve whatever it can to try to stop the caterpillar eating it. In response, we would expect selection on the caterpillar to evolve ways of getting around the defences of the plant, which in turn would put pressure on the plant to develop new defence strategies. For adult butterflies, we might expect that there would be selection on their ability to see, taste and smell which plants would be the best to find food and to lay their eggs on.
Butterflies and moths, like many insects, have special hairs (called gustatory sensilla) that they can use to taste, not only near their mouths, but also on their antennae, legs and ovipositors (where the eggs come out from). In this species of butterfly, the females have sensilla on their legs, while the males do not. Each of these sensilla has five receptors that actually do the detecting of different tastes.
Although the genes and chemistry the host plants use to protect themselves from insects eating them have already been looked at, very little is known about the other side of this arms race: the butterfly’s adaptations. To learn more about this, the scientists looked both at the genomes of species of Heliconius butterflies and at those of other species of butterfly to see how the genes that code for receptors that detect taste and smell might differ between the different species.
What the researchers found was that in all the butterfly groups there has been more adaptive changes in the parts of the genome that relate to the gustatory and olfactory receptors that are used in detecting the host plants. In the Heliconius group specifically these genes related to tasting and smelling are expressed (switched on) more often in females than males. This fits with what we know about this species, and the fact that only the females have sensilla on their legs. Using the receptors on these sensilla, the females will be able to detect both plant attractants (i.e. nice smells) and toxic chemicals given out by the plants, allowing these butterflies to choose the most suitable plants to lay their eggs on.
From looking at the genome, the scientists could see precisely what went on in the genes to result in this evolved ability in female Heliconius butterlies: a chance event where a gene copies itself, so-called ‘gene-duplication’.
This study has shown how looking at an animal’s genome can give us another level of understanding about how it evolved and its behaviour. With the technology available today, scientists can use databases of an animal’s genes (in this case the Heliconius genome was already sequenced in 2012), as well as looking at how genes are switched on in different parts of the body. This ‘next generation sequencing’ has been available since 2005 and is becoming increasingly accessible to biologists, giving us new levels of understanding. In this study, it allowed the scientists to discover that the butterfly female behaviour was actually driving the evolution of the taste receptors on the sensilla on their legs, due to the pressure for them to find the best plants to lay their eggs on.
Below is an entertaining and informative cartoon summarising this experiment
Thanks to Gil Smith for bringing this article to my attention
For more information on this remarkable butterfly see the Heloconius homepage
References
Briscoe AD, Macias-Munoz A, Kozak KM, Walters JR, Yuan F, et al. (2013) Female Behaviour Drives Expression and Evolution of Gustatory Receptors in Butterflies. PLoS Genet 9(7): e1003620.doi:10.1371/journal.pgen.1003620
Heliconius Genome Consortium (2012) Butterfly genome reveals promiscuous exchange of mimicry adaptations among species. Nature 487: 94–98.
Photo credits
Heliconius: Wildcat Dunny
Heliconius with pollen: Chris Jiggins
cartoon: Jay Hosler
