This week’s BEACON Researchers at Work blog post is by MSU postdoc Liliana Lettieri.
“My what a red throat you have, and such a blue belly! You’ve got some impressive dance moves, and you’ve built a nice nest.”
If stickleback fishes could talk, these are some things that we might overhear females saying to males as they check out prospective mates. As a postdoctoral researcher in Jenny Boughman’s lab, I try to uncover which characteristics, or traits, make males competitive among other males and sexy to females.
Some of these traits are stylized behaviors that are meant to get the attention of females looking for a place to lay their eggs. These behaviors may be under strong sexual selection; choosy females determine who is successful, thereby imparting an evolutionary advantage to males with certain traits. We call traits like these signals; they have evolved to influence the behavior of another individual.
Signals are particularly interesting to me, because they determine the outcome of interactions between organisms when they are communicating to one another. An individuals’ environment is made up of the natural features in its surroundings – what we might call the abiotic components, like the light, temperature, currents, soils and minerals. It is also made up of other organisms, the biota. Both abiotic and biotic interactions may shape how signals are transmitted and received. Signals may be very easy to detect in some environments, but, in other environments, they may not stand out enough against the background. This means that the signal may not have the same behavioral outcome in a different environment, or when the primary receivers of the signal have shifted. Because the environmental background can be so important for signaling, researchers often explore environmental characteristics or constraints when trying understand how signals work. This is especially true when we see rapid or repeated evolution of signals; in these cases similar environments may promote the evolution of similar changes. For instance, in my graduate school research, I discovered that repeated shifts in bold stripe colors among cleaner goby fishes in the Caribbean may be an example of evolved signals that contrast best against a new habitat to signal to predators. These signals may have changed in response to strong selective pressure from the biotic factors in the environment, on a particular background – a great example of natural selection on signal evolution.
So, signals can be under strong natural selection to both minimize risk (avoiding predators) and maximize gain (attracting mates) in the backdrop of the environment. They can also be under strong sexual selection from females of their own species. This takes us back to the sticklebacks! Jason Keagy, another postdoc on team Boughman, and I are particularly interested in uncovering which traits, including signals, lead females to choose males in a novel freshwater environment (lakes formed after worldwide massive deglaciation around 10,000 years ago). Evolutionary biologists would like to know why and how because it may help us to understand the mechanisms underlying evolution with strong selection. Part of the key to uncovering which traits are important may lie in the fact that success can happen in stages, for example 1) establishing a territory, then 2) building a nest, then 3) attracting a female. Different traits may have different costs and benefits at each stage. In order to try to tease apart the importance of male traits in male competition versus female choice, we are using some different density treatments in large outdoor enclosures where males and females interact during the mating season (Figure 1).
Another really amazing part of stickleback evolution is that repeated instances of two species, occurring in pairs, have independently evolved in (estimated) 10,000 year old lakes; the two biologically similar forms that biologists call benthic (living near the bottom of the body of water) and limnetic (living in the open, well-lit areas of the body of water) species pairs occur in many, many lakes around the world. Females of both types in these lakes have strong preferences for suites of traits that help them choose a mate of their own kind. As they swim through breeding grounds, males of both species are building nests and signaling to attract females, who lay their eggs in the nest and leave the father to do all the parental care (Figure 2). Because females’ senses and preferences have also diverged in the different environments in which they live (up in the water column versus down near the vegetation on the bottom), the species pairs have pretty strong reproductive isolation – meaning that they are very unlikely to mate with each other. In trying to understand the mechanisms of evolution in action, we hope to uncover some of the selective agents that drive species divergence. In other words, we want to know why certain traits are particularly good at attracting mates.
We also want to know something about the genetic regions that control these traits, and to do this we use an approach called quantitative trait loci mapping (Fig 3). In order to do this, we have to make hybrids between a species pair. This method allows us to make a genetic mosaic between two genomes, each carrying sets of traits that best attract mates of their own kind. By asking females to choose among males with these patchworks of traits, we can measure the relative success of males with combinations of traits. We can then use genetic markers smattered across the genome to correlate parts of the genetic blueprint with certain traits, as well as with overall reproductive success. If suites of traits and reproductive success (a proxy for fitness) map to the same regions, that will give us an additional clue pointing toward traits that may be of particular importance. We are particularly interested in the potential importance of suites of traits because multiple pairs of limnetics and benthics differ in multiple traits, including shape, color, size, and behavior. Is the evolution of suites of traits, as opposed to just single traits, important for establishing reproductive isolation? We think it might be and we hope to use these sticklebacks to help us answer this question. They can’t talk, but they can certainly tell us a lot about evolution in action!
For more information about Liliana’s work, contact her at lettieri at msu