13.6.1 Prerequisites determine the environment

The environment can be considered as a set of prerequisites. An animal performs at its best if it possesses all prerequisites for that environment. If some are missing, the performance is reduced. Each environment has its own set of prerequisites. However, some environments are so much alike that animals can use the same set of prerequisites. The more different two environments are, the more important it becomes to have the specific prerequisites to manage in either of the environments. Sometimes, the prerequisites are even adverse: if you have the one, you can’t have the other. For example, if you have a thick fur to tolerate cold, you can’t stand the heat. These adverse prerequisites are called trade-offs. An animal can be capable of handling one prerequisite, for example digesting low quality feed, but that requires a type of digestion physiology which makes it impossible to grow very fast when the feed quality is good. You can be good at one thing, but it automatically means you are not so good at something else. Often meeting a prerequisite in one environment only causes a small negative effect on performance in the other environment. The opposite can be much more of a problem: not being resistant to an infection is quite problematic in an environment with that infection.

The size of genotype by environment interaction

To get insight in the size of GxE you can plot the performance of different genotypes across multiple environments in a graph. On the x-axe of such a graph is the environmental gradient, for example temperature, or protein content in the diet, or some other component in the environment of the animals. On the y-axe is the performance of the animals in the environments. The resulting plot is called a reaction norm. The slope of the reaction norm will tell you how sensitive the genotype is. A horizontal slope indicates absence of sensitivity to the environments that are considered. A slope indicates that performance in one environment is better than in the other. Parallel reaction norms indicate that the two genotypes are equally sensitive to changes in the environment across the environmental range. However, if one reaction norm is more steep than the other, this is an indication that one genotype is more sensitive to a change in the environment than the other and only then we call that a genotype by environment interaction. In the extreme situation the reaction norms even cross each other. That means that in the one environment the one population would be best, whereas in the other environment the other population would be best.

In the figure below is an example of both type of reaction norms to indicate GxE. In the top figure the reaction norms are non-parallel, but not crossing, indicating that one population remains superior to the other in both environments. In the bottom figure the reaction norms are crossing,  indicating that the genetic superiority changes with the environment change. The reaction norms in these figures are straight lines because only two environments are considered. They can become non-linear when more environments are included, and thus multiple performances are compared. Non-horizontal reaction norms indicate that some breeds are more capable of dealing with less optimized environments than others.

Open

Figure 4: Two examples of GxE in the sheep breeds Dorper and Red Masai. In the top graph the non-parallel reaction norms are an indication of the higher sensitivity of the Dorper to semi-arid conditions than the Red Masai, expressed in mortality rate. But the mortality rate of the Dorper remains higher in both environments. In the bottom graph the GxE is stronger and indicating that the Dorper is heaviest in hot and humid conditions, whereas the Red Masai is the heaviest in the semi-arid conditions.