Monarchs, migration, and mating
In the de Roode lab I study monarch butterflies (Danaus plexippus). Monarchs are well known for their chemical ecology and migration. I instead plan to develop monarchs as a model to study how variation in movement ecology drives ecological speciation. Monarch populations vary wildly in their movement behavior, ranging from 3000mi transcontinental migration to completely sedentary island populations. As a result, selection has driven divergent adaptations among monarchs all over the world. My work focuses on understanding how these local adaptations can drive reproductive isolation within this iconic species.
Monarchs can be reared in large numbers in the lab. Each tube contains a single milkweed plant and 1 to 2 caterpillars. Rearing conditions can be manipulated to influence adult morphology and behavior.
Influence of host plant On monarch mate choice
Monarch caterpillars are notoriously specialized for feeding exclusively on milkweed plants. These plants produce toxins that caterpillars sequester and use to deter predators. Preliminary data suggests that monarchs assortatively mate based on the plants they were raised on as caterpillars. Interestingly, females prefer to mate with males that were raised on a different milkweed species than they were. Why? One possibility is they are trying to avoid inbreeding. How can they tell what plants males were raised on? Stay tuned…
Monarch caterpillars feeding on milkweed.
wing color and mate choice
My preliminary data suggest that wing color may influence mate choice. In many insects, color is directly influenced by diet. This may also be the case with monarchs. I plan to manipulate wing color through diet and look for downstream effects on mate choice.
Natural variation in monarch wing color.
Monarchs display extreme variations in movement. Some populations migrate up to 3000 mi, while others have lost their migratory ability. Non-migratory monarchs have repeatedly evolved smaller body sizes and altered wing patterns.
We developed methods to experimentally manipulate both color and size of North American monarchs. We can create non-migratory phenotypes in the lab that allow us to pinpoint the traits that influence mating compatibility among populations that have diverged in migration ability.
Heparin injections target expression of melanin in the WntA pathway to remove the white spots on the wing margins. Increasing concentrations of heparin create darker monarch morphs. Wing pattern manipulation mimics the differences in wing pattern between migratory and non-migratory monarchs.
Experimentally created non-migratory phenotype (left) and wild-type monarch (right).
Body size and mate choice
Body size may play a major role in mating success. In monarchs, size can be dependent largely on food availability as a caterpillar. Size can also be influenced by the species of milkweed that caterpillars feed on. Mating trials show that smaller males have a mating advantage. Thus, the local environment may dictate the outcomes of matings, and have large effects on the population as a whole. I developed a method of experimentally reducing monarch body size. This allows me to test the how smaller males are able to compete for mates and has implications for island populations of monarchs that have evolved small body sizes.
Experimentally manipulated monarchs. Males on the left, females on the right.
Assortative mating based on size. Large males tend to mate with large females (left) and small males tend to mate with small females (right).