Not such an exciting title, but it’s a pretty cool project! I’m looking at the distribution of genetic variation across the population of monarch butterflies found east of the Rockies (during the summer) in North America. Monarch butterflies are found throughout North America during the late spring, summer and early fall, and this is prime breeding season. Because monarchs only lay their eggs, and monarch caterpillars only eat, milkweed plants, the abundance of monarchs is closely tied to the abundance of milkweeds. During the breeding season, most adult monarchs live for about 6-8 weeks. As conditions start to change in the fall, however, this shifts dramatically. New adults that emerge during this time do not become reproductively active. Instead, they enter a s non-reproductive state called diapause, and they migrate southward to spend the winter in huge colonies that are clustered in a small region of the mountains northwest of Mexico City.

This migration follows three major flyways, as shown in Figure 1. By far the largest flyway is through the Great Plains. A second flyway starts in northeastern Canada, funnels through Point Pelee to cross the Great Lakes, then flows down through the Ohio and Mississippi River valleys until merging with the Great Plains flyway in Texas. Lastly, the Atlantic flyway traces the coastline from New England to Florida, then along the Gulf Coast until merging with the other flyways. A small proportion of the butterflies following the Atlantic flyway cross over the Appalachians to join the Great Lakes flyway (which was discovered using the same stable isotope techniques I’m using). An open question (see my question #2 below) is if any butterflies following this Atlantic flyway actually “forget” to head west when they hit Florida, and instead continue southward along the Atlantic coast. Along the southern edge of the summer range, small pockets of monarch butterflies follow a different life cycle: they remain reproductively active all year, and they do not migrate. A resident population of monarchs is found in South Florida, for example.

Once the monarchs reach Mexico, they cluster into huge colonies high in the mountains northwest of Mexico City. Even though these colonies are fairly close together on a map, most are found on different mountains, so they are effectively far more separated than a quick glance at the map of our sampling sites shown Figure 2 would indicate.

Research Questions

Although monarch butterflies have been studied for decades, little work has focused on their genetics, and many unanswered questions remain. The focus of my dissertation is to answer a couple of these questions:

1. 1.Is monarch butterfly population structure explained by summer range?
   Because there are three distinct flyways that feed into the overwintering colonies, there is a possibility that different colonies are fed by different flyways. Since butterflies in the spring mate with other butterflies in the same colony, such an association could lead to small differences in the distribution of genetic variation between these colonies. Prior studies have found mixed results.
   

2. 2.To what degree is the resident population in South Florida distinct from the migratory population?
   Weather conditions in South Florida allow monarchs to live, and breed, throughout the year. One hypothesis for how this population avoids the genetic pitfalls that can prevent small populations from persisting is that an influx of migrants enter this population during the autumn migration, fall out of reproductive diapause due to the environmental conditions, and contribute genetic variation to the resident population. Ecological studies across multiple decades have shown evidence for monarchs entering this population, but no evidence for any monarchs migrating back northward from South Florida.
   

Methods

To answer these questions, I will use a combination of genetic and stable isotope analyses. Slight differences between genetic markers found in sub-groups of a population can show how these sub-groups are connected across many generations. Stable isotope profiles complement these genetic data by indicating where an adult butterfly was a caterpillar. Thus, integrating these two types of complementary data will provide a more complete picture of monarch butterfly population structure than either approach alone.

Figure 1. Southward Migration Flyways.

Figure 2. Mexican Overwintering Colonies.