The Monarch's Pump: Milkweed Toxin and Gene Changes

A Toxic Meal in the Butterfly Garden

1The Riverbend Zoo helps protect monarch butterflies, whose numbers have dropped in many parts of North America. Its butterfly garden is planted with several kinds of milkweed, the only plants monarch caterpillars eat. Each summer, teen volunteers count caterpillars, raise some indoors and tag the adults before they are released. This year eighth grader Teodora Baptiste joined the program and asked the zoo's insect specialist, Dr. Kwame Lindqvist, a question: how can a caterpillar live on a plant that is poisonous to most animals?

2Dr. Lindqvist started with cells. Nerve and muscle cells depend on a protein called the sodium pump. It sits in the cell's outer membrane and moves particles into and out of the cell, which nerves need to send signals and muscles need to work. Like every protein, the pump is built from instructions in a gene, a section of DNA, and its shape lets it do its job.

3Milkweed leaves and sap contain bitter toxins. In most animals, the toxin fits onto one spot on the sodium pump and jams it. A bird that swallows enough of the toxin usually vomits, and large amounts can harm the heart. Monarchs carry a few mutations in their pump gene, in the part of the gene that holds the instructions for that spot. Monarch caterpillars eat milkweed and store much of the toxin in their bodies, and the stored toxin stays in the adult butterflies.

4Dr. Lindqvist showed Teodora results from a university lab that tests pump proteins in dishes. The lab measured how well four pumps worked with no toxin and with three amounts of milkweed toxin (Table 1). The fruit fly pump has no changes at the toxin spot. Besides the usual monarch pump, the lab tested two rare versions of the monarch pump gene found in lab-raised butterflies. Version S has one changed DNA letter, but its pump has the same building blocks in the same order as the usual monarch pump. Version X has a change in a different part of the gene, which bends the part of the pump that carries particles through the membrane.

5Later, Teodora listed the other insects she found in the garden and what they eat (Table 2). Researchers have read the pump gene of each kind of insect, so Dr. Lindqvist could tell her which ones have changes at the toxin spot.