Thursday, October 24, 2019

Monarch butterfly: A Beautiful Creature That Feasts on Poison

A recent Facebook post by a work colleague regarding his son being a budding scientist caught my attention. The two of them had found a Monarch butterfly wing in their yard and were looking at it under a microscope.
Figure 1.  Female Monarch butterfly.
The Monarch butterfly (Danaus plexippus) is certainly one of the most beautiful of all butterflies and as its name suggests, is considered the king of the butterflies by many. As with all insects, Monarch butterflies go through four stages during one life cycle. The four stages of the Monarch butterfly life cycle are the egg, the larvae (caterpillar), the pupa (chrysalis), and the adult butterfly.
Life cycle stages of the Monarch butterfly
Figure 2.  Life cycle stages of the Monarch butterfly. (Composite of images provided by Wikipedia.)
At the start, the eggs are laid on milkweed plants. They hatch into baby caterpillars, also called the larvae. It takes about four days for the eggs to hatch. Then the baby caterpillar doesn’t do much more than eat the milkweed in order to grow. After about two weeks, the caterpillar will be fully-grown and find a place to attach itself so that it can start the process of metamorphosis. It will attach itself to a stem or a leaf using silk and transform into a chrysalis. Over the 10 days of the chrysalis phase, the body parts of the caterpillar undergo a remarkable transformation, called metamorphosis, to become the beautiful parts of the butterfly. The Monarch butterfly will emerge from the chrysalis and fly away, feeding on flowers for its remaining short life of about two to six weeks.

When I was child, it was fascinating to capture a few Monarch caterpillars from milkweed plants and watch them form a chrysalis and eventually turn into a butterfly. The observations took place at home or in the school classroom.
Figure 3. Milkweed (Asclepias syriaca) showing flowers and latex.
Figure 3. Milkweed (Asclepias syriaca) showing flowers and latex. (Courtesy of Wikipedia)
The interesting point of this is that milkweed (Asclepias), which the larva (caterpillar) feeds on exclusively is quite toxic. Asclepias is a genus of perennial flowering plants known as milkweeds, named for their latex, a milky substance containing cardiac glycosides termed cardenolides, exuded where cells are damaged. Cardiac glycosides affect the sodium (Na+/K+-ATPase) pump of cells. Sodium ion pumps create ion imbalances in cells critical for cardiac and nerve cell function. These are the type of compounds that Agatha Christie featured in a number of her murder mysteries such as Herb of Death, Postern of Fate, and Appointment with Death. Milkweed should kill the caterpillar, but it doesn’t. In fact, the caterpillars store the toxins in their bodies as a defense mechanism against birds that would like to eat them. The bright orange coloring of their wings is actually an “Unsafe to eat” message to animals. So what should kill them actually makes them stronger; the question is how? In a recent article in Nature, researchers used genome editing to retrace the evolution of toxin resistance in the Monarch butterfly. It turns out that only three gene mutations are necessary. These involve amino acid changes at position 111, 119, and 122 of the ATPĪ± subunit of the Na+/K+-ATPase pump. These gene mutations did not occur at once, but rather developed sequentially. Using CRISPR/Cas-9 to edit genes, the group was actually able to make Drosophila fruit flies resistant to milkweed. Studies showed that the mutant flies were 1000 times more resistant to milkweed than the wild type. Using fruit flies, biologists found that these adaptive mutations are not without a cost. It turns out that these mutations had to occur in a specific order. The first mutation, while altering the structure of the pump and conferring some resistance to milkweed also causes neurological problems. The second mutation amended the pump slightly and fixed the neurological problem. This allowed the last mutation, which confers most of the milkweed resistance. The third mutation alone resulted in intolerable neurological seizure issues. Only with the second mutation, would the neurological issues with the third mutation be alleviated.
Figure 4.  The budding scientist at work in his lab.
Figure 4.  The budding scientist at work in his lab.

Oh, back to the budding scientist. His Dad works with me at BioTek, where we manufacture a wide variety of research instrumentation including automated microscopes and imagers, and the software necessary to capture and process image files. He is also a camera buff who is obviously sharing his passion with his son. They managed to take a number of microscopic images of the Monarch butterfly wing, and using BioTek’s Gen5™ software, stitched the individual images into an amazing composite. You can see their work in Figure 5. I think he has a future in science!

Figure 5.   Composite stitched image of wing cells from a Monarch butterfly wing using Gen5.
Figure 5.   Composite stitched image of wing cells from a Monarch butterfly wing using Gen5. 

By: BioTek Instruments, Paul Held PhD, Laboratory Manager

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