Pyrrolizidine alkaloids are as bitter and toxic as they are difficult to pronounce. They are produced by several different types of plants and are among the leading causes of accidental death in livestock.
Plants containing these alkaloids have made it very clear that they do not want to be consumed, but that has not deterred bella moths (Utetheisa ornatriz). These day-flying moths feed exclusively on the alkaloid-laden leaves and seeds of rattlesnake plants. They then use the toxin to protect their eggs and deter predators later in their life. They even use it to produce pheromones that attract their mates.
Exactly how bella moths and related species evolved the ability to safely consume pyrrolizidine alkaloids is still unknown.
In a new study published in the journal PNAS, researchers sequenced the beautiful moth’s genome, which they used to identify specific genes that may confer immunity to these toxins. They also sequenced genomes from 150 museum specimens (some more than a century old) to determine where bella moths and their close relatives originated. Finally, they combed through genetic data for clues that could help explain how the intricate wing patterns of bella moths evolved over time—the first study on moths or butterflies to do so using dried specimens from museum collections.
“We have been able to demonstrate that museum specimens can be used to answer genetic questions that normally require complicated laboratory techniques,” said study co-author Andrei Sourakov, collections coordinator at the McGuire Center for Lepidoptera and Biodiversity at the Florida Museum of Natural History. “This opens a window for future research of this type.”
Sourakov has been studying bella moths for 15 years and said sequencing the genome of this species was a natural next step in the research he has done so far. Much of the knowledge he gained during that time came from his work with undergraduate and high school students, whom he helped conduct short experiments, analyze data for science fairs, and interpret the results in peer-reviewed articles.
In one such project, a student set out to determine the average lifespan of adult beautiful moths and inadvertently stumbled upon the Methuselah of the moth world. “To our great surprise, they can live up to 50 days, which is four to five times longer than an average moth,” Sourakov said.
Longevity is not a critically important trait in most moth species. Many reproduce once and then die soon after, either from senescence or predation. But bella moths are not limited by the latter, making it more likely that genes conferring greater longevity will be beneficial and passed on to the next generation.
“It makes sense that something that is chemically defended lives longer, because even if they are caught, the predator usually breaks free and the moth can continue flying.”
Bella moths live throughout much of eastern North America, Central America, and the Caribbean and are typically active during the day. Instead of using darkness as a veil to avoid predators, bella moths strive to be seen. Its wings are adorned with radiant pink, pearl, onyx and sulfur yellow scales, which birds and carnivorous insects can easily spot from a distance. Any predator unlucky enough to catch a beautiful moth quickly corrects its mistake.
“Banana spiders will tear them out of their webs,” Sourakov said, adding that wolf spiders and birds will do everything they can to avoid them. “When caught, they produce a foamy liquid that tastes bad and is made almost entirely of alkaloids.”
When ready to mate, females release a plume of aerosolized alkaloids derived from the plants they ate as caterpillars. Males are attracted to this scent, which they follow to its source. There, they perform a brief but elaborate ritual in which they gently touch the female’s head with two spongy, retractable structures that closely resemble dandelions. Each filament of these structures is intertwined with pyrrolizidine alkaloids.
If the female decides that the male has sufficient quantity and quality of stored alkaloids, the pair will mate. When he’s done, the male leaves a parting gift called a spermataphore, which contains sperm and, yes, more alkaloids. The female will use this and alkaloids from her own reserve to infuse toxins into the resulting eggs. This type of biparental egg protection in insects is rare. In fact, when it was first observed in 1989 among adults of the bella moth, it was the only known example of a male moth or butterfly investing chemical resources in its progeny.
Bella moths can avoid the harmful effects of pyrrolizidine alkaloids by using a special enzyme that oxidizes the molecule, rendering it harmless. However, if a predator eats a moth, the process is reversed and the alkaloid regains its potency.
Pyrrolizidine alkaloids probably first evolved as a defense mechanism in plants, which later became a commodity for moths. Sourakov and his colleagues wanted to know how bella moths acquired this detoxifying enzyme and how they maintained it during an arms race that lasted millions of years between plant and moth.
The authors discovered that bella moths have not one but two copies of the gene that encodes their unique detoxifying enzyme. They may have acquired the second through a process of genetic duplication, by which other species, including many plants, have evolved new traits.
They also found two copies of a gene partially involved in the production and defense of antioxidants. Sourakov suspects that these genes may be related to both the beautiful moths’ ability to detoxify alkaloids and their remarkable longevity.
“Certain types of stress in biological systems result in longer lifespans. It could be that the interaction that bella moths have with alkaloids is not only the reason it makes sense for them to live long lives, but also one of the mechanisms behind this.”