tags: evolutionary biology, behavioral ecology, biochemistry, biophysics, magnetoreception, photoreceptor, cryptochromes, geomagnetic fields, butterflies, Monarch Butterfly, Danaus plexippus, birds, migration, signal transduction, bpr3.org/?p=52,peer-reviewed research, peer-reviewed paper
Every autumn, millions of monarch butterflies, Danaus plexippus, each weighing less than one gram (one US penny weighs 2.5 grams), migrate nearly 4000 kilometers (3000 miles) between their summer breeding grounds in the United States and their wintering areas either in southern California or in the mountains of Mexico (Figure 1).
Figure 1: Migratory circuits of Monarch Butterflies, Danaus plexippus.
Image: Karl Kahler, San Jose Mercury News, 3 January 2007.
There are at least two migratory populations of monarch butterflies; one winters in eucalyptus trees in Pacific Grove, California while the other winters in the endangered Oyamel fir tree forest that only grows at high elevations in Mexico's Transvolcanic Mountain Range. With the exception of the overwintering group of butterflies that hibernate between six and eight months of the year, each generation lives between two and six weeks, so it takes four or five generations of monarchs to complete their annual multi-stage migratory circuit. Thus, returning butterflies are the great, great grandchildren of the previous generation that overwintered in the same area during the previous year.
Previous research indicates that instinct guides the migrating monarchs on their journey as they follow their host plant, the milkweed. But what is the underlying biological mechanism that monarch butterflies rely upon to sense where they are while they travel along their migratory flyway?
A team of neurobiologists that has investigated the mysteries of monarch migration for many years now reports that photoreceptor proteins found in monarch butterflies are linked to animal navigation. Their research finds that two types of photoreceptor proteins not only allow the butterflies to see UV light (light that is less than 420nm long, and thus, is invisible to humans), but also allows them to sense the Earth's geomagnetic field. These photoreceptor proteins are known as cryptochromes.
Cryptochromes are very old and highly conserved proteins. They evolved from and are closely related to the bacterial enzyme, photolyase, which is activated by light and participates in DNA damage repair. Two types of cryptochromes are widespread throughout the animal kingdom, but there is considerable variation in their distribution: insects have either type 1 Cry (as seen in fruit flies, Drosophila melanogaster), type 2 Cry, or both (as seen in monarch butterflies), whereas vertebrates have only type 2 Crys.
To learn more about the functions of the two types of cryptochromes, the team studied laboratory-bred fruit flies, Drosophila melanogaster, that had been specially engineered to lack their own cryptochrome (drosophilaCry1) gene. In fruit flies, the CRY1 protein acts as a blue-light photoreceptor, directly modulating light input into their circadian clock. CRY1 also allows the flies to sense magnetic fields. But when an animal expresses both the Drosophila-like cryptochrome and the vertebrate-like cryptochromes, as do monarch butterflies, the precise function of each type is not known with certainty. To learn more, the researchers cloned monarchCry1 and monarchCry2 and inserted one or the other of these genes into the Cry-deficient flies' genomes.
After inserting either monarchCry1 or monarchCry2 into the genomes of Cry-deficient flies, the team used a behavioral assay to evaluate their tiny subjects' magnetosensitivity. They placed the genetically "rescued" fruit flies into an illuminated black maze shaped like the letter "T". At the end of one arm of the "T" was a magnetic field while the opposite arm of the "T" lacked a magnetic field.
Because cryptochromes are activated by light, the researchers evaluated which wavelengths of light awaken each of the two types of monarch CRY proteins. The T-maze was illuminated with different wavelengths of light and the team found that fruit flies with either type of monarchCry responded to a magnetic field under full-spectrum light (about 300-700 nm) and under UV-A/blue light (less than 420 nm), but their responses were abolished when only long-wavelength light (more than 420 nm) was available. Thus, both types of butterfly cryptochromes have the molecular capability to sense magnetic fields -- but only when activated by UV-A/blue light.
"We believe we are on the trail of an important directional cue for migrating monarchs in addition to their well-defined use of a sun compass," states co-author Steven Reppert, chair of the Department of Neurobiology at the University of Massachusetts Medical School (Figure 2).
Figure 2: Proposed monarch butterfly circadian clock mechanism. The main gear of the clock mechanism is an autoregulatory transcription feedback loop in which CLK and CYC heterodimers drive the transcription of the per, tim, and cry2 genes through E box enhancer elements; in addition to per, there are CACGTG E box elements within the 1.5 kb 5' flanking regions of the butterfly tim and cry2 genes (data not shown). TIM (T), PER (P), and CRY2 (C2) form complexes in the cytoplasm and CRY2 is shuttled into the nucleus where it shuts down CLK:CYC-mediated transcription. PER is progressively phosphorylated and likely helps translocate CRY2 into nucleus. CRY1 (C1) is a circadian photoreceptor which, upon light exposure (lightning bolt) causes TIM degradation to gain access to the central clock mechanism. The thick gray arrows represent output functions for CRY1 and for CRY2.
"These findings suggest that there is an unknown photochemical mechanism that the Crys use," explains Robert Gegear, lead author on the paper and research assistant professor of neurobiology at the University of Massachusetts Medical School. "One that we are hotly pursuing."
This study highlights how the unique biology of monarch butterflies continues to advance our understanding of general biological principles, particularly the connection between genes and behavior as well as the origin, evolution and function of the circadian clock. Since some insects, including the monarch butterfly, have both a Drosophila-like and a vertebrate-like copy of the CRY protein, this indicates an ancestral clock mechanism involving both light sensing and transcriptional repression roles for cryptochromes.
Additionally, because vertebrates possess cryptochromes, this research provides insight into migration, especially avian migration.
"Because the butterfly Cry2 protein is closely related to the one in vertebrates, like that found in birds, which use the Earth's magnetic field to aid migration, [our] finding provides the first genetic evidence that a vertebrate-like Cry can function as a magnetoreceptor," asserts Dr Reppert.
This research has important biomedical implications, too. As scientists learn more about the workings of the circadian clock, psychiatrists and psychologists likewise deepen their knowledge of how clock gene mutations contribute to sleep disorders and mental illnesses, particularly major depression and seasonal affective disorder. Further, this information could lead to development of new medicines and strategies for treatment of jet lag and shift-work ailments, and of clock-related sleep and psychiatric disorders.
Reppert's group is currently developing behavioral assays to show that monarch butterflies can actually use geomagnetic fields during their spectacular fall migration.
Gegear, R., Foley, L., Casselman, A., & Reppert, S. (2010). Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism. Nature DOI: 10.1038/nature08719
Journey North's Monarch Butterfly Migration Tracking Project.