How vitamin A plays a role in monarch migration

(Credit: Texas A&M)

New research links the monarch butterfly’s uncanny ability to sense the changes in day length to circadian clock genes and clock-regulated molecular pathways.

Day length, or photoperiod, is an environmental cue that signals them to migrate and triggers the reproductive dormancy they exhibit in the process. The finding establishes a clear connection between clock genes and the vitamin A pathway within the brain of the insects.

The research team reports their finding in the Proceedings of the National Academy of Sciences.

“Nearly all organisms adapt to the seasons by adjusting their physiology and behavior to changes in day length, or photoperiod,” says biologist Christine Merlin of the Texas A&M University biology department.

“Despite decades of research, the molecular and genetic mechanisms by which changes in photoperiod are sensed and translated into seasonal changes in animal physiology and behavior have remained poorly understood. While much remains to be learned, our findings pave the way for understanding the mechanisms by which vitamin A operates in the brain to translate day length encoding into seasonal physiological and behavioral responses in animals.

“Given that seasonal changes associated with this pathway have also been reported in the mammalian brain, it is tantalizing to speculate that the function of vitamin A in animal photoperiodism may be evolutionary conserved. If this turns out to be the case, our work in the monarch could have implications for better understanding seasonal changes in the human brain that could lead to ailments such as seasonal depression.”

Editing monarch genes

For the past six years, Merlin’s lab within the Texas A&M Center for Biological Clocks Research (CBCR) has been using the majestic monarch as a model to study animal migration, the role of circadian clocks in regulating daily and seasonal animal physiology and behavior, and the evolution of the animal clockwork.

With the help of CRISPR/Cas9 technology, her group already has succeeded in altering key biological clock-related genes in the monarch in order to study their effect on daily circadian rhythms and seasonal migratory responses.

“Despite significant advances our lab has made in developing genetic tools to knock out virtually any genes in the monarch genome—which has been key in this study to demonstrate the central importance of the vitamin A pathway in photoperiodic responses—the genetic toolbox in the Monarch is still far from rivaling with the one available in more conventional genetically tractable model organisms, such as Drosophila and the mouse,” Merlin says.

“While the use of CRISPR/Cas9 has facilitated the generation of full-body knockouts, knocking out genes in a tissue-specific manner in the Monarch remains a challenging task.”

Blind butterflies

One of the complications the Merlin lab had to overcome in this study is the fact that vitamin A is necessary for visual function of the Monarch’s compound eyes, meaning that their ninaB1 full-body knockouts would be rendered blind. As a fail-safe, Merlin’s team had to find a non-genetic way to eliminate the potential function of the compound eyes as a possible tie-back to the lack of photoperiodic responses observed in these new mutant butterflies.

“We had to be creative, so we turned to arts and crafts experiments,” Merlin says. “By painting the compound eyes of wild-type adult butterflies with black paint, we demonstrated that visual function was not necessary for photoperiodic responses, thereby supporting the idea that the vitamin A function in the brain and not the eyes is responsible for photoperiodic sensing and responses.”

The National Science Foundation, Merlin’s 2017 Klingenstein-Simons Fellowship in Neuroscience, and start-up funds from Texas A&M University supported the work.

Source: Texas A&M University