Moths may use disco gene to regulate day/night cycles

How does one species become two? If you’re a biologist, this is a difficult question to answer. The consensus is that, in most cases, the process of speciation occurs when individuals in the same population become geographically isolated. If they remain separated long enough, they lose the ability to interbreed.

A new study published in the journal Proceedings of the Royal Society B: Biological Sciences shows what happens when a less common form of speciation occurs. Rather than being separated by a physical barrier, such as a mountain range or an ocean, members of a species can become separated over time.

The researchers focused on two closely related moth species with overlapping ranges in the southeastern United States.

“These two species are very similar,” said lead author Yash Sondhi, who conducted the research for the study while at Florida International University and then at the Florida Museum of Natural History. “They differentiated along one axis, which is flight.”

Pink maple butterflies, of the genus Dryocampa, look like something you would get if Roald Dahl painted a fever dream. They sport thick lion’s manes over their heads and abdomens, and their vibrant scales are the color of strawberry and banana taffy. Both male and female pink maple butterflies fly exclusively at night.

The pink-striped butterflies of the genus Anisota are less showy, with subtle shades of ochre, umber and marl. While females of this species are active at dusk and early evening, males prefer to fly during the day.

Sondhi knew from previous research that these two groups, Dryocampa and Anisota, emerged from a single species about 3.8 million years ago, which is relatively recent on evolutionary timescales. There are a handful of species in the genus Anisota, all of which are active during the day. Rosy maple moths are the only species in the genus Dryocampa.

Sondhi specializes in the biology of insect vision and saw the moth pair as an ideal opportunity to explore how vision evolves when a species changes its activity pattern.

But things didn’t go as planned.

“I looked for differences in color vision. Instead, we found differences in their clock genes, which in hindsight makes sense,” Sondhi said.

Clock genes control the circadian rhythm of plants and animals. The ebb and flow of proteins they create causes cells to become active or dormant over a period of about 24 hours. They affect everything from metabolism and cell growth to blood pressure and body temperature.

For any organism that reverses its activity pattern, it’s almost certain that clock genes are involved. “It’s a system that’s been conserved in everyone from fruit flies to mammals to plants. They all have some type of timing mechanism,” he said.

Sondhi compared the transcriptomes of the two butterflies. Unlike genomes, which contain the entirety of an organism’s DNA, transcriptomes contain only the subset of genetic material actively used to make proteins. So they’re useful for exploring differences in protein levels throughout the day.

As expected, Sondhi found a number of genes expressed in different amounts in the two moth species. The pink maple moths invested more energy in their sense of smell, while the oak moth produced more genes associated with vision.

There are, however, no differences in the genes that confer the ability to see colors. This does not necessarily mean that their color vision is identical, but if differences exist, they are likely in the tuning and sensitivity, not in the structure of the genes themselves.

Another gene stood out. The Disconnected, or disco, gene was expressed at different levels during the day and night in both species. In fruit flies, the disco gene is known to indirectly influence circadian rhythms by producing neurons that transmit clock enzymes from the brain to the body.

The disco gene that Sondhi found in his moth samples was twice as large as its fruit fly counterpart and had extra zinc fingers, active parts of a gene that interacts directly with DNA, RNA, and proteins. It seems likely that changes in the disco gene are at least partly responsible for the shift to nocturnal flight in sugar maple moths.

By comparing the disco gene of sugar maple moths with that of oak caterpillars, he found 23 mutations that made them distinct from each other. The mutations were also located in active parts of the gene, meaning they likely contribute to the physical differences observed between the moths. Sondhi was studying evolution in action.

“If this is confirmed functionally, it will be a really concrete example of the mechanism behind how they speciated at the molecular level, which is rare to find,” he said.

This study also represents an important step forward in our understanding of the different ways in which life is maintained and propagated. When genetics first became a field of study, researchers focused most of their efforts on a few representative species, such as fruit flies or laboratory mice. This was done mainly for efficiency, but it limits our knowledge of large biological models. Just as a human is not a laboratory mouse, a moth is not a fruit fly.

“As species continue to decline due to climate change and other anthropogenic changes, we will need to genetically modify more of the remaining species to enable them to tolerate drought, for example, or to be active in light pollution regimes. To do this consistently, it is essential to have a larger pool of functionally characterized genes across organisms. We can’t just use Drosophila,” Sondhi said.