Clock Genes Drive Evolutionary Split in Two Moth Species

How can one species split into two? This is a complex question for biologists. Most agree that the process of speciation begins when members of a population become geographically isolated. If they remain separated for a long time, they eventually become incapable of interbreeding, leading to the formation of two distinct species.

A recent study illustrates what happens when a less frequent type of speciation takes place and is published in the journal Proceedings of the Royal Society B: Biological Sciences. Rather than being divided by a physical barrier, such as a mountain range or an ocean, members of a species can become separated in time.

The study concentrated on two closely related moth species with overlapping ranges in the Southeast region of the United States.

These two are very similar. They have differentiated along this one axis, which is when they fly.”

Yash Sondhi, Study Lead Author, Florida International University

Sondhi later worked at the Florida Museum of Natural History.

The Dryocampa genus of rosy maple moths resembles something Roald Dahl might paint if he were inspired by a fever dream. Their vivid scales are the color of strawberry and banana taffy, and they have a thick lion's mane above their head and abdomen. Rosy moths, both males and females, only fly at night.

The genus Anisota contains the less conspicuous pink-striped oak worm moths, which have delicate ochre, umber, and marl grades. Males of this species prefer to fly throughout the day, while females are active in the late afternoon and early evening.

From earlier studies, Sondhi was aware that these two taxa, Dryocampa and Anisota, descended from a single species about 3.8 million years ago, a relatively recent time in evolutionary terms. The genus Anisota contains a few species, all of which are active during the day. The genus Dryocampa contains only one species, the nocturnal rose maple moths.

Sondhi is an expert in the biology of insect vision, and she saw the moth pair as the ideal chance to investigate how vision changes as a species modifies its behavioral pattern.

However, things did not go as expected.

I went in looking for differences in color vision. Instead, we found differences in their clock genes, which in hindsight makes sense.”

Yash Sondhi, Study Lead Author, Florida International University

Animals and plants have circadian rhythms that are regulated by clock genes. Over around 24 hours, the proteins they produce fluctuate, making cells either active or dormant. Their effects range from blood pressure and body temperature to metabolism and cell proliferation.

Clock genes are almost a given when an organism reverses its pattern of activity.

Sondhi said, “It is a system that is been retained in everything from fruit flies to mammals and plants. They all have some kind of time-keeping mechanism.”

Sondhi examined the two moths' transcriptomes. Transcriptomes only include the portion of genetic material that is actively employed to build proteins, as opposed to genomes, which contain all of an organism's DNA. Therefore, they can be used to investigate variations in protein levels throughout the day.

Sondhi discovered several genes that were expressed differently in the two moth species, as was to be expected. While day-flying oakworm moths developed more genes linked to eyesight, nocturnal pink maple moths focused more of their energy on developing their sense of smell.

However, there were no variations in the genes responsible for color vision. This does not imply that they see color in the same way, but if there are variances, they most likely relate to tuning and sensitivity rather than the actual makeup of the genes.

There was one other gene that was particularly notable. In both animals, disco, or disconnected, was manifested to varying degrees both at night and throughout the day. Disco is known to indirectly affect circadian rhythms in fruit flies by influencing the development of neurons that carry clock enzymes from the brain to the body.

Sondhi discovered that the disco gene in his moth samples was twice as large as the one found in fruit flies. It also possessed extra zinc fingers, which are active sections of a gene that directly interact with proteins, RNA, and DNA. The shift in rosy maple moth behavior to night flight appeared to be mostly caused by modifications in the disco gene.

Upon comparing the disco gene of oak worms and pink maple moths, he discovered 23 alterations that differentiated the two. Additionally, the mutations were found in regions of the gene that were actively expressing, indicating that they probably have a role in the moths' discernible physical variations. Sondhi was seeing the process of evolution.

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

Additionally, the finding represents a significant step toward improving knowledge of the diverse mechanisms by which life forms and spreads. When genetics initially gained attention as a subject of study, scientists mostly concentrated on a small number of model animals, including lab mice or fruit flies.

This was mostly done for practical reasons, but it reduces understanding of general biological patterns. A moth is not a fruit fly, just as a human is not a lab mouse.

As species continue to decline due to climate change and other anthropogenic changes, we will need to genetically engineer a greater number of the ones that remain to enable drought tolerance, for example, or to be active in light polluted regimes. To do that consistently, having a broader pool of functionally characterized genes across organisms is crucial. We cannot just use Drosophila.”

Yash Sondhi, Study Lead Author, Florida International University