How do related species keep themselves separate? A study now shows how “switch-like” the mechanisms can sometimes be: Researchers have identified a single gene that determines whether the wings of two butterfly species reflect UV light. The system with signal effect provides UV iridescence only in the males of one of the two species. Accordingly, the genetic trait apparently restricts females from “falling in love” with other species, the scientists explain.
Be it due to geographical barriers or different specializations – in the course of evolution, species have split into new ones. For example, donkeys, horses or zebras emerged from common ancestors. The evolutionary process is also particularly evident in butterflies: different species developed from ancestors by adapting to different food sources and environmental conditions. As is well known, the often closely related species often differ in certain colors and patterns. It is assumed that these external characteristics also serve to avoid hybridization between species. Because this can lead to genetic incompatibility effects that affect the fitness and fertility of hybrids. However, little is known about the genetic background of such “identity features”.
On the trail of male splendor
“As evolutionary biologists, we are interested in identifying and understanding hereditary factors that account for the physical differences between species,” says Arnaud Martin of George Washington University in Washington DC. As part of their study, he and his colleagues therefore examined two North American butterfly species with interesting characteristics: the Orange Sulfur (Colias eurytheme) and the Clouded Sulfur (Colias philodice). The two related species, some of which occur together, look very similar at first glance – but there is one striking distinguishing feature that only becomes clear to us with special cameras: the wings of the male Orange Sulfur butterfly reflect UV light, which but not his females. In contrast, the Clouded Sulfur moths of both sexes show no iridescence in UV light. So it stands to reason that the UV iridescent wings of the male Orange Sulfur butterflies help the females to identify the best-matching males, the researchers explain.
In the study, the researchers have now investigated the background of the visual effect through genetic comparisons. Their analyzes showed that the two butterfly species apparently hybridized, which led to a partial exchange of genetic traits. The properties of the chromosomes of the butterflies are therefore also very similar – with one exception: the sex chromosomes differ comparatively clearly. This suggested that the sex chromosome harbors key genes that keep the two species separate, including UV staining.
A gene for species recognition
Through further genetic analysis, the researchers finally targeted a special gene on the sex chromosomes that is involved in the formation of the microscopic scales on the wings of butterflies. The features of these structures, in turn, are responsible for the appearance. As has been shown, the so-called bric-a-brac gene is active in the wings of the moths – with one exception: it is switched off in the male orange-sulphur butterflies, according to the genetic studies. Apparently, the lack of gene function thus leads to the formation of their UV-iridescent scales.
The researchers were then able to confirm this using the CRISPR genome editing technique: they removed the bric-a-brac gene from the genome of all butterflies. It turned out: “Large parts of the butterflies’ bodies were now covered with UV-reflecting scales, including those of the Clouded Sulfur butterflies. Scales that are normally only yellow or orange now reflect UV light,” reports co-author Vincent Ficarrotta. Co-author Joe Hanly adds: “Right before our eyes, a gene was emerging that makes these species look different.” Using a high-power electron microscope, the team was also able to show how the presence or absence of the gene alters the nanostructure of the scales, thereby affecting the butterflies’ visual characteristics.
“Our results document how a single gene can determine whether ultraviolet coloration is on or off in two different butterfly species. Since the geographic ranges of these two species overlap, this visual distinction is all the more important when it comes to finding mates,” summarizes Martin.
Through further investigations, he and his colleagues now want to clarify how the activity of the bric-a-brac gene is naturally regulated. The scientists also hope that the interesting butterfly species duo can also provide further insights into evolutionary biology.
Source: George Washington University, professional article: PNAS, doi: 10.1073/pnas.2109255118