How the drone has its stripes

Researchers have discovered a gene responsible for color differences in a bumblebee species. This discovery makes it possible to explain the very diverse color patterns among the bumblebee species as well as the evolution of mimicry (individuals from an area adopting patterns of similar colors). A study describing the gene, which occurs in a highly conserved region of the genome and provides plans for segmentation, was conducted by Penn State researchers and appears on April 29, 2019 in the journal Proceedings of the National Academy of Sciences.

"There is an exceptional diversity of coloring of bumblebees," said Heather Hines, an assistant professor of biology and entomology at Penn State and senior researcher of the study. "Of the approximately 250 species of bumblebees, there are over 400 different color patterns that mix and match the same colors on different segments of the bee's body."

The most common color patterns of bumblebee bees include red around the tail of the bee, meant to herald its dangerous sting. In spite of the great diversity available, color patterns tend to converge towards similarity in a given geographical area as they constitute an important and effective warning sign. This is an example of Müllerian mimicry, where similar, often vivid, color patterns are used among several species to warn predators of a dangerous feature such as toxicity or sharp spines.

"Mellérian mimicry is an example of evolution since the days of Darwin," Hines said. "The study of these mimicry systems – similar color models in many different species – allows us to see how natural traits evolve, and it can also inform us about how characters are encoded in the genome: n & rsquo; Is there only one way to create a trait species reach similar traits through different genetic pathways? "

The research team has studied the genetic basis of color in the species Bombus melanopygus, which presents two regional color patterns. Bumblebees that live in the Pacific Rim are black in the middle of the abdomen, while those in the Rocky Mountain region are red. Previous studies have suggested that a single gene was causing this color change, although the identity of this gene is unknown.

The researchers conducted a genome-wide association study during which they studied the variation of the DNA sequence associated with the variation of a trait. They identified a regulatory region that alters the expression of a gene called Abdominal-B, responsible for the color change.

Abdominal-B is located in a region of the highly conserved genome that contains important structuring genes called "Hox genes," which act as plans for segments of a developing honeybee. "said Hines. "Usually any modification made to these Hox genes triggers many important changes in the development of animal segments, but we have not observed them." It turns out that the modified expression of the Hox genes is noticeable. Abdominal-B intervenes so late in development that it can have very specific effects on color without other consequences. "

The change in the regulatory region resulted in abdominal-B being expressed in an unusual place late in pupal development; At the same time, in development, the pigment begins to appear in the bee. The abdominal-B region is usually expressed closer to the tail of the bee, but in red-striped bees, it is expressed in higher segments, which causes a color change in the center of the bee. abdomen.

"It is possible that the expression of this gene and other Hox genes will shift late in development, thus allowing mixing and matching within segments to create the color pattern. modular that we see on bumblebees, "said Hines.

According to the researchers, this particular genetic change in the regulatory region was not responsible for color changes in ten closely related bumblebees with similar red and black color variation. It is likely that these other species will undergo genetic modifications that entirely target other regulatory regions of the same gene or different genes. Researchers continue their research on the genetics of color changes between species.

"This tells us that the basis of color structure in Bombus melanopygus was born independently of that of other species," said Hines. "Bumblebees do not share this similarity in the color pattern because they all inherited from a common ancestor, so there must be many different ways to mimic."