The causes behind the development of female and male traits in nature are much more interesting and complex than scientists had realized, according to a new study by a Colby professor.
In a paper published in February in Proceedings of the Royal Society B, a team led by Dave Angelini, associate professor of biology, demonstrated that sexual differences in the milkweed bug, a black-and-orange insect, are produced by a surprisingly diverse set of genetic mechanisms.
The paper was based on the honors thesis of one of the coauthors, Josefine Just ’19. Angelini’s team explored the genetic makeup of the bug, which is commonly found eating milkweed seeds across eastern North America. Angelini has long been interested in the genetic mechanisms underlying the bugs’ physical characteristics. What, wondered Angelini and the team, lay behind the anatomical differences between males and females?
Geneticists had long assumed that different female and male traits arose from a single mechanism. If one crucial gene was removed, it was thought, an insect would not develop any sexually distinctive characteristics.
The reality, the research showed, is a lot murkier.
The genetic triggers for the expression of female and male traits, it turns out, are highly complex. Lacking sex-determination genes, an insect can still develop some sexual traits, but, interestingly, will not grow certain others. Female and male characteristics can coexist in a single insect, a reality that upends established thinking.
Angelini recently spoke about the findings. This interview has been edited for space and clarity.
What led to your interest in the genetics of sexual differences in insects?
Insects are little Swiss Army knives, with an appendage for every function. Most of my research has focused on looking at how different insect appendages are patterned genetically.
I have been involved in a whole series of projects where we looked at the genes that structure those appendages. We’ve gone from the head all the way down to the back end. And we finally got to the genitalia, which, obviously, are different in males and females.
Then we had a thought. What if we tried to eliminate the function of one of these genes, to see if we can disrupt the development of female and male identity? Eliminating the function of a gene is a good way to understand what the gene is doing.
Why did you choose the milkweed bug for this experiment?
Convenience. It’s easy to keep milkweed bugs in a laboratory. Biologists have used them for more than 60 years to learn about different aspects of insect physiology and genetics. The milkweed bug continues to tell us a lot about the larger principles of life.
How do you tell apart the female and male bugs?
The differences are subtle. The most obvious differences are in the genitalia. Females have a structure they use to lay eggs called the ovipositor. Males have two little appendages that they used to hang on to the female during mating. There are also some differences in pigmentation.
How did you go about eliminating the function of a gene?
It’s relatively straightforward. If you know the sequence of a gene, you can take a bug, and do a little chemistry in the lab to knock out the gene’s function.
We do that at an early stage, before we see differences in female and male development. Then, we give them a little time and look to see how they’re different. We can use that difference to infer what that gene is normally responsible for.
What did you find?
An unexpected layer of complexity in the story of sexual development.
A reasonable assumption is that different traits between males and females are down to the same underlying genetic mechanisms. So, if you eliminate the functions of the genes, the insect wouldn’t develop sexual characteristics.
But we found that this wasn’t the case. Eliminating the function of the genes caused effects in some traits but not in others. The insects ended up with both female and male characteristics. That was so unexpected. I didn’t think it would be that complicated. It dawned on us that there’s a lot more going on than we thought.
Do your findings have any relevance to the concepts of sex and gender more broadly?
It can be dangerous to draw direct connections from nature to human society. Having said that, the fact that sex in the natural world is more complex than we previously appreciated is, I think, very important for people to recognize.
What’s next for this research area?
My lab is continuing to look at the milkweed bug, but we are also studying sex determination in other organisms, such as butterflies.
I hope that our work will spur people to look more carefully at other species, and not assume that they’re going to develop the same mechanisms. I also think people need to be careful about labeling individuals as female or male in their entirety. We should be looking, instead, at specific traits.
The field is moving in this direction, which is great.
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