To prepare for an unusual photo shoot two years ago, scientists pulled the wings of thousands of fruit fly larvae and placed each flake of iridescent tissue on glass plates. The wing would often tear or fold, or an air pocket, or errant bit of dust, got caught with it. This could lead to the loss of the sample. Madhav Mani, an engineer and applied mathematician at Northwestern University, who was the lead of the project, stated that fly wings are not like Saran wrap. He said they are fragile like gold leaf foil.
They persevered until they mounted approximately 2,000 pairs of wings correctly. The scientists then took high-resolution photos and systematically compared them in 30,000 locations.
This was not a simple exercise in taxonomy. The study, published in the journal eLife recently, provides a detailed look at species variation. These results help to end a long-standing tension within biology.
One, even though there are dramatic mutations in the genes of individuals and in the environments they grow up in, members of a species can still develop into strikingly similar beings. This robustness makes it possible for almost all individuals to be functional. However, evolution can only occur if members of a species have different traits that natural selection can exploit. These two forces, robustness and evolvability, pull in opposite directions. One desires less variation and one seeks more.
What is the best way to strike a balance?
Biologists believed that genetics and environment would produce significant heterogeneity around 20 years ago. This was consistent with natural selection, according to Alex Lancaster, an evolutionary biologist from the Ronin Institute in New Jersey. He wasn't part of the new study. However, recent observations have shown surprising similarities across populations, he stated.
Mani began to think about how individuals can vary physically and how much they can, after a dinner with Richard Carthew. What options are available within the confines of embryonic and postnatal growth? Mani asked, "What's the menu for a growing organism?" Science is often referred to as a method. But science began with a simple, childish question.
Prior studies had only focused on a few points for comparison of an organism. But Mani, Carthew, and their co-authors Vasyl Alb and James Carthew, Richard Carthew's son and member of his laboratory, wanted more detailed information. The algorithms align sections of the genetic code to allow for comparison when sequencing genomes. The scientists also aligned the wings of Drosophila melanogaster fruit fly wings.
The thousands of wing photos look like line drawings, with dark veins that stand out against light backgrounds. These images were standardized by a computer program that fit them into a circle and preserved features like the angles of veins. Then, the program digitally stacked thousands of photos on top of each other. Researchers were able to compare every pixel of the stacked image with around 30,000 points instead of just a handful of landmarks. Its more comprehensive, said Lancaster. It captures features that would not normally be captured.
This rich data revealed surprising simplicity. Scientists saw only a limited range of possible appearances of the wings. They mostly differed in a few characteristics. This variation was most prominent near the hinge of a wing, and it showed up in certain spots such as the shape or the position of the frontmost vein. These variable traits are linked. If one trait on a wing is not average, all other traits will be. This held true regardless of the environmental or genetic modifications that fly had, which suggests that each factor has very limited influence.
Richard Carthew anticipated that more of the complexity of the flies' developmental process would be captured in their physical form. He said that the fact that all of the variations could be filtered into a small number of menu options was quite remarkable. Flies, as adults, have the ability to fix any differences and create strong final forms.
Photos of fly wings did not reveal any clues about the mechanisms that limit the possibilities for morphologies. These guardrails are powerful, as the results showed. Natural selection should primarily focus on the diversity of the few linked, variable traits while robustness constrains the rest.
Some scientists find the simplicity surprising. Luisa Pallares is an evolutionary biologist and ecologist at Princeton University. She expected that there would be restrictions on variation based on biological theories about robustness. This means that for species members to appear as similar as possible, their variation must be restricted. She said that this study provided a stunning quantification. Scientists can now see the theory in action more clearly.
Alba is a Northwestern biologist, but he studied physics before he was attracted to the guiding principles which can predict much of the physical universe. He said that biology doesn't have these principles. I have always wondered if it was possible to create a set of principles that can explain biology in its entirety or simplify our understanding.
More species will be needed to test if the limited variation Alba and his colleagues found holds across animals. Carthew hopes that paleontologists will test it on fossilized bones and teeth, while Pallares would like to see how it performs on three-dimensional structures. Michalis Barkoulas, Imperial College London, stated that understanding the relationships between genes and their physical form will help us unravel the workings behind diseases in the future.
The Northwestern team is currently working on a second study that examines thousands more fruit fly wings. They used their quantitative approach to identify differences in Drosophila melanogaster wings morphology. Now they are thinking about natural selection and how it might have affected their diversity to create new closely related species. They wanted to know if their method could predict which species would evolve from Drosophila. Mani stated that it seemed like we were about to say yes. It is possible to predict the future of nearby species.
Editor's Note: A grant from Simons Foundation partially funded the eLife study. This independent, editorially funded magazine also received this grant. Our coverage is not affected by Simons Foundation funding decisions.
