A Microscopic Tulip Field Blooms Within Fruit Fly Testes

A long, whip-like tail called the flagellum emanates from the sperm of most animals, which helps propel the cell toward the egg. Structural or functional defects in the flagellum can impede sperm movement, causing male infertility.

The human flagellum is about 50 micrometers in length. In comparison, the otherwise tiny fruit fly sperm flagellum can be as long as two millimeters, up to 40 times larger than the human sperm.^1,2 Despite this, fruit flies are a powerful cellular model for studying human flagella due to their conserved structure and motility proteins.³ Anthony Rabaté, a graduate student in Bénédicte Durand’s lab at Lyon 1 Claude Bernard University studies proteins in fruit fly appendages like flagella and cilia.

In addition to developing functional genetic and biochemical platforms, Rabaté and his colleagues apply super-resolution microscopy approaches to study the genes involved in cilia and flagellar assembly. This results in stunning images detailing the cellular morphology of these appendages and could help researchers understand diseases driven by their dysfunctions.

To study the fruit fly sperm, Rabaté dissects and fixes the insect’s testes. He then treats the sample to anchor the proteins to a polymer gel that can absorb water. Exposing the sample to water results in swelling of the gel, physically expanding the proteins attached to it. “The sample [and] the gel will grow by a factor of four,” explained Rabaté. “And then you can do your staining on this gel.”

In a recent experiment, he used different dyes to stain the spermatids’ nuclei, microtubules that make up the axoneme—which forms the core of the sperm flagellum—and the structural foundation that anchors the flagellum. The resulting image looked like flowers on a stalk, prompting Rabaté to title it “A tulip field in the Drosophila testis.”

Not used to seeing spermatids at such a high resolution, he was surprised to see the architecture. “I found them quite amazing to observe. I was quite impressed.” While scientists have observed such structures using immunofluorescence microscopy, expansion microscopy—pioneered by Massachusetts Institute of Technology neuroscientist Edward Boyden—offers a better resolution, said Rabaté. “It's fascinating to [see] how this technique has been improved to gain resolution for studying the cilia field,” he said, adding that this technique can help him compare the architecture of flagella containing wild type and mutated proteins.

The stunning view of what looked like tulips against a black background prompted Rabaté to sign up for an image competition organized as part of the European Molecular Biology Organization conference to be held next year. To his surprise, he won third place. “It made me proud of my work and put it into perspective. It was quite a good feeling,” he said.

Going forward, he hopes to figure out the roles of certain ciliary and flagellar proteins in eukaryotes, gathering striking images along the way.