Animal genitalia typically exhibit limited size variation relative to overall body size, a pattern known as negative allometry. The “one-size-fits-all” hypothesis suggests that genital compatibility between sexes constrains the evolution of extreme genital sizes, yet direct evidence remains scarce. Drosophila suzukii presents a unique opportunity to test this hypothesis from the female perspective. This species has evolved an enlarged, sclerotized ovipositor capable of piercing intact fruit skins. However, this innovation necessitated an altered genital coupling mechanism, as the modified ovipositor—also functioning as part of the female genitalia—posed a mechanical obstacle to copulation. In contrast, D. subpulchrella, the closest relative of D. suzukii, retains the ancestral coupling mechanism, which depends on genital size compatibility between sexes. Allometric analyses revealed that D. subpulchrella ovipositors exhibit shallower scaling slopes than other body parts, consistent with the negative allometry rule. Conversely, D. suzukii ovipositors display significantly steeper slopes, suggesting that the new coupling mechanism has relaxed the constraint on genital size. These findings support the one-size-fits-all hypothesis from a novel perspective at an unprecedented resolution.

Two pairs of sympatric populations of D. suzukii and D. subpulchrella, collected in Tokyo and Yamagata, Japan, were used to confirm the consistency across populations of different localities. For the Tokyo populations, the adults that emerged from wild berries (Rubus hirsutus, R. palmatus, R. crataegifolius, R. microphyllus, Cerasus jamasakura, Morus australis) were directly used for morphological measurements. Ripe fruits were collected from a mountainous area in Hachioji, Tokyo, once a week from May to June in 2024. They were individually maintained in glass vials at 20°C in the laboratory until adults emerged. For the Yamagata populations, laboratory-maintained strains were used for the measurements. These strains were originally established from multiple pairs that emerged from Chinese dogwood fruits (Cornus kousa) collected in Minorigaoka, Yamagata, in 2021. They have since been maintained on standard Drosophila medium consisting of cornmeal, glucose, and dry yeast at 20°C with a 12L:12D light cycle.

For each female, head width, thorax length, ovipositor length, and ovipositor width were measured. The head, thorax, and terminal segments of the abdomen were dissected from the rest of the body. Images of the head and the thorax were captured with a digital camera (FLEXACAM C1, Leica Microsystems, Wetzlar, Germany) mounted on a dissection microscope (MZ FL3, Leica Microsystems). The abdominal segments containing the ovipositor were immersed in 10% potassium hydrate overnight to clear the tissues before being mounted on slides with Hoyer’s mountant (Neo-Shigaral, Shiga Insect Company, Tokyo, Japan). The ovipositor consists of a pair of hypogynial valves. They were dissected apart so that both could be measured. Images were captured with a digital camera (FLEXACAM C1) mounted on a phase-contrast microscope (CK X31, Olympus, Tokyo, Japan). To obtain the trait length in pixel number, the obtained images were analysed using a custom MATLAB script that returns the dimensions of a rectangle drawn on the image. The same process was applied to the images of a micrometre to convert pixel values into real length.