The relationship between morphology and flight in Drosophila: a first approach to its genetic basis

Flight is a crucial activity for winged insects, involving diverse behaviors, and wing morphology has often been proposed as a key factor influencing flight capacity. Traits such as wing loading, wing:ratio, and wing aspect ratio have been suggested as targets of natural selection, exhibiting environmental and genetic variability. Here, we evaluate the relationship between morphological traits and two aspects of flight performance: flight duration (PTF) and its robustness (CVPTF) in Drosophila melanogaster. Additionally, we included wing conformation (i.e., wing shape without the component explained by size), using geometric morphometrics. We analyzed whether variation of flight capacity can be attributed to morphological variation, employing 53 lines of Drosophila melanogaster whose genome is fully sequenced. 

Methods
For the morphological analysis, 10-20  flies of each sex per line were randomly selected. Wings and thoraxes were dissected and mounted in a standardized way, then photographed using a binocular microscope. Morphological traits were measured from these images using tpsDig. For wing traits, 15 landmarks were placed on the ventral side of the left wing. Wing shape was estimated using the Morpho package, allowing shape to be decomposed into size and conformation components. 

The images were used to obtain also the following lineal measurements: thorax length (TL, the distance between the anterior margin of the thorax and the tip of the scutellum), total wing length (TWL, the distance between landmarks 6 and 13; Figure S1), and wing width (WW, the distance between landmarks 12 and 15; Figure S1). Then, we estimated the compound traits wing loading (WLD), wing:thorax ratio (WTR), and wing aspect ratio (WAS): WLD as (TL)3/(WAR), WTR as WAR/TL , and WAS as TWL/WW. Additionally, for each morphological trait, we estimated its coefficient of variation at the line level.

We estimated flight capacity using two different traits: the proportion of time in flight (PTF) over two minutes, and its coefficient of environmental variation, which represents a measure of its robustness (CVPTF). To evaluate PTF, we used a previously tested flight chamber associated with a camera set at a 1080 x 720 resolution and a frame rate of 30 frames per second.

All flight capacity measurements were carried out in controlled temperature conditions (25±1°C), employing virgin flies of 3–7 days old. We defined flight time as the time spent by flies flying above a height of five cm inside the chamber. Finally, each recorded video was analyzed using specially designed motion-tracking software (https://github.com/galimba/drosophila). Flies that were damaged by the rod or those that could not fly were removed from the analysis. CVPTF was estimated for each sex separately as σ/μ, where μ stands for the PTF line’s mean and σ represents the standard deviation for the corresponding line.