microscopic view of fruit fly compound eyes
Wild-type, hexagonal ommatidia (A) versus tetragonal patterning caused by loss of function genetic mutations of different regulatory pathways (B and C)
HAYASHI ET AL, Current biology 2022

For animals with compound eyes to get a complete view of their environment, each optical unit needs to be arranged in a regular pattern. It is most common for these so-called ommatidia to have six sides and to be nestled together in a hexagonal tessellation, much like a tiled floor. Certain mutations in fruit flies can cause tetragonal patterning, which is the tessellation more commonly found in marine animals as well. If both shapes can exist, what causes one pattern over another?

A recent study published in Current Biology investigated the development of Drosophila’s compound eye by comparing the wild-type arrangement of ommatidia with two mutations that inhibit regulatory developmental pathways in the eye. These so-called small-eye mutations resulted in significantly fewer ommatidia and thus required each ommatidium to migrate further to reach its neighbor, resulting in four-sided patterns to fill the space. In the wild-type, hexagonal shapes arose from being so densely packed. In the end, room to grow (or not) appears to be the key factor determining Drosophila ommatidia tiling patterns.