Natural genetic variation in Drosophila spans full range of predicted photoreceptor rhodopsin expression phenotypes

A fundamental question in evolutionary genetics is how much phenotypic variation standing genetic variation can generate without new mutations. A preprint posted to bioRxiv in June 2026 addresses this using the mutually exclusive Rhodopsin 5 (Rh5) and Rhodopsin 6 (Rh6) expression pattern in Drosophila R8 photoreceptors — a neuronal differentiation system in which a finite and enumerable set of qualitative phenotypic states can be defined.

Using wild flies and wild-derived inbred lines from the Drosophila Genome Reference Panel 2 (DGRP2), the authors report extensive variation in Rh5/Rh6 expression ratios, including shifts in the relative proportions of the two rhodopsin subtypes across the eye. Crucially, the observed variation was found to span essentially the full range of phenotypes predicted by the system's logic — suggesting that natural genetic variation alone can access the complete phenotypic space, without requiring new de novo mutations.

The findings contribute to ongoing discussions in population genetics and evolutionary biology about the role of standing variation in adaptation. The work is most relevant to researchers in evolutionary genetics, Drosophila biology, and quantitative genetics, as well as educators and students engaging with concepts of genotype–phenotype mapping and natural selection.