A study published in BMC Biology reveals how tiny changes in the expression of a single gene can lead to big differences in eye size. The study was carried out in two closely related fruit fly species, Drosophila mauritiana and Drosophila simulans, which are model organisms for studies on evolutionary developmental biology. The study is led by experts from the University of Barcelona, the University of Göttingen (Germany) and the universities of Durham and Oxford Brookes (United Kingdom).
The team found that a slight change in the expression phase of a gene called orthodenticle (otd) can cause a significant difference in the size of the ommatidia, individual hexagonal-shaped units that form a compound eye in these insects. In D. mauritiana, the otd gene is expressed earlier in the process of eye development than in D. simulans. This is associated with an increase in the size of the omatidia in D. mauritiana and, therefore, the eyes of this species are larger.
"We were surprised to find that such a small change in gene expression could have such a dramatic impact on eye size", says Professor Isabel Almudi, one of the lead authors of the study and a member of the Faculty of Biology and the Biodiversity Research Institute (IRBio) at the University of Barcelona.
"This suggests that the evolution of eye size may be much simpler than we previously thought", says Almudi, also member of the UB's Department of Genetics, Microbiology and Statistics.
Changes in the pace of biological development
Gene expression is the process by which the information encoded by a gene is capable of generating a functional outcome, e.g. the production of a protein. In this context, phenomena related to heterochrony, i.e. modifications or changes in the rate of developmental processes that generate major evolutionary changes, are of great importance. However, very little is known about the molecular and genetic mechanisms that drive these changes during development.
"In our study, we identify the heterochronic activity of a single transcription factor or activator that ultimately has an effect on the size of the ommatidia and eyes in these closely related Drosophila species", says Montserrat Torres-Oliva, from the University of Göttingen and one of the first authors of the paper.
"Insects exhibit great variation in the shape and size of their eyes, which contributes to adaptive differences in their vision. Our study, which compares Drosophila species that differ in eye size, reveals one of the genetic mechanisms that may make their eye facets larger and potentially allow for greater sensitivity to contrast", explains Professor Alistair McGregor, from Durham University.
How other organs evolve
The researchers believe their findings could have implications for our understanding of how other organs evolve. They now plan to study whether similar changes in gene expression can lead to differences in the size of other organs, such as the brain.
Reference article:
Torres-Oliva, Montserrat; Buchberger, Elisa et al. "Heterochrony in orthodenticle expression is associated with ommatidial size variation between Drosophila species" . BMC Biology, February 2025. DOI: 10.1186/s12915-025-02136-8
