Microscopy image of an ant soldier head with the active gene ocelli-less (magenta). Image credit: Vasquez-Correa et al. (CC BY-4.0)
The re-emergence of traits lost over evolutionary time has long fascinated biologists and sparked debate. Yet we still do not fully understand the developmental and evolutionary mechanisms that allow lost traits to reappear. This question is particularly relevant in species that exhibit polyphenism – the ability of the same genome to produce different forms in response to environmental signals.
Ants are a striking example of polyphenism in action. Within a single colony, the same genome can produce morphologically distinct queens, males and workers. One of the most remarkable polyphenic traits is the presence of ocelli, three simple light-sensing eyes on the top of the head. Queens and males consistently possess ocelli, but in workers they have been repeatedly lost and gained during evolution. Across different species, workers may have one, two or all three ocelli, suggesting that the trait has evolved independently multiple times.
This remarkable evolutionary flexibility led Vasquez-Correa et al. to propose that the developmental programme for forming ocelli remains dormant in workers, making it easier for the trait to reappear. To test this idea, the researchers reconstructed the evolutionary history of worker ocelli across the hyperdiverse ant subfamily Formicinae.
Their ancestral state reconstruction revealed a single evolutionary gain of worker ocelli, followed by multiple losses and independent reappearances. To understand how this repeated re-evolution occurred, the researchers examined ant head development, focusing on the eye-antennal disc. This tissue gives rise to the eyes, antennae, ocelli and head capsule.
Using hybridization chain reaction, a technique that visualizes gene activity in fixed tissues, they tracked the expression of genes involved in ocellus development throughout development. Remarkably, even in workers that lacked ocelli, key developmental genes were briefly active, suggesting that the underlying gene regulatory network remained intact. This retained developmental programme appears to have enabled the repeated evolution of ocelli in Formicinae workers and soldiers.
Polyphenism is widespread in nature, occurring at many levels of biological organization, from individual cells to complex societies. The findings of Vasquez-Correa et al. suggest that dormant developmental programmes may commonly persist after traits are lost, providing hidden evolutionary potential. If similar latent gene activity exists in other organisms, it could help explain how lost traits repeatedly re-evolve in new forms, offering fresh insights into the origins of evolutionary novelty.
