Abstract
Venom-secreting glands are highly specialized organs evolved throughout the animal kingdom to synthetize and secrete toxins for predation and defense. Venom is extensively studied for its toxin components and application potential; yet, how animals become venomous remains poorly understood. Venom systems therefore offer a unique opportunity to understand the molecular mechanisms underlying functional innovation. Here, we conducted a multispecies multi-tissue comparative transcriptomics analysis of 12 marine predatory gastropod species, including species with venom glands and species with homologous non-venom-producing glands, to examine how specialized functions evolve through gene expression changes. We found that while the venom gland specialized for the mass production of toxins, its homologous glands retained the ancestral digestive functions. The functional divergence and specialization of the venom gland were achieved through a redistribution of its ancestral digestive functions to other organs, specifically the esophagus. This entailed concerted expression changes and accelerated transcriptome evolution across the entire digestive system. The increase in venom gland secretory capacity was achieved through the modulation of an ancient secretory machinery, particularly genes involved in endoplasmic reticulum stress and unfolded protein response. This study shifts the focus from the well-explored evolution of toxins to the lesser-known evolution of the organ and mechanisms responsible for venom production. As such, it contributes to elucidating the molecular mechanisms underlying organ evolution at a fine evolutionary scale, highlighting the specific events that lead to functional divergence.