Redistribution of Ancestral Functions Underlies the Evolution of Venom Production in Marine Predatory Snails

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.

Keywords: ER stress; Neogastropoda; UPR; ancestral reconstruction; secretion; toxin; transcriptome evolution; venom.

Fig. 1.

Species and tissues investigated. Overview of the anatomy of a marine predatory gastropod with the sampled tissues and the phylogeny of the species used in this study based on Fedosov et al. (2024). The abbreviations used for the tissues and species used in other figures are shown. The anatomy drawing is modified after (Modica and Holford 2010), while the schematic of the foregut apparatus of caenogastropods with their mid-esophageal glands was modified after (Page 2011).

Fig. 2.

Overview of tissue-specific gene sets and gland secretomes. a) Number of genes within each tissue-specific set across all organs (x axis) and species (y axis). Species abbreviations as in Fig. 1. The venom glands have a significant higher number of tissue-specific genes (mean N = 1,424) compared to the gland of Leiblein (one-tailed t-test: mean N = 771; t = 3.6, df = 6, P = 0.005) and esophageal gland (mean N = 699; t = 5.6, df = 3, P = 0.005). Missing tissue samples are marked with a “-”. b) Gene Ontology (GO) enrichment results of the OEG-, LEG-, and VG-specific gene sets. c) Mean expression levels as Transcript per Million (TPM) of genes possessing a signal peptide, therefore comprising the secretomes, expressed in the organs (x axis) of each species (y axis). Species abbreviations as in Fig. 1. Missing tissue samples are marked with a “-”. d) GO enrichment results of the esophageal gland, gland of Leiblein, and venom gland secretomes.

Fig. 3.

Transcriptome similarity and shared tissue specificity. a) Heatmap of Pearson correlation coefficients between tissues and species. The expression tree was made using neighbor-joining based on the correlation matrix. b) Average number of OEG-, LEG-, and VG-specific OGs shared with other tissue-specific OG sets, where species have been grouped by their gland type (OEG-, LEG-, VG- species and glandless).

Fig. 4.

Gene expression dynamics across the phylogeny. a) Number of OEG-, LEG, and VG-specific OGs decreasing and increasing their expression levels in the ancestral salivary glands, esophagus, or mid-esophageal gland at each internal node of the gastropod phylogeny. The expression changes were calculated based on gene expression reconstruction at each node of the phylogeny. b) Species phylogeny with the number of the internal nodes. c) Number of OEG-, LEG-, and VG-specific OGs which were tissue-specific in the ancestral mid-esophageal gland, salivary glands, and esophagus at each node of the phylogeny. d) Ancestral reconstruction of gene expression of a galectin (OG2631) and selenoprotein F (OG465) in the mid-esophageal gland and esophagus. The missing tissues are marked with a *.