Axillary gland gene expression and potency reveal the nature of mimetic relationships in Corydoradinae catfishes

The evolution of Mullerian or Batesian mimicry depends on the relative unpalatability of participating taxa, yet the mechanistic basis of such unpalatability remains poorly explored in many systems. Unpalatability in the Corydoradinae catfishes, known for their striking mimetic diversity, is underpinned by sharp, lockable spines, armoured bodies and the production of venoms/toxins. Here, we assess the contribution of the axillary gland, a structure at the base of the pectoral spine, to toxicity in two Corydoradinae genera, Corydoras and Hoplisoma. Using a brine shrimp cytotoxicity assay, we demonstrate that axillary gland extracts are significantly more toxic than muscle extracts in both genera, but toxicity does not differ significantly between genera. Transcriptomic analyses identified 539 candidate toxin genes upregulated in axillary gland tissue, relative to scute tissue, containing signal peptides and had predicted toxin functions based on amino acid sequences. Notably, candidate genes include domains typical of piscine venoms, such as lectins and peptidase S1. Although significant differences in gene expression were detected between genera in candidate toxins, log-fold changes were small, and predicted toxin potency was not significantly different between genera. Together, our findings indicate that axillary gland toxins are likely to contribute to unpalatability in Corydoradinae catfishes and provide support for Mullerian mimicry in this system.