Acontia, a Specialised Defensive Structure, Has Low Venom Complexity in Calliactis polypus

Abstract

Phylum Cnidaria represents a unique group among venomous taxa, with its delivery system organised as individual organelles, known as nematocysts, heterogeneously distributed across morphological structures rather than packaged as a specialised organ. Acontia are packed with large nematocysts that are expelled from sea anemones during aggressive encounters with predatory species and are found in a limited number of species in the superfamily Metridioidea. Little is known about this specialised structure other than the commonly accepted hypothesis of its role in defence and a rudimentary understanding of its toxin content and activity. This study utilised previously published transcriptomic data and new proteomic analyses to expand this knowledge by identifying the venom profile of acontia in Calliactis polypus. Using mass spectrometry, we found limited toxin diversity in the proteome of acontia, with an abundance of a sodium channel toxin type I, and a novel toxin with two ShK-like domains. Additionally, genomic evidence suggests that the proposed novel toxin is ubiquitous across sea anemone lineages. Overall, the venom profile of acontia in Calliactis polypus and the novel toxin identified here provide the basis for future research to define the function of acontial toxins in sea anemones.

Genomic, functional and structural analyses elucidate evolutionary innovation within the sea anemone 8 toxin family

“…As none of the peptides identified in the milked venom of A. tenebrosa had significant similarity to SA8 sequences, the SA8 sequences in this species are probably non-venom peptides. This suggests that the inversion event, in concert with tandem duplication, may have facilitated the recruitment of this SA8 peptide into the venom of T. stephensoni , with SA8 only previously found in the acontial proteome of C. polypus [ 62 ]. Furthermore, the inverted gene is found in a well-supported clade on a branch sister to a Metridioidea-specific clade (Fig.…”

“…Previous research provided evidence for minimal transcriptional activity within T. stephensoni acontia, with maturation of nematocysts occurring in proximity to the acontia-mesenterial filament junction [ 58 , 59 , 61 ], which may explain why the venom SA8 peptide is associated with high gene expression in the mesenterial filaments. Furthermore, a SA8 peptide was recently discovered in the acontia proteome of C. polypus [ 62 ]. Overall, MSI data provide evidence for acontial, mesenterial and pedal disc localisation of the venom SA8 peptide in T. stephensoni and are consistent with the higher expression of the inverted SA8 gene in the gastrodermis and epidermis.…”