Histology and transcriptomic analyses of barnacles with different base materials and habitats shed lights on the duplication and chemical diversification of barnacle cement proteins

Abstract

Background: Barnacles are sessile crustaceans that attach to underwater surfaces using barnacle cement proteins. Barnacles have a calcareous or chitinous membranous base, and their substratum varies from biotic (e.g. corals/sponges) to abiotic surfaces. In this study, we tested the hypothesis that the cement protein (CP) composition and chemical properties of different species vary according to the attachment substrate and/or the basal structure. We examined the histological structure of cement glands and explored the variations in cement protein homologs of 12 barnacle species with different attachment habitats and base materials. Results: Cement gland cells in the rocky shore barnacles Tetraclita japonica formosana and Amphibalanus amphitrite are eosinophilic, while others are basophilic. Transcriptome analyses recovered CP homologs from all species except the scleractinian coral barnacle Galkinia sp. A phylogenomic analysis based on sequences of CP homologs did not reflect a clear phylogenetic pattern in attachment substrates. In some species, certain CPs have a remarkable number of paralogous sequences, suggesting that major duplication events occurred in CP genes. The examined CPs across taxa show consistent bias toward particular sets of amino acid. However, the predicted isoelectric point (pI) and hydropathy are highly divergent. In some species, conserved regions are highly repetitive. Conclusions: Instead of developing specific cement proteins for different attachment substrata, barnacles attached to different substrata rely on a highly duplicated cementation genetic toolkit to generate paralogous CP sequences with diverse chemical and biochemical properties. This general CP cocktail might be the key genetic feature enabling barnacles to adapt to a wide variety of substrata.