Jackknife-corrected parametric bootstrap estimates of growth rates in bivalve mollusks using nearest living relatives

Inhibition of growth in juvenile manila clam Ruditapes philippinarum: Potential adverse outcome pathway of TBBPA

Tetrabromobisphenol A (TBBPA) is ubiquitous and its contents showing an increasing trend in the coastal environment. In order to investigate the effects of TBBPA on marine bivalves, juvenile manila clams Ruditapes phillipinarum were exposed to TBBPA for 28 days. The results showed that shell growth rate of juvenile clams after exposure to 62.5-1000 μg L^-1 TBBPA for 28 d were significantly inhibited (p < 0.05). Then in order to link the changes in filtration rate, mRNA expression of insulin-like growth factor homologue (IGF) and tissue thyroid hormone (TH) contents to growth, juvenile clams were exposed to 62.5 and 500 μg L^-1 TBBPA for 14 days. The transcriptional levels of neuroendocrine signals (NPF and insulin homologue) associated with filter feeding regulation, and genes of TH synthesis-related enzymes were also examined. The results showed that filtration rates was significantly reduced to 44.1% and 14% of controls after 14 d of exposure. In parallel, exposure to TBBPA significantly increased the expression levels of insulin which may elicit the filter feeding inhibition. TBBPA exposure caused alterations in tissue content of THs and mRNA expression of TH synthesis-related enzymes. However, the data showed increased T₃ content, T₃/T₄ ratio and mRNA expression of IGF. These data demonstrated that the most important key event of TBBPA could be linked to growth impairment in juveniles was the reduction of filtration rate. These results provide a robust framework towards revealing the underlying mechanism of the growth inhibition caused by TBBPA on bivalves and understanding the adverse outcome pathway across taxonomic phyla.

A Microarray Study of Carpet-Shell Clam (Ruditapes decussatus) Shows Common and Organ-Specific Growth-Related Gene Expression Differences in Gills and Digestive Gland

Growth rate is one of the most important traits from the point of view of individual fitness and commercial production in mollusks, but its molecular and physiological basis is poorly known. We have studied differential gene expression related to differences in growth rate in adult individuals of the commercial marine clam Ruditapes decussatus. Gene expression in the gills and the digestive gland was analyzed in 5 fast-growing and five slow-growing animals by means of an oligonucleotide microarray containing 14,003 probes. A total of 356 differentially expressed genes (DEG) were found. We tested the hypothesis that differential expression might be concentrated at the growth control gene core (GCGC), i.e., the set of genes that underlie the molecular mechanisms of genetic control of tissue and organ growth and body size, as demonstrated in model organisms. The GCGC includes the genes coding for enzymes of the insulin/insulin-like growth factor signaling pathway (IIS), enzymes of four additional signaling pathways (Raf/Ras/Mapk, Jnk, TOR, and Hippo), and transcription factors acting at the end of those pathways. Only two out of 97 GCGC genes present in the microarray showed differential expression, indicating a very little contribution of GCGC genes to growth-related differential gene expression. Forty eight DEGs were shared by both organs, with gene ontology (GO) annotations corresponding to transcription regulation, RNA splicing, sugar metabolism, protein catabolism, immunity, defense against pathogens, and fatty acid biosynthesis. GO term enrichment tests indicated that genes related to growth regulation, development and morphogenesis, extracellular matrix proteins, and proteolysis were overrepresented in the gills. In the digestive gland overrepresented GO terms referred to gene expression control through chromatin rearrangement, RAS-related small GTPases, glucolysis, and energy metabolism. These analyses suggest a relevant role of, among others, some genes related to the IIS, such as the ParaHox gene Xlox, CCAR and the CCN family of secreted proteins, in the regulation of growth in bivalves.