Abstract
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.
Highlights
Growth is one of the most characteristic features of biological entities, and has a significant importance for food production
While it is clear that many molecular processes that act at the level of cell growth, cell division, and organogenesis are common to mammals and flies, and should be acting in other zoological groups as well, the fact that mammals and insects show usually determinate growth while most other invertebrates usually do not (Sebens, 1987; Charnov et al, 2001) suggests that some fundamental aspects of growth regulation will be different
No significant gene ontology (GO) term was significantly enriched (P < 0.05) in the subset of differentially expressed genes (DEG) upregulated in F clams, but 14 terms were enriched in the subset of DEGs upregulated in S clams (Table 5)
Summary
Growth is one of the most characteristic features of biological entities, and has a significant importance for food production. Non-model organisms, especially invertebrates, are far from this level of understanding. While it is clear that many molecular processes (e.g., signaling pathways) that act at the level of cell growth, cell division, and organogenesis are common to mammals and flies, and should be acting in other zoological groups as well, the fact that mammals and insects show usually determinate growth (i.e., growth stops when they reach a certain size) while most other invertebrates usually do not (Sebens, 1987; Charnov et al, 2001) suggests that some fundamental aspects of growth regulation will be different. Bivalve mollusks make an interesting experimental material for studying growth in invertebrates with indeterminate growth. Since several mollusk species are important as food, the study of the growth rate differences among individuals and species has a special relevance in this zoological group because it can help in designing adequate husbandry procedures and to increase yield (Wilbur and Owen, 1964)
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