Abstract

The invertebrate immune response is a non-adaptive system, based on both cellular and humoral components. Cellular defense comprises a variety of hemocyte types, carrying out phagocytosis process, and cytotoxic or inflammatory responses, while humoral defense involves a receptor-mediated recognition of molecular patterns associated to microorganisms. Herein, the receptors allow the identification of self and non-self components, being their molecular signaling pathways capable of triggering the transcriptome changes involved in the innate immune responses. Thus, invertebrates have a broad suite of possible innate immune responses against pathogens and marine environmental stressors. Despite our quite extended knowledge about innate immune system in invertebrate species in response to pathogen challenges, only very scarce studies investigate immune responses of bivalves exposed to harmful phytoplankton species, capable of having lethal and sublethal impacts on bivalves. Within this framework, there are strong evidences of the effects produced by harmful algal blooms (HABs) on bivalve physiology (e. g. decreasing clearance rate, growth rate of soft tissues, and condition index). Further, it has been shown that harmful algae can produce histopathological lesions and impair immune responses in commercially harvested bivalves exposed to marine toxins. Cellular and humoral immune responses of several bivalve species have indeed been inhibited by HAB species of the genus Alexandrium. Yet, there are pivotal questions unresolved regarding the genomic basis involved in immune response to HABs. For instance, could transcriptome changes in bivalves be triggered by marine toxins such as saxitoxin (SXT)? Is it possible to identify candidate genes involved in innate immune responses to SXT? Are there differences among gene expression from tissues and hemocytes under exposure to SXT? Is there any relationship between physiological responses and immune-related gene expression of bivalves exposed to SXT? Are there variations of immune-related gene expression among populations and species? To answer these questions, our proposal aims to analyze the transcriptomic changes in Mytilus chilensis and Mesodesma donacium challenged by A. catenella, a PSP producer, and get novel insights about immune-related genes in bivalves exposed to saxitoxin. Furthermore, we propose to determine gene expression and cell defense using short-term hemocyte cultures challenged to HAB. Herein, the goals will be to (1) Perform a transcriptome characterization by 454 pyrosequencing, (2) Perform an in silico analysis to discover and identify candidate genes associated to immune response against HAB. (3) Determine the association of immune-related genes expression in several bivalve tissues exposed to A. catenella. (4) Establish expression patterns of candidate genes and cell responses on short-term hemocyte cultures challenged to both A. catenella and SXT, and (5) Determine the variation of gene expression among individuals from different populations for M. chilensis and M. donacium. To accomplish these goals, the experimental design will be carried out by challenge trials (in vivo and in vitro exposure to analyze hemocytes), followed by High-throughput transcriptome sequencing (RNA-seq) analysis. This proposal will be conducted in collaboration with Dr. Allisson Astuya and Dr. Daniel Varela, who have expertise respectively in hemocyte cell cultures and ecophysiology of phytoplankton. In parallel, this study will have the international cooperation of Dr. Hélène Hégaret (IUEM/UBO, France). Her expertise in bivalve physiology exposed to HABs is recognized worldwide through her publications on this area. The expected outcomes is to establish how bivalve organisms may respond to marine toxins, and thus, develop molecular tools to improve our ability of predict the possible impacts of HAB on native mollusk species, but also on commercial activities such as aquaculture and fisheries.