248
M.L. Cancela et al.
was identified (Gonzalez et al. 2007a). The Cg-BPI expression is constitutive in
the epithelial cells of most tissues and induced in hemocytes after bacterial challenge in adult oyster (Gonzalez et al. 2007) and during development (Tirape et al.
2007). In addition, after infection of oyster with the pathogen Vibrio aesturianus,
the expression of the antioxidant enzyme Cg-SOD, specifically produced in hemocytes (Gonzalez et al. 2005), was strongly decreased in parallel to an increase in
Reactive Oxygen Species (ROS) production (Labreuche et al. 2006b). These results
are in contrast with other works where bacteria were reported to cause a reduction of
ROS production by hemocytes (Lambert et al. 2003). Labreuche et al. (2006b) suggested that these events could lead to an oxidative stress and allow Vibrio to escape
host cellular responses. In scallop, a member of the catalase family, which includes
enzymes involved in eliminating ROS by degradation of hydrogen peroxide, was
characterized and found to be up-regulated following challenge with Vibrio (Li et al.
2008). Concerning the antimicrobial peptides, modulations of their expression were
reported after bacterial challenges (Mitta et al. 1999a, Mitta et al. 2000a,b, Zhao
et al. 2007; Gonzalez et al. 2007, Gestal et al. 2007) suggesting a possible action of
peptides against Vibrio. Thus, the specific role of effectors in the immune response
to Vibrio remains to be elucidated. It will be very interesting to understand the role
of inducible and constitutive genes in the immune system and to determine how
bivalves discriminate between pathogen bacteria and various commensal microflora.
7.6.4.5 Status of Transcriptomic Tools
Until recently, most studies aiming at elucidating bivalve-parasite interaction were
done using the Pacific oyster. Nowadays, cDNA libraries, EST collections and
genomic sequencing of a wide range of bivalve species are becoming common practices in most laboratories and research projects, thus increasing genomic resources
worldwide. Major achievements in bivalve genomics resulted from joint research
performed within the scope of large consortiums like Marine Genomics Europe
(www.marine-genomics-europe.org) or American Marine Genomics consortium
(www.marinegenomics.org), whose major purposes were the massive sequencing
of EST and BAC libraries in order to identify candidate genes with putative functions in cellular and biochemical processes. Using this approach, hemocyte genes
related with immune defense of bacteria-challenged Pacific oysters (Gueguen et al.
2003) and Perkinsus-challenged Manila clam (Kang et al. 2006) have been identified. The end of Marine Genomics Europe project in 2008 left a huge legacy in
terms of bivalve EST collections, including ESTs from C. gigas, R. decussatus,
R. philippinarum, Mytilus edulis, and Bathymodiolus azoricus (Tanguy et al. 2008).
As of August 2008, EST collections from 19 different species have been posted in
public databases (www.ncbi.nlm.nih.gov). The most represented species is C. gigas
with 29,018 ESTs available, followed by Mytilus californianus and C. virginica with
23,871 and 14,560 ESTs, respectively. Hence, the most represented organisms are
oysters, mussels, clams and scallops, all of which are farmed species. In parallel,
ESTs from 2 freshwater mussels Dreissena rostriformis bugensis and Dreissena
polymorpha, with no commercial value but becoming increasingly problematic
invasive species in many different countries, were collected in order to attempt to
M.L. Cancela et al.
was identified (Gonzalez et al. 2007a). The Cg-BPI expression is constitutive in
the epithelial cells of most tissues and induced in hemocytes after bacterial challenge in adult oyster (Gonzalez et al. 2007) and during development (Tirape et al.
2007). In addition, after infection of oyster with the pathogen Vibrio aesturianus,
the expression of the antioxidant enzyme Cg-SOD, specifically produced in hemocytes (Gonzalez et al. 2005), was strongly decreased in parallel to an increase in
Reactive Oxygen Species (ROS) production (Labreuche et al. 2006b). These results
are in contrast with other works where bacteria were reported to cause a reduction of
ROS production by hemocytes (Lambert et al. 2003). Labreuche et al. (2006b) suggested that these events could lead to an oxidative stress and allow Vibrio to escape
host cellular responses. In scallop, a member of the catalase family, which includes
enzymes involved in eliminating ROS by degradation of hydrogen peroxide, was
characterized and found to be up-regulated following challenge with Vibrio (Li et al.
2008). Concerning the antimicrobial peptides, modulations of their expression were
reported after bacterial challenges (Mitta et al. 1999a, Mitta et al. 2000a,b, Zhao
et al. 2007; Gonzalez et al. 2007, Gestal et al. 2007) suggesting a possible action of
peptides against Vibrio. Thus, the specific role of effectors in the immune response
to Vibrio remains to be elucidated. It will be very interesting to understand the role
of inducible and constitutive genes in the immune system and to determine how
bivalves discriminate between pathogen bacteria and various commensal microflora.
7.6.4.5 Status of Transcriptomic Tools
Until recently, most studies aiming at elucidating bivalve-parasite interaction were
done using the Pacific oyster. Nowadays, cDNA libraries, EST collections and
genomic sequencing of a wide range of bivalve species are becoming common practices in most laboratories and research projects, thus increasing genomic resources
worldwide. Major achievements in bivalve genomics resulted from joint research
performed within the scope of large consortiums like Marine Genomics Europe
(www.marine-genomics-europe.org) or American Marine Genomics consortium
(www.marinegenomics.org), whose major purposes were the massive sequencing
of EST and BAC libraries in order to identify candidate genes with putative functions in cellular and biochemical processes. Using this approach, hemocyte genes
related with immune defense of bacteria-challenged Pacific oysters (Gueguen et al.
2003) and Perkinsus-challenged Manila clam (Kang et al. 2006) have been identified. The end of Marine Genomics Europe project in 2008 left a huge legacy in
terms of bivalve EST collections, including ESTs from C. gigas, R. decussatus,
R. philippinarum, Mytilus edulis, and Bathymodiolus azoricus (Tanguy et al. 2008).
As of August 2008, EST collections from 19 different species have been posted in
public databases (www.ncbi.nlm.nih.gov). The most represented species is C. gigas
with 29,018 ESTs available, followed by Mytilus californianus and C. virginica with
23,871 and 14,560 ESTs, respectively. Hence, the most represented organisms are
oysters, mussels, clams and scallops, all of which are farmed species. In parallel,
ESTs from 2 freshwater mussels Dreissena rostriformis bugensis and Dreissena
polymorpha, with no commercial value but becoming increasingly problematic
invasive species in many different countries, were collected in order to attempt to