World Aquaculture 41 titer. Whereas no significant difference was reported in total serum protein concentration, antiprotease activity and bacterial agglutinin level between the two lines. Expression of few immune genes (viz. transferrin, complement factor C3 and TLR 22-like transcripts) were significantly greater in liver samples of the susceptible line, while no such difference was found in β2 microglobulin and lysozyme gene expression between the two lines. Research on molecular markers, like SNPs and microsatellites associated with aerononiasis-resistance, is also being conducted by the group in collaboration with Nofima, Norway. Clarias catfish Aquaculture plays a special role in the economy of Thailand. Among cultured species (including tilapias, Puntius, and Pangasius), two of the catfishes Clarias macrocephalus and C. batrachus are the most popular. Hybrids such as “Big-Oui” (a hybrid of broadhead catfish Clarias macrocephalus and African sharptooth catfish Clarias gariepinus) can reach market size faster than either parental species. However, reported disease outbreaks also increased in conjunction with increased use of hybrid catfish (Areechon 1992). The resistance and immunological response to A. hydrophila infection by hybrids was compared with that of the parent species (Areechon and Karoon 1995). The researchers conducted challenge studies with these three types of fishes (both vaccinated and control groups) and studied the immune response with an antibody titer of the vaccinated group. Hybrids had greater resistance than broadhead catfish but African catfish were the most resistant among the three. There was an active immune response and less mortality in vaccinated groups, signifying that more research on vaccines is needed. Unfortunately, no further work has been reported from the group. Developments in Selection Procedures Previously it was believed that selection could be performed on two bases: directly by challenge survival results and indirectly by immunological markers. With the advancement of molecular techniques, molecular markers are now used in Marker Assisted Selection (MAS) programs. Direct Challenge Test The challenge test is the traditional method for selection with some limitations. Injection with a LD50 dose of bacteria intramuscularly (Lio-Po et al. 1996) or intraperitoneally (Angka 1990, Sahoo and Sahoo 1997) and calculation of survival 48 hours after infection was most practicable. However, this approach bypasses the natural defense barriers of skin and mucous membranes, the two most important barriers against bacterial entry. Challenging experimental fish with cohabitants of bacteria-injected fishes of the same size is also not possible for aeromoniasis because there is complete mortality of cohabitants within 48 hours, while no or low mortality occurs in control groups, making it difficult to develop a proper coFig. 2. Spleen sections of aeromoniasis-infected rohu showing focal necrosis and increased numbers of melanomacrophage centers (H & E × 100). habitation challenge model (Mahapatra et al. 2008, Jeney et al. 2011). For selection of breeders, fish that survive after challenge can be more efficiently utilized. However, with the hypothesis that these fish could serve as carriers of pathogens to the next generation, selection of fish from the same family remains preferable. Based on the challenge test, two lines of fish, resistant and susceptible, can be generated through Bulk Segregant Analysis (BSA). The segregation here would be on the basis of phenotypic character (resistance or susceptibility) of the animals. Immunological Markers Wiegertjes et al. (1996) detailed a comparative approach to immunogenetic studies of disease resistance in various fish species and discussed its application to selective breeding, suggesting the immunological parameters that function as a response to selection. Immune responses to A. hydrophila injection were correlated with resistance. Although Sahoo et al. (2004) found no significant difference in specific immune response measured through haemagglutination titer against sheep red blood cells and in a few nonspecific immune responses, such as lysozyme activity and neutral heamolysin titer. Only serum bacterial activity was positively correlated with survival from exposure to aeromoniasis. In catfish, there is no correlation of antibody titer with resistance but the immune response to A. hydrophila infection varies among groups (Areechon and Karoon 1995). Total IgM level can be used as an indirect marker for selection in common carp against aeromoniasis (Jeney et al. 2009) and in Atlantic salmon against furunculosis (Lund et al. 1995). There are significant differences in immune response, as measured by phagocytic and lysozyme activities and specific antibody levels, between resistant and susceptible groups of common carp challenged with A. hydrophila (Ardo et al. 2010). However, there was no correlation between respective control groups. Sahoo et al. (2008) observed no corre-
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