Physiological and genetic aspects of channel catfish immunity: I. Effect of low temperature stress on the distribution of epidermal mucous cells as a factor in saprolegniosis and II. Linkage and function of major histocompatibility genes

Two different aspects of immunity in the channel catfish (Ictalurus punctatus) were studied. The first involved the effect of an acute drop in water temperature on epidermal mucous cell distribution and its role as a contributing factor to saprolegniosis in catfish. Catfish were subjected to a temperature drop from 22 to 10°C in 24 hours. Three days after the temperature drop, there was a marked decrease in the mucous cell numbers in the superficial epidermis. The number of mucous cells returned to normal after fish were maintained at 10°C for six days. If catfish were subjected to a temperature drop and challenged with Saprolegnia , the superficial epidermis mucous cell numbers did not recover to normal levels but continued to decline until the fish died from saprolegniosis. These results explain the dry skin appearance of catfish exhibiting saprolegniosis. Since Saprolegnia cysts are shed in sloughed mucus of normal fish, low temperature-induced mucus loss may explain how Saprolegnia cysts attach and infect catfish skin.; The second study examined the linkage and function of catfish major histocompatibility complex (MHC) genes. Forty to fifty sibling catfish from four different families were analyzed by restriction fragment length polymorphism (RFLP). These analyses revealed that catfish MHC class I and class II gene loci are both polygenic and polymorphic. Comparison of MHC RFLP patterns among siblings demonstrated that MHC class II A and B gene loci segregate together, but independently from MHC class I A gene loci. In contrast to mammals, this finding indicates that catfish MHC class I and class II genes are not linked. With respect to MHC class I and II RFLP patterns, four types of fish were identified from within a family, those with (1) the same MHC class I and II patterns, (2) different MHC class I and II patterns, (3) same MHC class I but different MHC class II patterns, and (4) different MHC class I but same MHC class II patterns. Preliminary functional studies using these fish show that spontaneous nonspecific allogeneic cytotoxic responses are mediated by differences in the MHC class I, but not class II molecules.