Endocrine Responses of Fast- and Slow-Growing Families of Channel Catfish

—Sixty-eight families of USDA303 channel catfish Ictalurus punctatus were evaluated for growth performance for 30 d. The fastestand slowest-growing catfish families were further evaluated to examine the hypothesis that genes or gene products associated with the growth regulatory and stress axes can be used to describe differences in growth performance. Research examined mRNA levels of genes involved in the growth hormone–insulin-like growth factor (GH–IGF) network in fast(family A) and slow-growing (family H) USDA303 catfish. Fish (59.0 6 2.4 g) were fed for 7 weeks, weighed, and had tissues for RNA extraction. The remaining fish were subjected to an acute 10-min dewatering stress. Insulin-like growth factor-II mRNA was higher in the muscle of fast-growing fish, while the levels of IGF-I receptor (IGF-IR) and IGF-II receptor (IGF-II) were similar. Muscle IGF-IIR mRNA was two-fold higher than muscle IGF-IR mRNA. There were no differences in liver and muscle IGF-I and GH receptor mRNA or pituitary GH mRNA between the fastand slow-growing fish. Fast-growing fish consumed 135% more feed than slow-growing fish, though the abundances of ghrelin mRNA in the gut and neuropeptide Y mRNA in the hypothalamus were similar. Cortisol levels were negatively correlated to weight gain. These results suggest that the variation in growth between fastand slow-growing USDA303 catfish is explained, in part, by the variation in the GH–IGF and stress axes. The relationship between cortisol and weight gain warrants further investigation for possible exploitation in our selective breeding program. The USDA103 research strain of channel catfish Ictalurus punctatus was developed by family selection for improved growth rate. Continued selection within this line is focused on growth, fillet yield, and resistance to Edwardsiella ictaluri, the bacterium that causes enteric septicemia of catfish. The current method of identifying superior broodstock for growth is to conduct growth studies with large numbers of catfish families. This method is expensive and labor intensive owing to the numbers of families that are evaluated each year (approximately 100). The impetus to the current research is the desire to understand the physiological and genetic basis for the variation in growth phenotypes that is observed in our catfish families. We are interested in identifying markers that can be used to identify individuals with superior growth characteristics. Genetic variation that affects growth will probably be reflected in the growth hormone–insulin-like growth factor (GH–IGF) regulatory axis. Pérez-Sánchez and Le Bail (1999) first suggested the use of the GH–IGF axis as a marker for growth performance in fish. In support of their hypothesis, GH levels were shown to be higher in faster-growing domestic Atlantic salmon Salmo salar than in a wild population (Fleming et al. 2002). Variations in growth performance among broodstock rainbow trout Oncorhynchus mykiss families were explained by variations in the resting levels of GH and IGF-I (Lankford and Weber 2006). In channel catfish, IGF-II mRNA levels were higher in both muscle and liver of faster-growing fish (Peterson et al. 2004b). These results suggest a genetic basis for the variation in the growth performance of the broodstock and families of fish. In addition, genes and gene products related to the GH–IGF axis partially explained differences in superior growth performance. Thus, these molecules could be used as markers for selective