Although polymorphism in major histocompatibility complex (MHC) genes has been thought to confer populations with protection against widespread decimation by pathogens, this hypothesis cannot explain the type of large allelic diversity in classical MHC class I (Ia) in rainbow trout. Based on expression of Onmy-UBA (MHC class Ia) in trout neurons, we hypothesized that polymorphism in trout class Ia may contribute to polymorphism in behavioral traits. The present study examined whether polymorphism in Onmy-UBA was associated with behavioral variation in Donaldson rainbow trout (Oncorhynchus mykiss) using experiments on food competition, lure-catch, fright recovery, diel locomotor activity and activity characterized as dominance or aggression. These behavioral traits were investigated in fish having Onmy-UBA*401/*401 or*4901/ *4901 homozygous, or Onmy-UBA*401/*4901 heterozygous genotypes (referred to as BB, FF and BF, respectively). The BB fish exhibited boldness, aggression, faster growth and crepuscular activity, while the FF fish showed little boldness, smaller body size, and diurnal activity with no aggressive behavior. The BF fish displayed traits intermediary to those of the BB and FF fish. These results are consistent with polymorphism in a single MHC class Ia locus driving variation in neural circuits, thereby creating behavioral variation in the trout. This is the first study in any animal to show a potential correlation between polymorphism in MHC class Ia genes with polymorphism of behavioral traits such as aggression.
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