Social Hierarchy and Differential Expression of drd2, mao and nnos in Juvenile Rainbow Trout (Oncorhynchus mykiss)

Juvenile rainbow trout form social hierarchies in the wild where dominant individuals monopolize food and territories through aggressive behavior. Populations of rainbow trout reared in wild and captive environment show variation in social behavior, and this may be influenced by differential expression of associated genes. We tested the establishment of social hierarchies and associated gene expression in F1 hybrids of semi wild (SW) and domesticated clonal lines (HC and AR) of juvenile rainbow trout crossed with heterozygous domesticated females. Dyads of juveniles derived from each clonal line were observed over six to ten days to study the formation and stability of social hierarchies. Fish were categorized as dominant in the dyad if they fed first and more frequently; and they occupied an area of the tank nearest the food source.  Their brain specimens were analyzed for potential differential gene expression using quantitative polymerase chain reaction. mRNA of dopamine receptor (drd2) showed significantly higher expression in dominants across all hybrid populations. mRNA levels of mao and nnos was significantly higher in dominant fish across populations except in HC where subordinate and dominant fish showed similar expression level. These results support previous studies that show differential expression of these genes with respect to social interaction and aggression. This study highlights the modification of behavior associated with gene-by-environment interaction in rainbow trout from different genetic backgrounds.