Duplicated NELL2 genes show different expression patterns in two rainbow trout strains after temperature and pathogen challenge

Mammalian neural epidermal growth factor-like-like 1 and 2 genes (NELL1 and NELL2) encode multifunctional glycoproteins involved in cell growth regulation and differentiation. We isolated two closely related NELL2 transcript sequences from rainbow trout (Oncorhynchus mykiss). These conceptually translated NELL2a and -b sequences share 83% identical residues and the NELL-typical structure. Phylogenetic analyses suggest that bony fish possess two NELL genes, though these are either present as NELL1/2 pair corresponding to their mammalian orthologs or as a NELL2a/2b combination that might have arisen by a fish-specific duplication event. Both trout NELL2 genes are highly expressed at early developmental stages. In adult rainbow trout, NELL2a copies were detected in each tissue analyzed, whereas NELL2b is abundantly expressed only in brain. Cerebral NELL2a/b gene expression seems to be temperature-independent, whereas NELL2a gene expression is clearly down-regulated in gill and up-regulated in muscle tissue after temperature elevation. Infection with Aeromonas salmonicida leads to a considerable increase in NELL2a/b copy number in trunk kidney at day 7 p.i. Moreover, comparative qRT-PCR revealed different NELL2a/b expression pattern in two rainbow trout strains, imported rainbow trout TCO and a local trout selection strain BORN that is known to be resistant to several biotic and abiotic stressors.