To Sea, or Not to Sea: Exploring the Life Histories of Steelhead and Rainbow Trout Through Gene Expression

The functional mechanism behind why some individuals of a salmonid population become anadromous while others remain resident remains a relative mystery.  Studies comparing the anadromous and resident life history forms of several salmonid species show no significant genetic difference between individuals of the same species that express different life history strategies. This is the case for Oncorhynchus mykiss (resident: rainbow trout, anadromous: steelhead) (Beacham and Pollard, 1999; Narum et al. 2004) and is not unexpected given that mating between alternate life history forms is known to occur.  Confirmed cases of anadromous salmonid progeny remaining resident and resident progeny becoming anadromous are also common. Several studies have focused on the effect of environmental conditions (stream temperature, stream flow, food availability, etc.) on juvenile development into different life history strategies; however there have been relatively few studies that focus on how environmental conditions impact gene expression (Lee and Powers 1976; Metcalf et al. 1989; Hendry et al. 2004).  We used microarray profiling to investigate gene expression patterns that are associated with alternate life histories in juvenile O. mykiss from the John Day River Basin (Oregon).  Expression profiles were generated for gill and brain tissue from hatchery stocks of juvenile steelhead and rainbow trout as well as from similar size fish caught in a number of tributaries of the John Day River known to produce different ratios of anadromous and resident life history variants.  Probes generated from this RNA were applied to Agilent 44K oligonucleotide arrays (release date 06/08) developed from Atlantic salmon sequence information and commonly used for all salmonid species.  Our results suggest that pre-smoltification juvenile O. mykiss can be identified as anadromous or resident based on their gene expression patterns.  We are currently using these data to develop biomarkers that could be used in the field to identify different life history variants, as well as investigating the molecular basis of this major life history decision.