If you’ve watched any number of nature shows in your lifetime, you’ve seen the astounding migrations made by salmonid fishes. You can count on seeing a shot of salmon darting against the current and catapulting themselves over turbulent falls (like this!). These migrations between freshwater streams and the ocean are spectacular for both their magnitude and difficulty, but the changes that happen within each fish to get them to migrate in the first place might be just as interesting.
The two forms of Oncorhynchus mykiss: the anadromous steelhead (top) and the resident rainbow trout (bottom)
This month’s issue of Molecular Ecology includes a new study from Garrett McKinney and colleagues that compares the gene expression patterns within brains of rainbow trout that are resident or migrant forms. The rainbow trout form that completes long migration events to the ocean and back, called Steelhead or anadromous, undergo striking changes in phenotype to make these journeys. This includes a different body shape, different coloration, and various physiological changes to deal with saltwater. These developmental changes have been previously associated with genetic differences, but little is known about how and when those genetic differences manifest themselves.
Currently, studies of transcriptome-wide patterns of gene expression in salmonids have largely ignored ontogenetic changes during early development and little is known about the timing of activation of molecular pathways that regulate phenotypic differentiation.
McKinney and colleagues generated transcriptomes from the brain tissue of trout that were migratory or resident types. This sampling happened at multiple points over a year, and the authors showed that major differences in gene expression happen at around eight months, especially in males.
The majority of differentially expressed genes between migrants and residents were unique not only to a single time point but also to a single sex, indicating possible temporal differences in gene expression during development and significant sex
differences. This raises the possibility that males and females may be developing at different rates or utilizing different molecular pathways during development.
At eight months old, these fish are still a year away from the big phenotypic differences that aid in migration, but their expression pathways are already cranking up proteins that are specific to those physiological differences. In addition, the authors map these expression differences to previously-documented QTLs and chromosomes that are associated with migration phenotypes.
As with other transcriptome-based research on non-model organisms, the authors are limited in what genes that can actually annotate, so who knows how many undescribed genes are also determining what fish “just keep swimming”.
McKinney G.J., Hale M.C., Goetz G., Gribskov M., Thrower F.P. & Nichols K.M. (2015). Ontogenetic changes in embryonic and brain gene expression in progeny produced from migratory and resident Oncorhynchus mykiss , Molecular Ecology, 24 (8) 1792-1809. DOI: http://dx.doi.org/10.1111/mec.13143