For some young fish, early gene expression is a clear harbinger of fated lifestyle

As juveniles, individuals of many fish species face a developmental choice that will profoundly affect their future: whether to adopt a sedentary or migratory lifestyle. Sedentary (or “residential”) individuals remain in the region of their birth, while their migratory compatriots set forth on long open-water journeys. The developmental choice of the residential versus migratory “life history” is known to be influenced by environmental factors, but is not well understood at the genetic level. Researchers now report that fish that are very closely related genetically show dramatically different patterns of actual gene expression if they have adopted different lifestyle fates. Moreover, and perhaps more surprisingly, less-related individuals from geographically different populations nonetheless exhibit very similar patterns of gene expression if they have adopted the same fate–residential or migratory. Thus, the researchers found that levels of expression of a great many genes depend primarily on an individual’s future lifestyle.

The findings, which illuminate how programs of gene expression have evolved to control profoundly different developmental outcomes, are reported in the April 18th issue of Current Biology by Drs. Thomas Giger, Carlo Largiadèr, and Laurent Excoffier of the University of Bern, along with colleagues from France, Ireland, Denmark, and the UK.

Salmonid fish, which include trout and whitefish as well as salmon, show exceptional levels of life-history variation–that is, residential and migratory types often co-occur within a single population of young fish. Before reaching sexual maturity and leaving their natal stream, migratory individuals undergo dramatic morphological, physiological, and behavioral changes that prepare them for adulthood in open fresh and salty waters.

In their innovative work, which is based on studying the gene expression profiles of hundreds of genes at a time in different fish populations, the researchers studied gene expression in two species–strains of the brown trout, Salmo trutta, and a strain of the Atlantic salmon, Salmo salar. The researchers showed that many genes of genetically similar sedentary and migrant populations living in the same river were expressed at different levels. At the same time, two sedentary brown trout populations from Denmark and France, despite having diverged half a million years ago, showed very similar gene expression profiles. This remarkable similarity in gene expression between populations sharing the same life history–but being genetically very divergent and occupying different habitats–suggests that the genetic program of a given life history has been a highly selected attribute during the evolution of brown trout populations.

The findings also indicate that such striking differences in gene expression profiles are probably controlled by only a few major genes.

In addition, the authors showed that while there is a large diversity in gene expression levels between individuals from the same population, the different expression profiles associated with lifestyle fates were so distinct that by measuring the expression levels of relevant genes, it was possible to predict the future lifestyle of fish at the juvenile stage.