The PI and collaborators will use a genomic approach to determine the genetic basis of an important life history polymorphism in rainbow trout. This species has the most complex suite of life-history traits of any species of Pacific salmonid. The rainbow trout has two primary life history types. The anadromous form of rainbow trout is called steelhead. Steelhead spawn and rear in freshwater streams or lakes and then migrate to the ocean for growth and maturation in the more productive marine environment. The resident form of rainbow trout spends its entire life history within freshwater. These two life histories (anadromous and resident) often occur as a life history polymorphism within a single reproductive population. This life history polymorphism is one of the most extraordinary examples of a phenotypic polymorphism within natural populations. This research will be done on natural populations of rainbow trout from two tributaries of the Copper River of Alaska and from two rivers on the Kamchatka Peninsula of Russia. Genetic comparisons of resident and anadromous fish from the same reproductive populations at many loci throughout the genome will allow the PI to identify regions associated with the life history differences. Females are generally in excess in the anadromous form while males are in excess in the resident fish. Thus, it is likely that natural selection is acting in opposite directions for these life histories in male and female rainbow trout (this has been termed sexually antagonistic selection). Models of sex chromosome evolution predict that differential selection on males and females will cause the accumulation of genetic differences on the sex chromosomes. This research will test these predictions. Salmonid fishes are a valuable and fascinating taxon to study evolutionary genetics. All species in the family Salmonidae are descended from a single tetraploid (chromosome doubling) event that occurred approximately 25 million years ago. Salmonids are a valuable model system to study the evolution of duplicate genes and the genetic basis of diploidization of a polyploid genome. The process of diploidization, the evolutionary process by which a polyploid genome turns into a diploid one, remains one of the major unsolved challenges of understanding genome evolution. The objective of this SGER proposal is test for differences in allele frequencies between males and females at sex-linked loci. Intellectual Merit: Understanding the mechanisms responsible for the maintenance of phenotypic and genetic variation in natural populations is one of the primary goals of population genetics. This research will provide valuable information aimed at understanding the genetic basis maintenance of one of the most extraordinary examples of a life history polymorphism within a natural population. In addition, understanding the evolution of sex chromosomes is crucial for understanding molecular mechanisms responsible for sex determination and sexual dimorphisms. This is an excellent model to study the evolution of sex chromosomes because of the presence of undifferentiated sex chromosomes in rainbow trout and the large differences in natural selection between males and females for these two life histories. This research will test existing hypotheses for the evolution of sex chromosomes. Broader Impacts: Many salmonid populations and species are threatened with extinction. Ten groups of rainbow trout are listed as threatened under the U.S. Endangered Species Act. Understanding the genetic basis of complex life history differences in this species is crucial for conservation. In addition, an estimated 25% of all wild salmon in the Pacific return to Kamchatka. The proposed research will provide important biological information to support conservation efforts of these fishes. Even negative results of this pilot study would have important significance for conservation. That is, if we cannot find evidence for a genetic basis of this polymorphism on the sex chromosomes or the autosomes, this would suggest the loss of the SH life history types would not be expected to have a major genetic effect on the population.
