WCT Multi-State Assessment February 10, 2003 



fitness of hybrids and the extent to which the hybridizing populations depart from Hardy- 

 Weinberg expectations of an ideal population. 



For example, if 20 rainbow trout (breeding aduhs) are introduced onto a cutthroat population (80 

 breeding adults, no other individuals), before any mating, the sample offish is composed of 20% 

 rainbow trout (RBT) alleles and 80% westslope cutthroat trout (WCT) alleles. If the introduced 

 RBT randomly mate with the WCT and the subsequent hybrids are as fit as the parents, then the 

 percentage of RBT alleles and WCT alleles will not change from generation to generation. What 

 will change, early on, is the number of hybrids in the population. Before any mating the number 

 of hybrid individuals is zero. As random mating progresses, the number of hybrids in the 

 population increases each generation until eventually all of the individuals are hybrids and the 

 RBT alleles are randomly distributed throughout the population (a hybrid swarm). The 

 percentage of RBT alleles does not increase, however (the potential effects of drift are ignored 

 for this example). Sample observations would indicate 20% RBT alleles and 80% WCT alleles, 

 which is the true frequencies for the population. If enough diagnostic genetic markers are 

 available to detect introgression in the individual (requiring -15 loci, 30 alleles to detect 20% 

 RBT introgression) then a genetic screen will likely demonstrate that all indi\'iduals sampled are 

 hybrids to some degree and the level of introgression among the individuals will be consistent 

 with a binomial distribution of RBT alleles across the population. The more diagnostic loci 

 available, the greater power to detect introgression at low levels in the population and individual. 



The increase or decrease of RBT introgression (the percentage of RBT alleles within a 

 population) depends on whether new RBT alleles are continually introduced into the population, 

 the relative fitness of hybrid genotypes, genetic drift, and the potential for the increased mating 

 among related individuals (phenotypic advantage). As new RBT alleles enter the population 

 (stocking) and if hybridization and introgression occurs, the percentage of RBT alleles in the 

 population will increase. If hybrid genotypes/RBT alleles are more fit than WCT 

 genotypes/alleles (outbreeding enhancement or heterosis), then the percentage of RBT alleles in 

 the population will increase even after stocking has stopped due to this selective advantage. 

 Alternatively, if hybrid genotypes/RBT alleles are less fit than WCT genotypes/alleles 

 (outbreeding depression or negative heterosis), then the percentage of RBT alleles in the 

 population will decrease after stocking has stopped, depending on the level in which they are 

 expressed and selected against within the population. Genetic drift (change in allele frequency 

 from generation to generation due to statistical chance) may also change the percentage of RBT 

 alleles within a population, especially if the population is small. However, genetic drift is non- 

 directional, providing equal opportunity for RBT or WCT allele frequencies to change 

 significantly. Rainbow trout alleles will also increase in the WCT population if rainbow trout or 

 hybrid phenotypes are preferred partners for mating (both equally or unequally among sexes). 

 The increase in mating success will result in an overall increase in RBT alleles in the population 

 and a departure from random mating evidenced by examining linkage and/or gametic 

 disequilibrium among individuals. 



Whether the number of populations that are introgressed in an area increases, depends on a 

 number of factors including the stocking history (how long ago were non-native trout stocked, 

 whether non-native trout are stocked in places now that they were not in the past), whether the 

 stocking of non-natves has resulted self-sustaining populations, the dispersal of stocked trout 



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