WCT Multi-state Assessment February 10, 2003 



arbitrary limit of admixture (introgressive hybridization) below which a population will be 

 considered 'pure" (e.g. 1%, 10%) are problematic because, as cited above, the amount of 

 admixture in many WCT populations is rapidly increasing. Research reporting the rapid spread 

 of introgression is significant and will have to be considered carefiilly by the agencies 

 responsible for managing these particular WCT populations. However, as reviewed previously, 

 it is highly unlikely that every WCT population that has experienced some level of hybridization 

 and introgression would experience an increase in the percentage of RBT introgression over time 

 or that introgression would spread rapidly from one population to many populations throughout a 

 drainage. Importantly, the reportedly continuing spread of RBT introgression within the 

 Flathead River system is likely due to the establishment of self-reproducing populations of 

 introduced rainbow trout and the dispersal of hybrids into areas containing pure cutthroat 

 populations (Hitt et al. submitted). In the case of the observed increase in hybridization and 

 introgression within the tributaries of the upper Kootenai River, those authors mention that 'the 

 most likely reason for the apparent increase is the continued and expanded introductions of 

 rainbow trout into the Koocanusa Reservoir and adjacent tributaries" (Rubidge et al. 2002). 



It is also important to separate out two different issues with regards to setting limits of 

 introgression. One issue would be the scientific rigor and precision associated with estimating 

 the level of introgression in a population using molecular genetic information. It may be 

 reasonable to set a limit of introgression below which a population will be considered 'pure' if it 

 is appropriate to be conservative due to imprecision associated with the genetic markers. 

 Genetic markers used to detect introgressive hybridization are often assumed to be "fixed" 

 between RBT and WCT (meaning that a certain marker is only observed in RBT and never 

 observed in WCT or vice versa). However, markers continually have to be tested to ensure that 

 they are in fact fixed within populations. The recent work by Rubidge et al. (2001) reports that 

 the nuclear DNA marker Ikaros (IK) digested with Hinf-I yields fixed differences between RBT 

 and WCT. Work by IDFG on WCT populations in the Middle Fork of the Salmon River 

 indicates that the IKJHinf-I marker is not fixed within these populations, stressing the importance 

 of using multiple diagnostic genetic markers when assessing introgressive hybridization. 



Hitt (2002) (using dominant PFNE markers) described procedures for being conservative in 

 describing a population as admixed or not following procedures outline by Forbes and Allendorf 

 (1991). When individuals from a population only show a "RBT" band (based on its 

 electrophoretic mobility through a gel) at one marker/locus, then the population is considered 

 pure and the observed "RBT" band is considered to be a WCT allele with the same 

 electrophoretic mobility as the true diagnostic RBT allele. Hitt (2002) described 6 populations as 

 being unhybridized WCT populations despite that fact that they exhibited "RBT" bands. These 

 "RBT" bands were used as evidence for RBT introgression in other populations when other 

 diagnostic markers also demonstrated RBT introgression. 



A second issue regarding setting limits of admixture involves the setting of introgression levels 

 at some level from which populations should be prioritized and conservation and management 

 decisions made (e.g. Cutthroat Trout Management: A Position Paper. Genetic Considerations 

 Associated with cutthroat trout management UDWR 2000; 



http://www.nr.utah.gov/dwr/PDF/cuttpos.PDF ). This document was developed by the states of 

 Colorado. Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming, to help guide managers 



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