YCT Multi-state Assessment February 1 0, 2003 



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 IK/Hinf-I marker is not fixed within these populations, stressing the importance 

 of using multiple diagnostic genefic markers when assessing introgressive hybridization. 



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

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

 (1991). When individuals fi^om 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 

 electrophorefic mobility as the true diagnostic RBT allele. Hitt (2002) described 6 populafions 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 ft-om 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: wu\\.nr.uiah.uu\/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 

 working with cutthroat trout. Cutthroat trout with a measured introgression level of less than 1% 

 are designated as "core conservation populations", and are considered pure. The less than 1% 

 limit allows for possible imprecision associated with genetic markers. A second category, 

 "conservation population", is used for populations with less than 1 0% introgression (but may 

 extend to a greater amount depending upon circumstances and the values and attributes to be 

 preserved). The less than 10% criterion is not suggesting that populations with introgression 

 levels between 1% and 10% be considered 'pure' or managed as a 'pure' populations, rather it is 

 an agreed upon decision to manage populations a certain way given that a particular level of 

 introgression is observed (in this case, <10%). Importantly, the primary management goal of the 

 "conservation population" designation is to protect and conserve populations that, while existing 

 in a introgressed condition, still contain a unique or essential portion of ecological, behavioral, 

 physiological, or genetic diversity found within the subspecies. 



A concern with setting such threshold criteria based on percentages is that those criteria may not 

 accurately describe the true hybridization status of a sample location. The percentage 

 corresponds to the number of non-native alleles observed among the total alleles examined, and 

 is only useful in situations where the researcher is using dominant markers and can determine 



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