Shortjaw cisco COSEWIC assessment and status report: chapter 7

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Patterns of Phenotypic and Genetic Variability

Shortjaw ciscoes exhibit morphological variability across their geographic range (Clarke 1973). Previous studies revealed population subdivision in Lake Superior (Todd and Smith 1980), but the possibility that more than one morph of shortjaw cisco existed in Canada had not been studied. Todd and Steinhilber (2002) found that two basic groups of shortjaw ciscoes exist across Canada. The most common morph is found in large lakes such as Lake Superior, Lake Nipigon, and Great Slave Lake and is characterized by more and longer gill rakers than a morph that seems more typical of small lakes such as Basswood Lake (Ontario), George Lake (Manitoba), and Barrow Lake (Alberta). Recent studies on Lake Athapapushkow (Manitoba) suggested that the few-rakered form ascribable to C. zenithicus may actually be a conspecific morph of C. artedi (Aoki 2003). However, most populations of shortjaw ciscoes from the Great Lakes across Canada to the Arctic clearly share a similar morphology.

Many genetic analyses on ciscoes have not proved very informative on population differences (Todd 1981b; Reed et al. 1998; Steinhilber et al. 2002). The cisco species as a whole have sufficient genetic variability to suggest that they contain distinctive populations, at least. However, distinct genetic markers for the individual species have been elusive. Recently, though, analyses of microsatellite polymorphisms from Lake Nipigon ciscoes (Turgeon et al. 1999) revealed that shortjaw ciscoes were genetically differentiated from sympatric lake herring (C. artedi), bloaters (C. hoyi), and blackfin ciscoes (C. nigripinnis regalis) even though a species-specific marker was not observed. In this paper, Turgeon’s findings supported the presence of two major colonizing groups in North American ciscoes (Smith and Todd 1984; Todd and Smith 1992; Turgeon and Bernatchez 2001a,b). However, a broad scale examination of genetic relationships among several cisco taxa throughout North America strongly revealed that five populations of shortjaw ciscoes (White Partridge Lake, Lake Superior, Lake Nipigon, George Lake, Barrow Lake) were genetically more similar to other sympatric or nearby populations of lake herring than they were to each other. Overall, Turgeon’s work strongly suggests that diversification of the cisco assemblage occurred in the postglacial period, as has been shown for several other fish species from postglacial lakes (Schluter 1999).