Morphological and genetic variation in Cisco (Coregonus artedi) and Shortjaw Cisco (C. zenithicus): multiple origins of Shortjaw Cisco in inland lakes require a lake-specific conservation approach

Abstract

The study of cisco diversity in inland lakes of North America has been plagued by taxonomic uncertainty linked to high phenotypic plasticity and an ongoing reliance on morphology to differentiate species. More recently, this uncertainty has hindered the development of conservation plans and status assessments of ciscoes. This study presents the first range-wide comparison of morphological and genetic variation between Cisco (Coregonus artedi) and Shortjaw Cisco (C. zenithicus). Using morphological and genetic data from 17 lakes, three sets of analyses were undertaken to evaluate alternate hypotheses explaining the pattern of cisco phenotypic diversity in inland lakes. Morphotypes (MTs) representing the two taxa were phenotypically distinct (largely reflective of differences in gill raker number and jaw morphology) within lakes but highly variable across lakes. Shortjaw Cisco was only recognizable when compared to sympatric Cisco and some populations were morphologically similar to Cisco from other lakes. Analysis of AFLP data revealed two genetic clusters that conformed to differences in geography (eastern and western groups), rather than hypothesized taxonomic boundaries. Genetic variation strongly suggests that each of these unique sympatric pairs of MTs originated recently and locally, in parallel, from the ancestral Cisco. Phenotypic and genetic distinctiveness between MTs were not related. MTs were sometimes clearly recognizable despite a lack of genetic differentiation, suggesting that the canalization of phenotypic plasticity is unevenly completed across lakes. These results provide evidence that the taxonbased approach is clearly inadequate for the protection of Shortjaw Cisco. In Canada, status assessment should aim to identify lake-specific designatable units (DU). Given the idiosyncratic nature of each instance of Shortjaw Cisco, it is expected that the strength of morphological, biological, ecological and genetic evidence for individual DUs will vary among lakes.

Evolution and diversity of two cisco forms in an outlet of glacial Lake Algonquin

“…This trend was noted for gill-raker counts by Bell et al (2019) and remains true with additional lakes, including L. Nipigon artedi (Figures 3 and 4a). This pattern is very similar to that uncovered when comparing cisco and shortjaw occurring in lakes over a vast area in Canada (Turgeon et al, 2016). Using local pelagic fish as a reference, comparisons with sympatric benthic ciscoes did reveal significant differences for some or all of the traits mentioned above.…”

“…Lake codes are as per Table 1 quite surprising given past results indicating the sole presence of the Atlantic lineage in a nearby APP lake with cisco and shortjaw (Turgeon et al, 2016). Lake codes are as per Table 1 quite surprising given past results indicating the sole presence of the Atlantic lineage in a nearby APP lake with cisco and shortjaw (Turgeon et al, 2016).…”

“…Ciscoes also display depth partitioning within lakes (Koelz, 1929) that can be associated with trophic position (Blanke, Chikaraishi, & Vander Zanden, 2018;Schmidt, Harvey, & Vander Zanden, 2011). In many instances, a cisco form may only be phenotypically recognizable when compared to a sympatric form (Clarke, 1973;Smith & Todd, 1984;Turgeon et al, 2016). In many instances, a cisco form may only be phenotypically recognizable when compared to a sympatric form (Clarke, 1973;Smith & Todd, 1984;Turgeon et al, 2016).…”

“…Our purpose is not to resolve the thorny issue of ciscoes taxonomy, and this informal naming of an apparent morphotype is not meant to provide a valid taxonomic designation. Both lineages reached the Great Lakes where extant cisco forms display a signature of admixture between these two glacial lineages (Turgeon & Bernatchez, 2001a, 2001bTurgeon et al, 2016). Among cisco forms, artedi is likely the ancestral form because it has the widest distribution across North America (Smith & Todd, 1984;Turgeon & Bernatchez, 2003).…”

“…It is used as a temporary convention, while official status and designation are formally examined. The evolutionary origin of cisco forms has been debated for decades (Behnke, 1972;Smith & Todd, 1984;Todd & Smith, 1992), but a consensus based on genetic evidence is emerging that cisco forms originated from repeated postglacial parallel and local divergences in the Great Lakes as well as in several other inland lakes (Eshenroder et al, 2016;Turgeon & Bernatchez, 2003;Turgeon et al, 2016). Two glacial lineages of artedi were probably involved in the continental postglacial recolonization: a western lineage from the Mississippian refugium in central Canada, and an eastern lineage from the Atlantic refugium (Turgeon & Bernatchez, 2001a, 2001b.…”