Cytotaxonomic Studies of the Coregonine Fishes of the Great Lakes, USA: DNA and Karyotype Analysis

Abstract

Coregonine fishes are difficult taxonomically because they exhibit great plasticity in both morphological and physiological characters. The purpose of this study, based on the Great Lakes coregonine fishes, was to determine from cytogenetic parameters the possible basis for phenotypic plasticity as an approach to understanding systematic relationships within the family Salmonidae."Nearness-of-relationship" among these fishes was judged by the percentage of guanine plus cytosine (G + C mole %) in DNA isolations (from livers) from eight Great Lakes plus one Canadian species of coregonines. Polyploidy as a factor in this group's evolution and speciation was examined by measuring and comparing, by microspectrophotometry, the amount of DNA per corneal epithelial cell for nine coregonine and one thymalline species. The karyotypes of seven whitefishes were determined from blastula materials as the morphology and count of chromosomes have been recognized as species-specific characters.There was a significant difference between the G + C values for species of Prosopium and Coregonus plus Stenodus. These data and those from other parameters cited possibly confirm the early dichotomy between Prosopium and Coregonus. There were no differences in G + C values for the species of Coregonus (true whitefish and cisco) and Stenodus. No one coregonine species had multiple units of the DNA amount nor chromosome number of another species. Therefore, polyploidy as a means of speciation in this group is not operating. A correlation between amount of DNA and chromosome arm number is apparent. Karyotype analysis revealed a stabilization at the diploid number of 80 for species of Coregonus. Prosopium coulteri and P. cylindraceum have a diploid number of 82 and 78, respectively. The major differences among the species-specific karyotypes were in the number of chromosome arms with the more primitive species having higher numbers than the recent species.A hypothesis concerning the possible derivation of cisco from whitefish karyotypes was stated. The similarity of whitefish karyotypes has probably permitted introgression and geneflow in this group. Coregonine genera (Coregonus, Prosopium, and Stenodus) were defined on the basis of the new parameters studied.