Evolutionary relationships of Eastern Palearctic Brown Frogs, genus Rana: paraphyly of the 24-chromosome species group and the significance of chromosome number change

Abstract

The Eurasian 'brown frogs' are a morphologically conservative assemblage consisting of the European Rana temporaria and a large number of similar species considered to be related. Although the chromosome number of the great majority of Rana species is 26, there are several species of brown frogs (including R. arvalis, R. chensinensis, R. ornativentris and R. dybowskii) which instead have 24 chromosomes. Yet comparative study of isozyme variation at 25 gene loci from these and seven other European, Caucasian, and east Asian species indicates that the 24-chromosome brown frogs are not a monophyletic group. To the exclusion of R. arvalis, the east Asian 24-chromosome species form a distinct clade related to the 26-chromosome species R. temporaria, R. dalmatina, R. japonica, and R. tagoi more closely than to R. arvalis. This result is reinforced by the comparative position of the nucleolar organizer region (NOR) in the karyotypes of these frogs, as shown by NOR-banding of metaphase chromosomes. The NOR is on chromosome No. 2 in R. arvalis but on chromosome No. 10 in the other three 24-chromosome species. The systematic positions of the Causasian species R. camerani and R. macrocnemis relative to the other species are unclear except that these frogs are distinct enough to be considered part of an outgroup clade. However, in general, the systematics of the entire brown frog group remains unresolved due to great genetic divergences between species and the evident homoplasy of the brown frog morphotype. The parallel origins of reduced karyotypes among the brown frogs demonstrate that caution should be exercised when judging systematic relationships based upon gross karyology in the absence of more detailed information.