1 Zoological Museum, University of Zürich (Switzerland), 2 Department of Ecology, Ethology and Evolution, University of Illinois, Urbana (USA), 3 Polish Academy of Sciences, Poznan (Poland)
A growing body of recent evidence suggests that many linkage groups (LGs) of protein-coding genes have been conserved during the evolution of vertebrate animals. Data obtained on European water frogs support such conservation. Using electrophoresis of backcross progeny of an interspecies hybrid male (Rana shqiperica x Rana lessonae) to Rana lessonae, 16 of 20 informative protein loci and sex could be assigned to five LGs, numbered arbitrarily: (1) peptidase D, carbonate dehydratase-2, esterase-4, mitochondrial aspartate aminotransferase; (2) fructose-biphosphate aldolase, cytosolic aspartate aminotransferase; (3) mannose-6-phosphate isomerase, sex, hexokinase-1, peptidase B, lactate dehydrogenase-B; (4) guanine deaminase, fructose-biphosphatase-1, albumin; (5) xanthine oxidase, cytosolic malic enzyme, mitochondrial superoxide dismutase. The enzyme loci a-glucosidase, glyceraldehyde-3-phosphate dehydrogenase, phospho-gluconate dehydrogenase, and phosphoglucomutase-2 each appear to be unlinked to any of the other loci. LGs 3, 5, and presumably 1 are shared with the sister group, the eastern Palearctic water frogs; several linkages of protein loci in LGs 1, 3, and 4 are shared with Nearctic Rana species. At least one linkage of protein loci is shared with (and none differs from those reported for) frogs of the genus Xenopus; several linkages are shared with mammals, and even with teleost fish. The data support evolutionary conservation of these linkages during ~60x10^6 years (Nearctic Rana), over 140x10^6 years (Xenopus), ~300x10^6 years (mammals), and ~500x10^6 years (teleosts). The degree to which such high evolutionary conservation reflects stringent selective pressure on the maintenance of co-adapted gene complexes, or some kind of cytogenetic constraints, remains to be determined.