The Pacific Giant salamander, Dicamptodon tenebrosus, from the Pacific Northwest of North America has variable numbers of supernumerary chromosomes, from 0 to 10, per individual. B chromosome frequency among populations varies geographically such that salamanders from the most southern and northern regions have lower average numbers of B chromosomes than salamanders in the middle of the range. This variation in B frequency may be correlated with both historical and climatic factors. To assess how the supernumerary chromosomes originated in D. tenebrosus, B chromosome DNA was isolated by microdissection and amplified by degenerate oligonucleotideprimed PCR. The B DNA hybridized similarly to genomic DNA from individuals of D. tenebrosus and the related species D. copei and D. ensatus demonstrating that the supernumerary chromosomes were derived from the normal chromosome complement. Unique hybridization bands in both D. copei and D. tenebrosus suggest that the shared sequences have evolved independently.

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