Structure and evolution of supernumerary chromosomes in the Pacific giant salamander, Dicamptodon tenebrosus

We examined the genetic make-up and plausible origins of the supernumerary (B) chromosomes of the Pacific giant salamander, Dicamptodon tenebrosus, fr om the Pacific Northwest of North America. These salamanders have variable numbers of B chromosomes, from 0 to 10 per individual. Salamanders from the most southerly and northerly regions of the species' range have lower aver age numbers of B chromosomes than salamanders in the middle of the range. T o assess how the supernumerary chromosomes originated in D. tenebrosus, B c hromosome DNA was isolated by microdissection and amplified by degenerate o ligonucleotide-primed PCR. The B chromosome DNA hybridized similarly to gen omic DNA from individuals of D. tenebrosus and the related species D. copei and D. ensatus, thus 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 e volved independently.