The Genetic Relationships of the Salamanders of the Genus Plethodon

Abstract

Highton, Richard and Allan Larson (Department of Zoology, University of Maryland, College Park, Maryland, 20742) 1979. The genetic relationships of the salamanders of the genus Plethodon. Syst. Zool. 28:579-599.-An electrophoretic comparison of proteins is used to evaluate the genetic relationships of the salamanders of the genus Plethodon. The genetic relationships are generally in agreement with the morphological data upon which recent classifications are based but several discrepancies are discussed. In particular, the discovery that the eastern small plethodons are paraphyletic suggests that the ancestral plethodon resembled the small species in size and that the large size of the other eastern species is a derived state. Plethodon neomexicanus, whose relationship to the other plethodons has long been ambiguous, is most closely related to P. larselli; these two species share, in addition to protein similarities, a derived osteological character state not found elsewhere in the genus Plethodon. Electrophoretic genetic distance (D) is highly correlated with albumin irnmunological distance determined for the same set of populations (r = .86) and both measures estimate similar divergence times for Plethodon lineages even when values of D are very large. Average net rate of speciation in Plethodon equals that previously determined for salamanders in general (0.12 speciation events/lineage/million years) but the rates vary considerably. Estimates of genetic distance are used in an analysis of the results of DNA annealing studies. Evolutionary divergence of total single copy and total middle repetitive DNA fractions is highly correlated with genetic distance indicating that both DNA fractions include sequences that diverge in a time dependent manner. Sequence divergence of single copy DNA, which is directly proportional to ATm, is calculated to be 0.17-0.18%/million years as a minimum estimate for Plethodon. The middle repetitive fraction as a whole appears to evolve rapidly in Plethodon as measured by changes in percentages of reassociation and by ATm in interspecific comparisons, but a small subset of the middle repetitive sequences is highly conserved. It is concluded that the eastern and western species of Plethodon diverged slightly over 40 million years ago (in the late Eocene). Compared to other North American salamanders, morphological divergence among plethodons has been slow. [Protein electrophoresis; species phylogeny; rates of speciation; DNA annealing; Amphibia; Plethodontidae; Plethodon.] Electrophoretic analysis of protein variation provides a measure of genetic distance among populations that reflects the evolutionary divergence of structural genes. The largely time dependent or clocklike nature of protein sequence evolution has been established by analyses of the relative amounts of change occurring along the diverging branches of molecular phylogenetic trees (Sarich and Cronin, 1977; Wilson et al., 1977; Carlson et al., 1978). Protein sequence comparisons (Kimura and Ohta, 1974) and immunological protein comparisons (Maxson et al., 1975; Wilson et al., 1977) 1 Present address: Department of Genetics and Museum of Vertebrate Zoology, University of California, Berkeley, California 94720. evaluated in the contexts of fossil evidence and continental drift, relate protein sequence evolution to measures of time. The generally high correlation of electrophoretic genetic distance with albumin immunological distance is used by Sarich (1977) to establish the time dependency of genetic distance and to date divergence of evolutionary lineages from genetic distance estimates. Another estimate of divergence time from electrophoretic data is described by Nei (1975). We again compare the electrophoretic and immunological estimates of genetic divergence because immunological comparisons are available for the same set of populations that we have studied electrophoretically (Maxson et al., 1979). Because of the largely time dependent na-