50 Stanley K. Sessions and John E. Wiley 



pattern, two subspecies of pickerels are thought to have spread north- 

 ward, one on each side of the Appalachians from a common origin at 

 the southern end, with secondary contact at the southern end producing 

 intergrades (Crossman 1966). Of relevance to the present distribution of 

 Necturus species as well as Esox americanus, however, is the contrast 

 between the relatively short, eastward and parallel flowing river systems 

 on the east side of the Appalachians and the vast Mississippi River sys- 

 tem flowing southward from Canada to Louisiana on the west side: 

 north-south spreading of such stream-bound, permanently aquatic organ- 

 isms was probably very slow along the Atlantic coast relative to south- 

 north spreading in the inland area. 



Presumably, karyological differentiation in Necturus, involving increas- 

 ing chromosomal asymmetry and progressive changes in sex chromo- 

 some morphology, gradually occurred as populations became estab- 

 lished farther south, around the southern end of the Appalachians, and 

 into the Mississippi River drainage system. Necturus maculosus (includ- 

 ing the southern form, N beyeri) is the culmination of these karyologi- 

 cal and geographic trends and represents the most derived state. It has 

 an extensive, fanlike distribution and probably represents one vast, 

 genetically and karyologically homogeneous population. The distribu- 

 tion of N maculosus was probably the result of a relatively recent and 

 explosive northward dispersal of this species in response to access to the 

 extensive Mississippi River drainage system. This hypothesis awaits 

 substantiation by further electrophoretic, karyological, and ecological 

 studies. 



ACKNOWLEDGMENTS.— We are indebted to Ray E. Ashton, Jr., 

 William Birkhead, Paul Moler, Richard Sage, K. Thomas, and Wayne 

 Van Devender for providing the specimens used in this study. The fol- 

 lowing people read the manuscript and offered valuable comments at 

 various stages of its preparation: Ray E. Ashton, Jr., Alvin Braswell, 

 Steven D. Busack, John E. Cooper, James Kezer, James Patton, Steven 

 Sherwood, and David B. Wake. This research was supported in part by 

 NSF grant DEB 78-03008. 



LITERATURE CITED 



Ashton, Ray E., Jr., A.L. Braswell and S.I. Guttman. 1980. Electrophoretic 

 analysis of three species of Necturus (Amphibia: Proteidae), and the taxo- 

 nomic status of Necturus lewisi (Brimley). Brimleyana 4:43-46. 



Brame, Arden H., Jr. 1967. A list of the world's recent and fossil salamanders. 

 Herpeton. J. Southwest. Herpetol. Soc. 2(1): 1-26. 



