Chromosome Evolution in Salamanders of 

 the Genus Necturus 



Stanley K. Sessions 



Museum of Vertebrate Zoology, 



University of California, Berkeley, California 94720 



AND 



John E. Wiley 



St. Paul's College, 



Box 751, Lawrenceville, Virginia 23868 



ABSTRACT. — All species of the salamander genus Necturus (N. ala- 

 bamensis, N. beyeri, N. lewisi, N. maculosus, and N. punctatus) have 

 19 pairs of chromosomes (2n = 38), and well-differentiated hetero- 

 morphic sex chromosomes of the XY male/ XX female type. Four dis- 

 tinct karyotypes are observed among the species in terms of C-band pat- 

 terns, the proportion of asymmetrical chromosomes such as telocen- 

 trics, and the degree of differentiation of the sex chromosomes. Nectur- 

 us beyeri appears to be identical to N. maculosus in these features 

 while the three other species have uniquely different karyotypes; N. 

 alabamensis may be polymorphic for an intermediate number of telo- 

 centric chromosomes. Interspecific homologies between the most 

 asymmetrical chromosomes, including telocentrics, are suggested by 

 the similar position of these chromosomes in the karyotypes and sim- 

 ilarity in C-band patterns. The species appear to exhibit sequential 

 stages of karyological differentiation in the order: lewisi-punctatus- 

 alabamensis-beyeri + maculosus. Karyology correlates with geographic 

 distribution in a simple pattern suggesting that gradual karyological 

 differentiation occurred as populations became established southward 

 along the Coastal Plain of southeastern United States, around the 

 southern end of the Appalachian Mountain Range, and into the Mis- 

 sissippi River drainage system. Thus, N. lewisi represents the most 

 primitive form and N. maculosus the most derived condition. 



INTRODUCTION 



Salamanders of the genus Necturus of eastern North America have 

 been used extensively for biological research for well over 100 years, but 

 the phylogenetic relationship of Necturus to other salamanders, and sys- 

 tematics within the genus, remain problematical. Necturus represents an 

 ancient lineage of neotenic (sensu Gould 1977), permanently aquatic 

 salamanders, with a generalized larval morphology that obscures affini- 

 ties to other living salamanders (Hecht 1957). Most workers consider 

 Necturus to be most closely related to the European blind salamander, 

 Proteus anguinus, and place both genera in the same family, Proteidae 

 (Brandon 1969; Larsen and Guthrie 1974; Naylor 1978; Noble 1931; 



Brimleyana No. 10:37-52. February 1985. 37 



