Necturus Chromosome Evolution 49 



in this investigation. Furthermore, the Y-chromosome of the 8-telocentric 

 male N. alabamensis differs in its C-band pattern from both N. puncta- 

 tus and N beyeri, suggesting that it is a distinct form. Although C-band 

 information is not available for the 10- and 12-telocentric specimens 

 collected in the range of N alabamensis, our tentative conclusion is that 

 N alabamensis is a chromosomally polymorphic species, possibly exhib- 

 iting clinal variation in number of telocentrics. Resolution of this prob- 

 ably complex problem awaits more extensive karyological and biochem- 

 ical investigations. 



Necturus maculosus may be the only totally allopatric species of 

 Necturus, except in regions where it may have been recently introduced 

 (Ashton et al. 1980), and has by far the largest range of any of the 

 species. Yet, virtually no molecular divergence is detectable between 

 populations of this species (Ashton et al. 1980). Specimens of N. punc- 

 tatus taken from two different river systems less than 200 km apart in 

 North Carolina showed more genetic divergence from each other than 

 did specimens of N. maculosus collected from widely disparate localities 

 in Wisconsin, Massachusetts, and North Carolina (Ashton et al 1980). 

 The electrophoretic and karyological patterns observed probably reflect 

 the streambound life style of these neotenic salamanders, which repres- 

 ent a lineage that may have been permanently aquatic since the Paleo- 

 cene (Naylor 1978). The isolation of populations of Necturus in parallel 

 river systems imposes a constraint on dispersal patterns,, and has proba- 

 bly encouraged in situ chromosomal and genetic differentiation. The 

 peculiar geographic distribution and pattern of genetic differentiation of 

 N. maculosus relative to its congeners is probably due to its occupancy 

 of the vast, highly branched, north-south flowing Mississippi River 

 system. 



The differences observed in the heteromorphic sex chromosomes 

 among the species of Necturus may reflect evolutionary differentiation 

 of these elements in a manner similar to that hypothesized by Ohno 

 (1967). If so, then the sex chromosomes of Necturus species can be used, 

 in conjunction with electrophoretic and distributional data, to recon- 

 struct certain aspects of the phylogenetic history of Necturus in North 

 America. 



From a karyological viewpoint, the geographic distribution of Nec- 

 turus species can be interpreted as a "karyomorphocline", with the south- 

 east coastal species showing increasing karyological differentiation south- 

 ward and then westward along the Gulf coast, and finally into the Mis- 

 sissippi River drainage system (Fig. 6). The Appalachian Mountains 

 form a natural barrier to westward dispersal of the northern coastal 

 populations and to eastward dispersal of N maculosus. A somewhat 

 analogous situation exists in subspecies of the pickerel, Esox america- 

 nus (Crossman 1966). In contrast to our interpretation of the Necturus 



