48 Stanley K. Sessions and John E. Wiley 



Table 2. Summary of chromosome morphology of four species of Necturus, 

 from Table 1 (disregarding subtelocentric Y-chromosomes). 



Chromosome morphology 













% asymmetric 



Species 



m 



sm 



St 



t 



chromosomes (t + st) 



lewisi 



10 



7 



2 







10.5 



punctatus 



9 



7 



3 







15.5 



alabamensis 



10 



4 



1 



4 



26.3 



maculosus 



8 



4 



1 



6 



36.8 



kind of non-Robertsonian mechanism of centromere shifts. This latter 

 process may represent an example of the phenomenon referred to by 

 White (1973) as "karyotypic orthoselection," though it is not clear how 

 selection could be involved in this case. 



We consider the karyotype of N. lewisi to represent the primitive 

 condition within the genus, since this species has the least differentiated 

 sex chromosomes. Necturus lewisi has the most restricted range of any 

 Necturus and may represent a relict. 



Necturus punctatus has more differentiated sex chromosomes than 

 N. lewisi; the X-chromosome is larger relative to the Y-chromosome, 

 and the Y-chromosome is more heterochromatic. In this species, all 

 chromosomes are bi-armed, but they show more asymmetry in centro- 

 mere position than is seen in N. lewisi (Table 2). The similar sizes and 

 C-band patterns of the most asymmetrical chromosomes in both species 

 suggest that they are homologous chromosomes (Fig. 3). The geogra- 

 phic distribution of N punctatus includes that of N. lewisi, but is much 

 larger, extending southward to possibly overlap with the range of N alabam- 

 ensis in the southern Gulf states (Fig. 1). 



Necturus alabamensis has more highly differentiated sex chromo- 

 somes than N punctatus, in terms of Y-chromosome heterochromatin, 

 and an even more asymmetrical karyotype with at least four pairs of 

 telocentric chromosomes (Table 2; Fig. 3). This species is found south of 

 the southern limit of the Appalachian mountain range and, with TV. 

 beyeri, is located at the southern end of the range of N maculosus (Fig. 

 1). Necturus beyeri appears to be identical to N maculosus in number 

 of telocentrics and in Y-chromosome differentiation. 



At least two and possibly three karyotypes were encountered in 

 specimens collected in the geographic range of N alabamensis, with 8, 

 10, and 12 telocentrics. Several interpretations of this apparent kary- 

 ological variability are possible. Perhaps N alabamensis represents a 

 karyological intergrade between TV. punctatus and TV. beyeri. If this is 

 so, then heterozygotes with heteromorphic telocentric/ bi-armed chrom- 

 osome pairs should occur. Such heterozygotes, however, were not found 



