40 Stanley K. Sessions and John E. Wiley 



somes of the XY male/ XX female type. In terms of chromosome mor- 

 phology and degree of differentiation of the X and Y sex chromosomes, 

 we discern at least four unique karyotypes in the genus (Fig. 2a-d). 

 These are represented by idiograms in Figure 3. 



Necturus lewisi has a completely bi-armed karyotype (Figs. 2a, 3; 

 Table 1), and the X-chromosome is a medium-size metacentric so that 

 the sex chromosome pair is in position 6 of the karyotype. The Y- 

 chromosome of this species is subtelocentric and mostly euchromatic 

 (unstained) after C-banding, except for two bands of heterochromatin, 

 one light and one dark, in the middle of the long arm, and the proximal 

 portion of the long arm which is lightly stained (Fig. 4). Two chromo- 

 somes (besides the Y-chromosome) are extremely asymmetrical (subte- 

 locentric) with large arm ratios (Fig. 3; Table 1). 



Necturus punctatus also lacks telocentric chromosomes (Figs. 2b, 3; 

 Table 1), but its X-chromosome is larger than that of N. lewisis and the 

 sex chromosomes are in position 3 or 4 of the karyotype. Necturus 

 punctatus has three pairs of subtelocentric chromosomes (besides the 

 Y-chromosome), two medium-size and one small, with large arm ratios 

 (Table 1). The medium-size subtelocentrics of N. punctatus are in sim- 

 ilar positions in the karyotype to the most asymmetrical chromosomes 

 of N. lewisi (Fig. 3). The subtelocentric Y-chromosome of N. punctatus 

 is considerably more heterochromatic than the Y-chromosome of N. 

 lewisi, but less heterochromatic than the Y-chromosome of N. maculo - 

 sus (Fig. 4). 



We have found at least two and possibly three different kary- 

 omorphs in specimens collected from the range of N. alabamensis, 

 which differ in the number of telocentric chromosomes. The karyotype 

 of a female from Georgia is identical to that of N. maculosus, with 12 (6 

 pairs) telocentric chromosomes. A male and female from Florida, on 

 the other hand, have karyotypes with only 8 (4 pairs) telocentric chromo- 

 somes (Fig. 2c). A third specimen from the same locality in Florida 

 may have 10 (5 pairs) telocentrics, although this count was based on 

 three mitotic preparations of mediocre quality. A male specimen with 

 eight telocentrics was used for the construction of an idiogram (Fig. 3). 

 In this karyotype, two of the telocentric chromosomes are medium-size 

 and two are small (Table 1; Fig. 3). The medium-size telocentrics are in 

 positions in the karyotype similar to the most asymmetrical medium- 

 size chromosomes in N. lewisi and A r . punctatus, suggesting interspecific 

 homology of these chromosomes (Fig. 3). The sex chromosomes of this 

 specimen fall into position 3 of the karyotype (Fig. 3), as in N. maculo- 

 sus (Sessions 1980; this paper). In addition, the subtelocentric Y- 

 chromosome is more heterochromatic with a more complex banding 

 pattern than in N. punctatus, but is somewhat less heterochromatic than 

 the Y-chromosome of N. maculosus (Fig. 4). 



