Necturus Chromosome Evolution 



45 





71 



.1 



v \J 



-> 



-\m 



u 



A 



4d2j 



•i" 



\Y: 



ri 



V 



T 



Fig. 6. Map showing relationship between geographic distribution and phy- 

 logenetic history in the genus Necturus. Karyological differentiation occurred as 

 populations of Necturus become established increasingly southward along the 

 coastal plain east and south of the Appalachian Mountains (designated by 

 hatching). Eventual arrival at the Mississippi River drainage system allowed 

 explosive northward dispersal of the karyologically most derived form, N. 

 maculosus. 



chromosome asymmetry, sex chromosome differentiation, and /or pro- 

 portion of telocentric chromosomes, N. punctatus and N. alabamensis 

 have seemingly intermediate karyotypes. Since these two species also 

 have a somewhat intermediate geographic distribution, we hypothesize 

 that karyological evolution within the genus involved progressive differ- 

 entiation of the X and Y sex chromosomes, through increasing hete- 

 rochromatinization of the Y-chromosome and increasing relative size of 

 the X-chromosome, with a concomitant increase in the degree of 

 chromosomal asymmetry through pericentric inversion or some other 



