508 



ANTONIE W. BLACKLER 



A review of the history of nucleolar mutants informs us that this kind of 

 mutation is not a particularly rare phenomenon in Xenopus laevis. When the 

 original Oxford mutant female was discovered, the Oxford colony numbered 

 some 90 animais; thèse had been assembled from various laboratories in 

 Great Britain, and ail had been checked for nucleolar number. The Geneva 

 mutant, if the initial hesitancy about this maie animal is discounted, was found 

 in a stock of about 90 toads, — most of which had not been checked for the 

 2nu condition. The two Cornell mutant females have been found during an anal- 

 ysis of 61 adults imported from South Africa. Therefore, although the numbers 

 of animais are small, we can hazard that the frequency of the mutation causing 

 total deletion of the nucleolar organizer région of one chromosome of a diploid 

 set is in the order of 1 J/ 2 %• 



There remains the question whether ail the toads involved originally derived 

 from a single small breeding population. Since wild-caught animais for export 

 are usually sent by the Division of Inland Fisheries at Stellenbosch, enquiries 

 have been made at this source (van Wyk 1967). It appears that Xenopus laevis, 

 for commercial export, are collected from ponds in the Western Province of 

 South Africa, the ponds being spread over an area of about 2500 square miles. 

 Adult toads, caught in the ponds, are temporarily stored at Stellenbosch prior to 

 shipment. Thus it is likely that some shipments of Xenopus are made up of indi- 

 viduals drawn from a number of différent ponds throughout the collecting area. 

 However, since as many as 5400 toads have been taken from a single 2|/ 2 acre pond 

 in one day (sic), it is quite possible that shipments made immediately after a collecting 

 trip may be made up entirely of animais from a single pond. Unfortunately,it has 

 not been possible to establish the précise geographical origin of the Cornell colony. 



Occasional matings of 2nu toads yield progeny, some of whose nuclei bear 

 one large and one variably smaller nucleolus. Genetic analysis of such toads 

 has never been performed, but it may well be that some of thèse toads are carry- 

 ing partial deletions of the nucleolar organizer région. This région, as recently 

 reported (Wallace and Birnstiel 1966, Brown 1967), has about 800 inter- 

 mingled ribosomal RNA gènes per chromosome for coding the synthesis of 

 28s and 18s ribosomal RNA. 



In any event, zoologists using Xenopus laevis may reasonably expect to find 

 new cases of the Oxford nuclear marker in their animais without much difficulty. 



REFERENCES 



Barr, H. J. and H. Esper, 1963. Nucleolar size in cells of Xenopus laevis in relation to 

 nucleolar compétition. Exptl. Cell Res. 31: 211-214. 



Blackler, A. W. and M. Fischberg, 1961. Transfer of primordial germ cells in Xenopus 

 laevis. J. Embryol. Exptl. Morphol. 9: 634-641. 



