OXFORD NUCLEAR MARKER 



507 



at the University of Geneva. This colony consisted, at that time, of the larger part 

 of the original Oxford collection, augmented by a number of wild-caught toads 

 imported from South Africa. The animal in question hailed from this wild-caught 

 part of the colony, but it was felt at the time of the discovery that the animal 

 may have been a stray from the original Oxford \nu stock, notwithstanding 

 indications to the contrary. It may also be of note that whereas the entire Oxford 

 part of the colony had been checked for the nucleolarity of its constituent members, 

 the imported addition had received little analysis, — it being assumed to be entirely 

 2nu. It was by a chance observation that Uehlinger found the second mutation. 



When the author, who had worked with both the Oxford and Geneva col- 

 onies, moved to his présent address, a new shipment of Xenopus laevis was 

 received from South Africa. Thèse toads were known to be wild-caught. The 

 new colony has been systematically checked for the nucleolarity of the animais 

 by means of tests of progeny. To date, 61 individual toads have been so analyzed, 

 and two of them, both female, have produced some heterozygous offspring 

 when mated to a known wild-type maie (\nux2nu cross). We may conveniently 

 refer to thèse mutant females as Cornell 1 and 2 in order to distinguish them, 

 in later discussion, from the Oxford and Geneva hétérozygotes. 



Cornell 1, mated to a 2nu maie, yielded a sample of progeny in the ratio 

 of 124 x Inu : 150x2/îw. Cornell 2 yielded a smaller sample of 34 x Inu : 38 x 2nu. 

 Both samples show good mathematical fit for a standard test-cross. 



In order to détermine the genetic relationship, if any, of the Cornell mutants 

 to the Oxford mutant strain, two maies of the latter were borrowed from Dr Brown 

 and Dr Gurdon and mated to the Cornell females. The appearance of a ratio 

 of 1 x 2nu \2x\nu : 1 x Onu could be taken as conclusive évidence of the identical 

 nature of the Oxford and Cornell deletions. 



Two methods of scoring the progeny of thèse matings were employed. A 

 sample of eggs from each mating was sorted at the blastula stage, allowed to 

 develop to the pre-feeding tadpole, and then scored for the number of animais 

 showing the Onu syndrome of abnormality. A second sample, also sorted as 

 blastulae, was allowed to develop to the late neurula stage, and then each embryo 

 was squashed under a coverglass and its nucleolarity " read " with the aid of 

 a phase contrast microscope. The results were as follows, 



Cornell la) Onu syndrome; 125 examples in a total sample of 497 tadpoles. 

 b) Squash analysis; 16x2nu : 169x Inu : 100 xOm* 



Cornell 2 a) Onu syndrome; 118 cases in a sample of 468 tadpoles. 

 b) Squash analysis; \26x2nu : 234 x Inu : 133 xOm/. 



Thèse figures are consistent with the conclusion that the Oxford and Cornell 

 markers are identical and involve the same deletion of part of the chromosome. 



Rev. Suisse de Zool., T. 75, 1968. 



34 



