506 



ANTONIE W. BLACKLER 



N° 18. Antonie W. Blackler. — New Cases of the Oxford Nuclear 

 Marker in the South African Clawed Toad. 1 



Division of Biological Sciences, Cornell University, Ithaca, New York, U.S. A. 



The Oxford nuclear marker (Elsdale, Fischberg and Smith 1958) in the 

 South African Clawed Toad, Xenopus laevis is a mutation which reduces the 

 number of nucleoli in the nucleus. In the wild-type, the nucleus of each cell 

 contains two nucleoli (2nu); sometimes thèse nucleoli fuse into a single body, 

 the proportion doing so varying from tissue to tissue. In animais heterozygous 

 for the Oxford mutant, ail nuclei, without exception, contain only one nucleolus 

 each (lnu); this nucleolus is approximately the size of a fused pair of the wild- 

 type (Barr and Esper 1963, Fischberg and Elsdale 1960). Hétérozygotes are 

 just as viable as wild-types, and their ribosomal RNA synthetic pattern during 

 development parallels that of the wild-type (Brown and Gurdon 1964). 



In the homozygous condition (Onu), the nuclei of the embryo do not con- 

 tain true nucleoli but are populated by a number of " blobs " of apparent nucleolar 

 nature (Wallace 1960, Esper and Barr 1964). Homozygous embryos die when 

 their brothers and sisters (2nu and lnu) begin to feed as tadpoles. The homo- 

 zygotes exhibit a syndrome of abnormality which is remarkably constant, — 

 they are retarded, are microcephalic and microphthalmic, bear bilatéral oede- 

 mas on the head and rectum, possess an abnormal gut, often haemorrhage in 

 the kidney, and usually have a down-turned tail-tip. The Onu syndrome is 

 associated with the absence of ribosomal RNA synthesis during development 

 (Brown and Gurdon 1964). 



The Oxford nuclear marker behaves as a single Mendelian factor in tests 

 of progeny (Fischberg and Wallace 1960), and has been shown to consist ol 

 the complète deletion of the nucleolar région of the chromosome (Kahn 1962). 

 It was found originally in a female toad of a colony maintained at Oxford Uni- 

 versity. A stock of lnu (heterozygous) progeny was obtained from matings ol 

 this female, and the marker employed in a number of experiments involvin^ 

 nuclear transplantation (Gurdon, Elsdale and Fischberg 1960) and primordial 

 germ cell transfer (Blackler and Fischberg 1961). 



A second mutation affecting the number of nucleoli in a similar manner waf 

 discovered by Uehlinger 1964, this time in a maie animal of the colony of Xenopui 



1 Investigation supported by grant HD-01663 from the National Institutes of Health 

 U.S. Public Health Service. The technical assistance of Miss Shelley Melkin is gratefulh 



acknowledged. 



