Section 10 — Developmental Genetics 



10.24. Alteration of Frog Zygote Nuclei by Macromo- 

 lecular Fractions obtained from Adult Liver 

 Cells. Heinrich Ursprung (Baltimore, 

 U.S.A.). 



Experiments were carried out to test the hy- 

 pothesis that differential gene function and cyto- 

 differentiation are based upon complexes for- 

 med between chromosomes and specific mac- 

 romolecules. Macromolecular fractions were 

 prepared from nuclei and cytoplasm of adult 

 Rana pipiens liver. Aliquots containing approxi- 

 mately 0.2 mg of protein were injected into 

 fertilized frog eggs. Two of the fractions, the 

 globulin-albumin fraction and the histones, pro- 

 duced cessation of cell division and arrest of 

 further development at the late blastula stage. 

 When nuclei from such arrested blastulae were 

 transplanted into enucleated eggs, the recipient 

 eggs developed to the late blastula stage, but then 

 were arrested also, indicating a persistent nu- 

 clear change. Cytological examination of such 

 embryos showed no primary gross chromosomal 

 abnormalities. 



The substance responsible for this alteration of 

 nuclear capacities is heat-labile and not dialyzable. 

 Specific activity, expressed as biological activity 

 per milligram of protein, is highest in the fraction 

 that precipitates between 57 and 65 per cent satura- 

 tion of ammonium sulfate. Removal of digestible 

 RNA by ribonuclease does not decrease the specif- 

 ic activity. DNA has not been detected. It is conclu- 

 ded that the active substance is or contains protein, 

 and it is speculated that its action consists in form- 

 ing complexes with the DNA portion of the chro- 

 mosomes in such a way as to impede those nuclear 

 activities that are required for continuation of cell 

 division and gastrulation. 



Supported by NSF Grant GB-298. 



10.25. "Brown Spots" (bsp), A Genetic Character 

 of Drosophila melanogaster which manifests 

 only after Copulation. Anna di Pasquale and 

 Laura Zambruni (Milan, Italy). 



"Brown spots", a new character arisen spon- 

 taneously in a wild stock of Drosophila melano- 

 gaster in 1959, is due to the presence of one or 

 more recessive factors localized in the 2nd 

 chromosome. It is transmitted by the male as 

 well as by the female, but the phenotypical 

 manifestation, which consists in the formation of 

 browncoloured areas on the pleurae, is limited 

 to the female. 



The manifestation shows a clear relation to 

 mating. Females kept apart from males and fe- 

 males isolated from them soon after courtship 

 phases but before copulation, never show spots. 

 A single copulation is sufficient to provoke 

 spotting. 



bsp females mated with males of nine different 

 stocks show spots with high frequency; the 

 mating with y w males, after which the inci- 

 dence of spots is particularly low, is a peculiar 

 case. 



Mating with sterile males (X/Y Lc and X/O: 

 non-motility of spermatozoa; X/X; trajtra: 

 females transformed into males = no sperma- 

 tozoa) also determine the appearance of spotting 

 in bsp females. 



Frequency of spotting increases with the age of 

 the copulating females. No relation is found 

 between aging of the male and spotting. 



From these data it is clear that the pheno- 

 typical manifestation of "brown spots" is strictly 

 dependent on copulation, irrespective of fer- 

 tilization and presence of spermatozoa in the 

 sperm fluid. The nature of the phenomenon 

 is not yet clear; recent experiments would 

 emphasize an activity of the sperm fluid: 

 mating with males lacking paragonial fluid owing 

 to repeated copulations, does not seem to cause 

 any spotting. 



10.26. Gene-environment Interactions and the Pene- 

 trance of Melanotic Tumour Genes. James 

 H. Sang and B. Burnet (Edinburgh, Great 

 Britain). 



The melanotic tumour phenotype of Droso- 

 phila melanogaster may be determined by any one 

 of a number of non-allelic, second chromosome, 

 recessive genes. Penetrance of some of these genes 

 depends on modifiers on other chromosomes, 

 usually the third, and also on the environment, 

 particularly the nutritional balance provided 

 to the larvae. The question examined in this 

 report is whether the same environmental mani- 

 pulations affect tumour penetrance in non- 

 allelic tu strains of different origin and, if they 

 do, what this implies with respect to gene action. 



Two inbred strains were examined, each of 

 which showed a low incidence of tumours when 

 cultured on live yeast or on complete, chemically 

 defined media. One strain (tu k ) carried a re- 

 cessive of very low penetrance on the second 

 chromosome (and probably an enhancer — 

 e-tu* — on chromosome three); the other also 

 carried a second chromosome tu gene but this 

 was hypostatic to a third chromosome recessive 



177 



