Section 2 — Recombination 



The genetical analysis demonstrated that the 

 sulf+ allele, stable in homozygous condition, 

 becomes unstable in presence of a sulf allele. 

 This type of allele-induced instability in somatic 

 cells is termed somatic gene conversion/ 1 ) 

 Further experiments have shown: 



(1) A two-element-system (as found in maize) 

 is not involved. 



(2) In hybrids between sulf homozygotes and 

 taxa of the subgenus Eulycopersicon the pet- 

 centage of variegation is only determined by the 

 "conversion activity" of the special sulf allele 

 present. The environmental conditions (tempera- 

 ture, nutrition, illumination) do not signifi- 

 cantly alter the percentage of variegation. Con- 

 version-table sulf + alleles have not yet been 

 found. 



(3) In hybrids between sulf homozygotes 

 (Lycopersicon esculent um) and L. hirsutum 

 (subgenus Eriopersicori) or Solatium pennellii 

 variegation occurred very seldom, thus indicat- 

 ing an influence of the genotypic milieu. 



(4) The somatic conversion is confined to the 

 sulf locus. But as to the different alleles, conver- 

 sion is not highly directed. The sulf allele, con- 

 verted from sulf + , need not be identical with the 

 sulf allele already present; e.g. in a sulf + sulf puTa - 

 90 per cent p i ant t h e S ulf+ allele is converted 

 to sulf va -z more frequently than to sulf puTa , 

 and even the sulf vu ™ alleles arisen may differ 

 in their conversion activity from s«// pura " 90 



per cent. 



performed. In the first type the marker asco that 

 gives rise to white ascospores was used. The 

 asci were dissected and second division segrega- 

 tions scored. In the second type biochemical 

 markers that were separated by various distances 

 were used and recombinants scored. 



The protein synthesis inhibitor, chloramphe- 

 nicol which inhibits rejoining and also inhi- 

 bits growth of Neurospora, did not affect re- 

 combination. We interpret this to mean that 

 protein synthesis is not involved in crossing- 

 over. 



The only inhibitor that gave consistently 

 higher recombination values when added at the 

 time of meiosis was 5-fluorodeoxyuridine 

 (FUdR) a compound that, in some systems, has 

 been shown to be a specific inhibitor of DNA 

 synthesis. 



Bromodeoxyuridine (BUdR) a compound that 

 is structurally similar to FUdR but is a thymi- 

 dine analog that becomes incorporated into the 

 DNA did not have any affect on recombination 

 nor did uridine which is usually added with 

 FUdR to prevent effects on RNA. 



The simplest explanation for these experi- 

 ments is that genetic recombination, which 

 entails the exchange of chromatids, is specific 

 for DNA and that a functional mechanism for 

 DNA synthesis is required when the bonds re- 

 joining the two pieces are synthesized. This is 

 unlike the rejoining or repair of X-ray induced 

 breaks that are non-polarized and are inhibited 

 by chloramphenicol. 



1. Z. Vererbungsl. 89, 587, 1958. 



2.10. Preliminary Experiments on the Chemistry of 

 Crossing-Over. Sheldon Wolff and Frede- 

 rick J. de Serres (Oak Ridge, U.S.A.). 



Experiments with metabolic inhibitors have 

 indicated that protein synthesis is necessary for 

 the rejoining of radiation-induced chromosome 

 breaks. Because of these experiments Wolff has 

 postulated that the bonds formed at rejoining 

 are in protein. Although crossing-over may in 

 some ways be likened to breakage and rejoining 

 there are some differences. We, therefore, 

 treated Neurospora crassa with specific metabolic 

 inhibitors at the time of crossing-over to see if 

 recombination is affected when certain cellular 

 synthetic processes are blocked. In particular, 

 the experiments were designed to see whether or 

 not protein synthesis inhibitors which can in- 

 hibit the rejoining of radiation-induced breaks 

 can affect crossing-over. 



Several experiments of two different types were 



2.11. Tetrad Analysis of Crosses of Neurospora 

 crassa Grown on Media Containing Various 

 Concentrations of 5-bromouracil and Cytidine 

 Sulfate. S. F. H. Threlkeld (Hamilton, 

 Canada). 



Crosses of two Neurospora crassa strains A> 

 arg-3 (30300), cr, tryp-2 (75001), ylo (Y30539y) 

 and a, pyr-1 (H263), pdx-1 (37803) were made on 

 various media, by conidiating 5-day-old cul- 

 tures of the protoperithecial parent, a pyr-1 

 pdx-1, with conidia from the other strain. 



Standard Neurospora reproductive medium 

 was supplemented in the following ways to give 

 four types of media: (i) cytidine sulfate 100mg/l., 

 5-bromouracil mg/1.; (ii) cytidine sulfate 

 100 mg/1., 5-bromouracil 100 mg/1.; (iii) cytidine 

 sulfate 45 mg/1., 5-bromouracil 100 mg/1.; 

 (iv) cytidine sulfate 45 mg/1., 5-bromouracil 

 mg/1. 



Crosses grown on media (i) and (ii) showed no 

 significant differences with respect to ascus 

 patterns or recombination frequencies. Com- 



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