Section 2 — Recombinanon 



pared with the crosses grown on media (i) and 

 (ii), crosses grown on media (iii) showed a 

 highly significant increase in asymmetrical te- 

 trads, and in the recombination frequency be- 

 tween the marker ylo and the respective centro- 

 mere. Crosses grown on media (iv) were infertile. 

 These results will be discussed. 



2.12. Changes of the Recombination Frequency by 

 Means of Different Concentrations of Ca and 

 Mg in Barley. Milos Ondrej (Prague, 

 Czechoslovakia). 



Fi generation of hybrids between cultivar 

 Stupicky plnozrny and the new-breeding Solary 

 38-50 were cultivated in sand cultures and 

 watered with these kinds of nutrient solution: 



1. with normal content of Ca and Mg (N) 



2. with 100 < decreased content ( — ) 



3. with 2 X increased content (+) 



The last batch of plants were cultivated in field 

 conditions (C). The content of the nutrient 

 medium influenced all the quantitative charac- 

 ters of the analysed plants. 



In F2 generation the recombination frequen- 

 cies for rough-smooth and long-short rachila 

 hairs were tested. 



The lowest recombination frequency was 

 found in the progeny of plants C, higher in N, 

 more higher in (-f) and highest in ( — ) nutrient 

 solution. 



The results are in controversy with those 

 authors which used EDTA to influence the re- 

 combination values. It is possible to explain it in 

 two different ways: 



1. The meiotic cells were not influenced by 

 the decreasing or increasing content of Ca and 

 Mg. They were influenced only by the general 

 physiological changes of the whole organism. 



2. EDTA acts on crossing over another 

 way other than direct binding of ions Ca and Mg 

 from chromosomes and their environment. 



2.13. Effect of Temperature on Recombination in 

 Sphaerocarpus. W. O. Abel (Heidelberg, 

 Germany). 



The effect of temperature on recombination 

 in Sphaerocarpus was tested in 3- and 4-factor 

 crosses. The position of the loci used in the 

 crosses enabled an analysis of recombination 

 almost in the whole now known left arm of the 

 linkage group I. The sporophytes grew during 

 the (for the recombination) important stages of 

 development in temperatures of 10, 18, 26 or 



33°C. The temperature of 33°C had to be re- 

 duced for physiological reasons to 19°C during 

 the night. The temperatures of 10, 26 and 33°C 

 caused for all chromosome sections higher re- 

 combination frequencies than the temperature 

 of 18°C. We found therefore a minimum at 

 18°C for the recombination frequency. This 

 minimum was very striking and statistically 

 significant in the chromosome section near the 

 centromere. The double crossing over frequency 

 was higher in the experiments at 10, 26 and 33°C 

 than at 18°C. It is very likely that the increase 

 of the double crossing over frequency was 

 caused through a decrease of crossover inter- 

 ference. This demonstrated the alteration of the 

 second division segregation for the locus 

 "crassa" from 91 per cent at 18°C to 69 per 

 cent at 33°C. 



The observation of a minimum for the re- 

 combination dependent on temperature, the 

 reaction of crossover interference and the high 

 sensitiveness to temperature of the chromosome 

 region near the centromere are comparable 

 with the findings of Plough and other authors 

 in Drosoplu'la. 



2.14. The Genetic Effect of Mitomycin C. Robin 

 Holliday (Hertford, Great Britain). 



All mutagenic agents which have so far been 

 tested are able to induce somatic recombina- 

 tion in diploid strains of fungi. Apart from their 

 specific mutagenic properties, such agents may 

 have a more general metabolic effect, in particu- 

 lar the temporary inhibition of DNA synthesis 

 relative to RNA and protein synthesis. In order 

 to determine which of these properties is respon- 

 sible for the induction of recombination, a 

 heterozygous diploid of Ustilago maydis and a 

 heteroallelic diploid of Saccharomyces cerevisiae 

 have been treated with the antibiotic mitomy- 

 cin C, which is known to be a specific inhibitor 

 of DNA synthesis. It was found that in Ustilago 

 mitomycin is somewhat more efficient than ul- 

 traviolet light in stimulating reciprocal mitotic 

 crossing over; whereas in Saccharomyces it is 

 less efficient than ultraviolet light in stimulating 

 nonreciprocal recombination or gene conver- 

 sion. Parallel experiments with haploid strains 

 of Ustilago have shown that mitomycin has 

 no mutagenic activity. It is proposed that agents 

 which induce recombination but not mutation 

 be termed recombinagens. Experiments are now 

 in progress which will attempt to determine 

 whether recombination can occur during mito- 

 mycin treatment, i.e. in the absence of genetic 



13 



