Section 2 — Recombination 



techniques described by Pontecorvo for de- 

 tecting mitotic recombination, namely the use of 

 <a) a morphological gene and (b) a recessive 

 resistance gene in heterozygous combination 

 with their wild-type alleles, and also the use of 

 recombination between "heteroallelic" mutants 

 in the same cistron to produce a heterozygote of 

 wild phenotype. Roman's (1956) finding, that 

 selection using the last system results in the 

 detection of exclusively non-reciprocal events 

 best explained by a copy-choice theory of re- 

 combination, is confirmed, but it is also found 

 that some of the recombinants so selected can- 

 not be explained so simply and may provide 

 evidence of either four-strand or inexact recip- 

 rocal recombination at mitosis. 



When homozygous recombinants from hetero- 

 zygous loci are selected, for in this diploid they 

 are found to fall into two classes: 90 per cent or 

 more are found to be homozygous at all the 

 linked marked loci, both proximal and distal to 

 the "selected" locus; the remainder are non- 

 recombinant at any other marked locus, either 

 proximal or distal. 



It is proposed that the second class arise by a 

 recombination process similar to that, already 

 described, which is observed in selecting for 

 recombination between heteroalleles. The other, 

 majority, class may be the result of chromatid 

 non-disjunction rather than mitotic crossing- 

 over. That marker genes unlinked to the selected 

 genes remain heterozygous in all these recom- 

 binants indicates that this non-disjunction is 

 not general, that is, that the recombination is 

 not due to the occurrence of the first division of 

 meiosis. 



2.22. U.V.-induction of Somatic Recombination in 

 Yeast. David Wilkie (London, Great Britain). 



In diploid yeast cells the change from the 

 heterozygous Ad2 ad2 (white colony, proto- 

 trophic) to the homozygous recessive ad2 ad2 

 (red, adenine-requiring) takes place spontane- 

 ously with a low frequency and is usually detected 

 as a red sector in a white colony. The change can 

 be induced by u. v. -irradiation and irradiated 

 cells when plated out give rise to colonies classi- 

 fiable as follows: 



(1) all white, showing no recombination; 



(2) half-sectored, or otherwise showing sec- 

 toring from the centre of the colony, 

 indicating segregation at the first mitotic 

 division; 



(3) all red. The relative frequency of this class 

 increases with dose suggesting the recip- 

 rocal white cell had a lethal defect; 



(4) a. showing a small red sector or sectors 

 mainly at the periphery, 

 b. mottled red and white. 



(4) a and b are interpreted as showing an 

 unstable condition of partial induction which is 

 apparently transmissable. In other words, it 

 appears that the u.v.-induction is not an all-or- 

 none event and cannot be attributed directly to a 

 photochemical change. Further evidence of the 

 complexity of the recombination process is seen 

 in the results of preliminary experiments which 

 show that u.v.-induction is dependent both on 

 the intensity of the irradiation and on the 

 temperature at which it is administered. It is 

 suggested a comparison along these lines be 

 made with heteroallelic systems where recombi- 

 nation appears to involve a different mechanism. 



2.23. Extrabasidial Nucleic Recombinations in a Basi- 

 diomycete Coprinus radiatus. Nicole Prud' 

 homme (Gif-sur-Yvette France). 



It is possible to make triheterokaryons of 

 Coprinus from three monokaryotic strains. In 

 these heterokaryons new nuclei have been 

 demonstrated, formed through an exchange of 

 genetic material between two of the original 

 nuclei. The study of these nuclei leads to the 

 following facts: 



(1) High frequency of aneuploid nuclei 

 (single or multidisomic). 



(2) Possible recombinations of the genes 

 located on non-homologous chromosomes of 

 the original nuclei. 



(3) Apparently total linkage between genes 

 located on homologous chromosomes, even if 

 these genes are 35 c.o. units apart from each 

 other. 



(4) More frequent appearance of some nu- 

 clear types than others. 



These data suggest a mechanism of parasexual 

 recombination analogous to that described in 

 various Ascomycetes, associated with possible 

 selection of some nuclear types. 



2.24. The Purple Suppressor System in Coprinus lago- 

 pus. D. H. Morgan (Hull, Great Britain). 



It has been established that certain suppressor 

 mutations in Coprinus may be functionally alle- 

 lic with one another, as judged by a complemen- 

 tation test, but at different chromosomal loca- 

 tions as judged by recombination data. (1 ) 

 Further work on the purple-suppressor system is 

 now reported. 



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