Section 2 — Recombination 



appear to be produced either by mitotic crossing- 

 over or by mitotic non-disjunction. Unusual 

 types of segregants have been isolated from cer- 

 tain diploids. All results obtained are consistent 

 with the hypothesis that these diploids contain 

 chromosomal aberrations, most probably trans- 

 locations. Such aberrations cause consistent 

 complete linkage of the markers of the two 

 linkage groups involved in the haploids resulting 

 from repeated steps of mitotic non-disjunction. 

 This is expected if near-haploids with unbalanced 

 chromosome complements, that is dis omic and 

 nullisomic for translocated segments, are inviable. 

 The frequency of haploids is therefore reduced 

 (evident, e.g. relative to the frequency of diploid 

 segregants resulting from more than one event 

 of mitotic recombination). 



Similarly, mitotic crossing-over in chromo- 

 some arms heterozygous for translocated seg- 

 ments regularly leads to the formation of unu- 

 sual segregants. Suitably marked diploids were 

 produced heterozygous for either or both of the 

 translocations T(VI-VII) and T(I-VII), as well as 

 normal and translocation homozygotes. Mitotic 

 crossovers selected in one of the involved arms 

 were found to be unstable and presumably unbal- 

 anced. They produce regularly, by further steps of 

 mitotic recombination, better growing diploid 

 and haploid sectors on complete medium. 

 Segregation patterns specific for each transloca- 

 tion were observed and segregants of each type 

 were found to show very characteristic pheno- 

 types. Information on the position of the 

 breakage points as well as the relative size of the 

 translocated segments was obtained. 



2.19. Somatic Recombination in Coprinus lagopus. 



K. M. Swiezynski (Warsaw, Poland). 



In a vegetative mycelium, arising from a 

 compatible or incompatible di-mon mating, 

 frequently new nuclei arise, containing markers 

 originating from two different nuclei. 



Dikaryotic mycelia containing such nuclei 

 fruit easily. In the progeny markers often appear 

 in the proportion 2:1. The 1 : 1 proportion of 

 alleles is also frequent. 



In common AB heterokaryons, containing 

 two types of nuclei with complementary growth 

 requirements, occasionally prototrophic nuclei 

 arise and may be isolated. Mycelia containing 

 such nuclei mate with compatible strains. In 

 the progeny of the fruibodies obtained from 

 such matings again the alleles often appear in 

 the proportion 2 : 1 or 1 : 1. 



The simplest explanation is that in hetero- 

 karyons nuclei may fuse and become diploid. 

 Such nuclei are unstable and turning back to 



the haploid state in vegetative mycelia give rise 

 to somatic recombinants. Hypothetical diploid 

 nuclei, containing different alleles of the A mating 

 type locus, are apparently very unstable and 

 were never isolated. On the other hand diploid 

 nuclei homozygous for the A locus are compara- 

 tively stable and are able to form dikaryons 

 with compatible haploid nuclei. 



The results will be published in detail in the 

 journal Genet ica Polonica. 



2.20. Somatic Recombination in Arabidopsis. 



Y. Hirono and G. P. Redei (Columbia, U.S.A.). 



Somatic segregation in heterozygotes may be 

 brought about by: deletion (breakage), sorting 

 out of extranuclear elements, non-disjunction 

 (reduction), mutation (gene conversion), trans- 

 location and crossing over. In higher organisms, 

 the distinction among these possibilities is 

 difficult partly because the lack of special cell 

 markers and mainly because of the determinate 

 differentiation of the cells. In Drosophila con- 

 vincing positive evidence is available for somatic 

 crossing over, though without breeding test but 

 with supporting cytological observation of 

 somatic pairing. Until now, only in certain fungi 

 has it been possible to demonstrate the occur- 

 rence of mitotic recombination with progeny 

 test, though the responsible chromosomal 

 mechanism is not amenable to cytological anal- 

 ysis. In Arabidopsis plants (crucifer) heterozy- 

 gous for three good linked markers (ch 1 : chloro- 

 phyll-b free; gi 2 : late-vigorous; and pa: dark 

 green, dwarf) somatic segregation was induced 

 by X-raysand the seed obtained from the pheno- 

 typically different two sectors has been subjected 

 to further genetic analysis. Though the irradia- 

 tion caused multiple hereditary alterations, in 

 the case of one plant the experimental data can 

 be interpreted only by genetic exchange at the 

 four-strand stage. Observations on somatic 

 metaphases of wild type plants indicate that 

 juxtaposition of the homologous chromosomes 

 occasionally occur in Arabidopsis. Full details 

 will be reported in Genetics. 



Contribution from the Missouri Agricul- 

 tural Experiment Station. Journal Series No. 

 2543. Approved by the Director. 



2.21. Mitotic Recombination in Yeast. B. S. Cox 



(Liverpool, Great Britain). 



A diploid yeast is described with which it has 

 been possible to apply three of the selective 



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