Section 7 — Cytogenetics 



have been offered previously (Schultz and 

 Redfield, 1951). 



The work of the senior author was done while 

 receiving the support of a National Science 

 Foundation Grant. 



7.29. Crossing-over in Irradiated Females and Males 

 of Megaselia (Phoridae). F. Mainx and 



E. Doschek (Vienna, Austria). 



Megaselia scalaris has become an interesting 

 object for genetic studies in many respects. A 

 new type of sex determination was discovered 

 in this fly and reported on recently. Crossing-over 

 occurs also in males. The crossing-over values 

 are considerably reduced in males. The differing 

 rates of this reduction depend on the localization 

 of a given zone in the chromosome. Crossing- 

 over values increase after irradiation of the 

 pupae to a certain extent. This increase again 

 differs for particular chromosomal zones in 

 female and in male. Established on these results 

 a new theory of crossing-over could be con- 

 sidered. 



7.30. Experimental Analysis of Interchromosome 

 Distribution and Genetic Significance of 

 Chiasmata. H. K. Jain and S. L. Basak (New 

 Delhi, India). 



Observations on chiasma formation in 

 structurally homozygous and heterozygous 

 plants of Delphinium ajacis and other species, 

 in which two or more groups of chromosomes 

 could be recognized on the basis of size differ- 

 ences, have indicated the factors which control 

 interchromosome distribution of chiasmata. An 

 intraplant analysis first showed that chiasma 

 frequency in an organism like Delphinium can 

 be divided into an autonomous fraction which 

 the different chromosome pairs claim indepen- 

 dently of one another and a correlated fraction for 

 which they show interrelationships. It has been 

 found that the relative magnitudes of these two 

 fractions depend primarily on the karyotype, 

 particularly its symmetry. A highly asymmetrical 

 karyotype such as in Delphinium, favours a 

 relatively large interrelated fraction, which 

 generates negatively correlated interchromo- 

 some distribution. A symmetrical karyotype on 

 the other hand, like the one in Chrysanthemum, 

 favours a disproportionately large autonomous 

 fraction which is associated with absence of 



interchromosomal effects, or with positively 

 correlated chiasma formation in different pairs. 

 It has been found possible to induce an inter- 

 chromosome redistribution of chiasmata in 

 Delphinium following elimination of one or two 

 of the chromosome pairs from forming chias- 

 mata — a condition brought about in a number of 

 plants, raised from X-irradiated seeds. The inter- 

 chromosomal effects for chiasma formation in 

 the normal and the Xi plants find a close 

 parallel in the "Schultz-Redfield" effect for 

 genetic recombination and the present analysis, 

 thus, provides further evidence for a corres- 

 pondence between chiasmata and genetic 

 recombination. A one to one relationship 

 between chiasma formation and chromatid 

 exchange has also been demonstrated in two 

 interchange heterozygotes of Delphinium, in 

 which it was found possible to record obser- 

 vations on chiasma formation in the interstitial 

 segments of the interchange multiple and on 

 anaphase separation of the chromatid parts 

 involved. 



7.31. Autosomal Behavior in Claret-nondisjunctional 

 Drosophila Females. Gale Davis (Oak Ridge, 

 U.S.A.). 



High frequencies of exceptional behavior of 

 chromosomes X and 4 were found by Lewis and 

 Gencarella (1952) in progenies of female 

 Drosophila melanogaster homozygous for ca Ild . 

 Studies reported here were undertaken to 

 determine the extent of chromosome-4 nondis- 

 junction and to clarify patterns of chromosome 

 behavior under the influence of ca nd . 



Mating systems were devised to test nondis- 

 junction and loss of chromosomes X and 4 

 simultaneously. Females having y/y; ca nd /ca nd ; 

 ci D /spa Cat were mated to males having a com- 

 pound chromosome 4. The resultant progeny 

 reflected 63.4 per cent nullo-4, 3.2 per cent 

 diplo-4, and 33.4 per cent mono-4 egg types, and 

 18.3 per cent nullo-X, 2.7 per cent diplo-X, and 

 45.5 per cent mono-X egg types among recovered 

 offspring. The X-chromosome classes contained 

 an apparently nonrandom distribution of 

 chromosome-4 types, leading to the conclusion 

 that X and 4 were involved coincidentally in 

 nondisjunction as well as loss. Nondisjunction 

 and loss of chromosome 2 was studied by 

 mating ca nd females to males having a compound 

 chromosome 2. Estimates of frequency of 

 chromosome-2 nondisjunction were not possible 

 because behavior of compound autosomes in 

 the male was poorly understood. The ratio of 

 recovered nullo-2: diplo-2 egg types (2:1) con- 



127 



