Section 4 — Gene action 



The validity of these concepts will be illustrated 

 by studies on position effects causing white varie- 

 gation in D. melanogaster and peach variegation 

 in D. virilis. 



4.13. Non-specific Genetic Control of Variegation in 

 Drosophila melanogaster. Janice B. Spofford 

 (Chicago, U.S.A.). 



The Su- V locus (IIIL, 41.4) has already been 

 described; one allele enhances, the other sup- 

 presses variegation of white, facet and diminutive 

 induced by Dp (1 : 3) w m 264.58a, a short inser- 

 tion from the X into the proximal heterochro- 

 matin of IIIL. Neither allele at the locus is do- 

 minant. The extent of variegation depends on both 

 the individual's own Su- V genotype and his 

 mother's. This reports a study of the effects of 

 the Su-V locus on position-effect variegation due 

 to other rearrangements, all X chromosome 

 inversions: sc s (scute-8), v 3p (yellow-3 of Patter- 

 son), w m4 (white-mottled-4), and rst 3 (roughest-3). 

 For sc s , the number of scutellar bristles was 

 counted; for v 3p , the amount of black pigment 

 in the core of the bristle was arbitrarily scored, 

 the scutella being mounted in euparol and ex- 

 amined immediately at 120x; for vv m4 , the total 

 drosopterin per head was measured by densito- 

 meter after paper chromatography; and for rst 3 , 

 the proportion of eye which was rough was 

 estimated. In all cases the results agree with those 

 already described above for white mottling due 

 to the duplication. Possible modes of action of 

 the suppressor locus will be discussed. 



The cell phenotype is not yet fixed at the time 

 of the replication. The expression is irreversible, 

 however, after the completion of cell different- 

 iation. The alternative functions of the im allele 

 can be induced or repressed by various physical 

 and chemical agents (temperature, irradiation, 

 metabolites of the cysteine pathway, etc.). The 

 transmission of this gene through the "germ 

 line" is clearly Mendelian and linkage has been 

 established. Spontaneous mosaicism in hetero- 

 zygotes was not observed. Seeds harvested from 

 white or green fruits are genetically identical and 

 repeat the mosaicism. Apparently the im alter- 

 ation is not due to mutation in a ,, structural" 

 gene but rather to a stable change of a "regu- 

 latory" element. A detailed report will be pub- 

 lished in Genetics. 



Supported by National Science Foundation. 



4.15. The Mode of Transmission of Exogenotic 

 Fragment in Bacterial Crosses. E. Calef and 

 L. Fischer-Fantuzzi (Naples, Italy). 



Structural organization in heterogenote of K12 

 can be explored by interrupted mating. 



The analysis of results of some crosses between 

 Hfr heterogenote and haploid F- show that 

 markers on the exogenotic piece are transferred 

 with the same timing as those of the endogenote. 

 Such mode of transmission is suggestive of an 

 organization involving some kind of attachment 

 between the exo- and the endogenote. 



4.14. A Genetic Oscillatory Mechanism in Arabidopsis. 



G. P. Redei (Columbia, U.S.A.). 



In homozygous condition the recessive X-ray 

 induced im gene periodically and repeatedly 

 causes the turning on and off of chloroplastid 

 differentiation, and consequently of leaf pigment 

 synthesis. Due to this oscillating function all 

 parts of the mutant plants exhibit sectors. The 

 individual cells are either all white or normal 

 green without any intermediates. The frequency 

 of the switch may be very high. The pattern of 

 variegation indicates non-randomness. It appears 

 that the altered cells transmit their temporary 

 character to their several mitotic offspring. 

 Though there is some resemblance to certain 

 mutable systems observed in other organisms, 

 there are several obvious differences. The change 

 of state of the cell lines dividing parallel to the 

 axis of differentiation is fairly well synchronized. 



4.16. Thermosensitive Mutations affecting the Re- 

 plicating Capacity of the Sex-factor of E. coli. 

 F. Cuzin and F. Jacob (Paris, France). 



It has recently been suggested that in bacteria 

 a self-replicating unit, or replicon, such as a 

 chromosome or an episome, determines its own 

 replication system. (1 > This hypothesis predicts 

 the existence of mutations affecting specifically 

 the replication of a given element such as an 

 episome or a chromosome. From bacteria car- 

 rying an F-Lac + , mutants have been isolated in 

 which the episome replicates normally at 30° but 

 not at 40°. At 40°, the episome is diluted out by 

 the bacterial multiplication. It is not destroyed, 

 however, since, when bacteria grown at 40° are 

 shifted down to 30°, the multiplication of the 

 episome in those cells, which will carry it, is 

 resumed. This thermosensitivity is genetically 



41 



