Section 5 — Mutagenesis 



5.58. Fitness of Irradiated Populations of Drosophila 

 melanogaster. K. Sankaranarayanan (New 

 York, U.S.A.). 



Males in experimental populations of D. mela- 

 nogaster were treated with acute doses of X-rays 

 (2000 r, 4000 r, 6000 r and 7000 r) and the effects 

 of irradiation on population fitness were studied 

 under conditions of relaxed selection, the relax- 

 ation being obtained mainly, by minimizing 

 larval competition. In one population, males 

 received an initial dose of 7000r units and no 

 further irradiation in subsequent generations. 

 At other dose levels, there were populations 

 where the males received X-irradiation (1) at 

 generation O only and (2) at generation O and 

 in every subsequent generation. From the latter, 

 sub-populations were derived where irradiation 

 was stopped after 5, 10, and 15 generations. The 

 components of fitness investigated are (1) 

 hatchability, (2) viability from the larval to 

 the adult stage, and (3) viability from the egg to 

 the adult stage. 



An important finding that confirms the results 

 of earlier investigators is the observation that the 

 radiation induced mortality (dominant lethal 

 effect) is mainly (but not exclusively) at the egg 

 stage. A second finding is the surprisingly rapid 

 recovery to the control level in hatchability and 

 viability rate following the cessation of ir- 

 radiation. 



A detailed analysis of the second chromo- 

 somes is in progress to get an estimate of the 

 radiation-induced genetic load and to gain an 

 insight as to how much of the genetic load is 

 adaptively incorporated into the gene pool of the 

 population. 



Work done during the tenure of a Columbia 

 University Pre-Doctoral Fellowship (1961-62) 

 and Boese Fellowship (1962-'63). 



5.59. Alterations of Viability Characters in an 

 Irradiated Population of Drosophila. V. Boch- 

 nig (Berlin, Germany). 



Alterations of viability of Drosophila melano- 

 gaster have been studied after X-raying a 

 population in its consecutive generations. The 

 population used had been derived from the 

 Berlin wild stock. In each generation 500 males 

 and 500 females (1-5 days old) have been irradi- 

 ated with an acute dose of 2100 r. Up to more 

 than thirty generations of irradiatian viability 

 characters have been studied in intervals of 

 about five generations by testing lifespan, 



fecundity, and fertility after application of 

 different X-ray doses ranging from 0-100 kr. 

 The results obtained show that in each of the 

 three characters studied the irradiated population 

 reached a significantly higher degree of viability 

 than the Berlin wild stock under all experimental 

 conditions. The higher fitness of the irradiated 

 population is explained by an effect of heterosis. 

 A great number of mutations must have been 

 induced by the irradiations and accumulated in 

 the population. As a consequence a higher degree 

 of heterozygosity must have been reached. 



5.60. Mutational Recurrence or Genetic Compen- 

 sation of Lethals in Drosophila Natural and 

 Irradiated Populations? M. L. Reguly and 

 A. R. Cordeiro (Porto Alegre, Brazil). 



The total frequency of 30.3 per cent lethal II 

 chromosomes of D. willistoni from an irradiated 

 "A" population do not differ from the control 

 "E" population, 30.5 per cent (x 2 = 2.17)(714 

 strains). This analysis was made 77.5 generations 

 after radiation. The "A" population exhibited 

 6 out of ten retested lethals and the "E" 3 of 

 these. 



Nevertheless, three lethal loci are exclusively 

 found in "A" irradiated population. The allelism 

 tests we performed and the calculated average 

 mutation rate per locus in the II chrom. of 

 willistoni, 0.000008 obtained from the data of 

 Dobzhansky et al (1) allows the estimation, with 

 Wright's formulae, of the equilibrium levels for 

 each lethal assuming complete recessivity. 



According these calculations the six more 

 frequent induced lethals, still present in popu- 

 lation "A" and the three spontaneous alleles of 

 "E" population are at 10 to 24.4 times above the 

 expected values. 



The lethal survival in nature, at such high 

 frequencies, can only be explained by mutation 

 pressures many times above the normal average 

 observed or by overdominant alleles providing 

 genetic compensation or both, by mutational and 

 balanced effects. We suggest that selection of 

 modifiers and or overdominant allelic forms, 

 tends to buffer (balance) the recurrent lethal 

 producing loci. 



1. Genetics 37, 650 1952. 



5.61 . Changes of Reproductive Performance of Droso- 

 phila willistoni at Two Inbreeding Levels. A. J. 



Centeno, M. L. Reguly and A. R. Cordeiro 

 ( Porto Alegre, Brazil). 



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