Section 9 — Population Genetics 



decreasing rate of average viability in homo- 

 zygous condition were estimated. 



Under the assumptions that the mutations in 

 question are distributed on the chromosome 

 according to Poisson distribution and that all 

 mutations are homozygously deleterious, the 

 average mutation rate was estimated to be at 

 least 0.1 per second chromosome pel generation 

 on the basis of counting about 1.8 million flies. 

 This mutation rate is of a surprisingly large 

 magnitude. 



When the survival rate of eggs is calculated on 

 the basis of the classical hypothesis and this 

 estimated mutation rate, it becomes about 58 per 

 cent in random mating populations, and is 

 inconsistent with the actual situation. Thus, the 

 result is unfavorable to the classical hypothesis. 



9.59. Let huls and the Parameters of a Southern 

 Population of Drosophila willistoni. M. Napp, 

 H. Winge, M. L. Reguly and A. R. Cordeiro 

 (Porto Alegre, Brazil). 



Random crossing, mutation pressure and the 

 progressive betterment of gene coadaptation in 

 complex genie balance due to natural selection, 

 builds the genetic structure of natural popu- 

 lations. 



Genetic analysis of a natural population 

 inhabiting an isolated, small wood in the grass- 

 land of Rio Grande do Sul State, disclosed sig- 

 nificantly different frequencies of II chromo- 

 somes bearing lethals or semi-lethals, averaging 

 44.7 ± 1.7 per cent for the period of March 

 1959 to April 1961 with 895 strains studied. The 

 III chromosome lethals + semilethals frequen- 

 cies showed no changes and averaged 29.5 ± 

 2.3 per cent in five samples from March 1958 

 to December 1959 (389 strains). The total 

 intrapopulation allelism frequency determined 

 is: 11=0.0102 111= 0.0029. 



With the aid of S. Wright's formulae we 

 calculated that the rate of lethal incorporation 

 for II chrom. is 0.0046 and 0.0049 for the III. 

 The elimination due to random homozygosis: 

 11=0.0007 and 0.0001 for the III. This allows 

 increments of II = 0.0039; HI = 0.0048. 

 Nevertheless, taking the same mutation rate (*) 

 the number of lethals, if completely recessives, 

 would be in the II, 2.6 — 4.6 times, andinthelll, 

 5.6 — 10.3 times greater than the actually observed, 

 averaging 2.6 and 7.6 times, respectively. 

 Selection coefficients opposed to lethals in heter- 

 ozygous condition of s (II) = 0.018 and s(III) = 

 0.031 would be necessary to account for the 

 discrepancy. Contrary to others' results the s 

 values for semilethals in heterozygous state are 



smaller than the ones for lethals. Local in- 

 breeding, F(II) = 0.01 65, might be another factor 

 of lethal or semilethal elimination. The estimated 

 effective population size: N= 4030 (ace. Wright's 

 formula) conforms with the values of six to ten 

 thousand individuals determined by release 

 experiments of marked flies in the same wood. 



1. Determined by Dobzhansky et al. 

 Genetics 37, 650. 1952, 

 Work to be reported full in Evolution. 



Work supported by the Rockefeller Foun- 

 dation, Conselho Nacional de Pesquisas and 

 the University of Rio Grande do Sul. 



9.60. The Peisistence of Deleterious Genes in 

 Natuial Populations of Drosophila melano- 

 gaster. Chozo Oshima (Misima, Japan). 



The distribution of viabilities manifested by 

 individual homozygous second chromosomes 

 isolated from the Japanese natural populations 

 of Drosophila melanogaster has been in the 

 course of several years repeatedly examined. 

 From the results of allelism tests between lethal 

 genes, some of them were proven to have been 

 retained for several years in the same population. 

 The pre-adult viability of heterozygous flies for 

 single lethal and double lethal chromosomes was 

 estimated and compared with that of normal 

 heterozygotes. A complementary effect between 

 lethal genes was recognized in the viability of 

 double lethal trans- or cis-heterozygotes and it is 

 assumed to be due to interaction between two 

 loci carrying lethal genes in the heterozygous 

 genetic background. The mean relative viability 

 of heterozygous flies for single semi-lethal and 

 double semi-lethal chromosomes was also 

 estimated. The deleterious effect of a single 

 semi-lethal chromosome on heterozygous con- 

 dition was almost none as compared with that of 

 a single lethal chromosome, but the detrimental 

 effect of double semi-lethal trans-heterozygotes 

 was emphasized. From the above mentioned 

 results, some mechanisms of persistence of 

 lethal genes in a natural population are consider- 

 ed and the difference in their frequency between 

 lethal and semi-lethal chromosomes will be 

 discussed. 



9.61. A Comparative Survey of Genetic Variabilities 

 between Second and Third Chromosomes from 

 Korean Populations of Drosophila melano- 

 gaster. Yong Kyun Paik (Seoul, Korea). 



138 second and 151 third chromosomes col- 

 lected in mid-July in a suburb of Seoul were 



163 



