Section 9— Population Genetics 



analyzed by Cy and TM3, Ser balancer chromo- 

 somes. The number of lethals and semi-lethals 

 are 13.0 per cent for second and 18.5 per cent 

 for third chromosomes. In a second collection 

 after three months, 160 second and 164 third 

 chromosomes were taken in mid-October and 

 17.5 per cent of the second and 16.6 percent of 

 the third chromosomes were found to be lethals 

 and semi-lethals. Another sample was taken in 

 a location at rural Kwangnung in mid-September 

 of the same year, and 16 per cent of 176 second 

 and 20 per cent of 221 third chromosomes 

 sampled were found to be lethals and semi-lethals. 

 Differences in lethal concentration between 

 second and third chromosomes as determined 

 by chi-square are not significant in time and 

 space. However, it is interesting to note that on 

 the average slightly more third chromosomes 

 than second carry lethals. Additional data 

 showed high value of homozygous viability of 

 both second and third chromosomes above 

 semi-lethals and of quasi-normals: on the aver- 

 age, mean viability of chromosomes above 

 semi-lethals appears to be 30 ± 0.3 per cent for 

 second and 31.0 ± 0.5 per cent for third chromo- 

 somes. Nearly complete tests showed compara- 

 tively high frequency of allelism among the 

 strictly lethals in both second and third chromo- 

 somes, showing no significant differences be- 

 tween collections in the same location or be- 

 tween locations (1 to 3 per cent for III and 1 

 per cent for II chromosomes). Total incidence of 

 visibles was low in each collection, but more 

 than half of the different visibles occurred more 

 than once within the respective populations. Many 

 of the wild males tested, which carried either bw 

 or p, were further proved to have more than one 

 lethal and/or other deleterious genes either on the 

 bw or p carrying chromosomes or on the other 

 part of the homologous chromosomes. 



As far as we may judge by the present data 

 available, the frequencies and allelism of sec- 

 ond and third chromosomes of lethals and other 

 genetic variabilities agree well each other, and 

 a batch of the results falls within the expected 

 range showing smallness of population size. 

 It may be safe to conclude that Asiatic popula- 

 tions of Drosophila melanogaster are really 

 different from the other geographical popula- 

 tions so far reported, in respect of population 

 size and genetic structure. 



9.62. (D.) Polymorphism in the Water Boatman, Tri- 

 chocorixella mexicana. W. Peters (Chicago 

 U.S.A.). 



Dextral and sinistral forms displaying situs 

 inversus occur in a 1 : 1 ratio in the wild popu- 



lation. Phenotypic characteristics are illustrated 

 and a probable mode of inheritance proposed. 



9.63. Color Polymorphism in Cage Populations 

 derived from Hybrids between Drosophila I. 

 lebanonensis and Drosophila I. casteeli. S. B. 

 Pipkin (Panama, Panama). 



Founders of four cage populations were Fi 

 hybrids between a homozygous pale strain of 

 D. I. lebanonensis, a species exhibiting a balanced 

 mesonotal color polymorphism in natural pop- 

 ulations of the Lebanon, and D. I. casteeli, 

 its dark subspecies, lacking a color polymorphism 

 in natural populations of Arizona, U.S.A. The 

 pale versus dark mesonotal color is dependent 

 upon a single pair of alleles, S,s, not associated 

 with a chromosomal inversion. 



After twenty-four generations, two popula- 

 tions maintain a Ss superiority. An initial s/S 

 ratio of 1 declined to 0.9 in the fifth generation 

 and 0.45 in the fourteenth generation, after 

 which there was a rise to 0.7 in one population 

 and 0.65 in the other. Presumably coadaptation 

 of gene complexes resulted in improvement of the 

 selective value of the Ss heterozygotes in these 

 two populations from the fourteenth through 

 the twenty-fourth generations. In two other 

 cage populations, the SS homozygotes proved 

 superior to Ss heterozygotes, although the 

 selective value of the latter improved somewhat 

 after the fourteenth generation. 



Mating propensity, known to be low in D. I. 

 casteeli, is high in cage populations derived from 

 hybrids between D. 1. lebanonensis and D. I. 

 casteeli, with no differences among the SS, Ss, 

 and ss genotypes. Culturing a high temperatures 

 of from 25 to 28°C of replicate half-gallon 

 bottle populations, derived from the two equilib- 

 rium cage populations at the thirteenth genera- 

 tion, resulted in a rapid selection strongly fa- 

 voring SS homozygotes. Length of egg to hatch- 

 ing life cycle in an equilibrium cage popula- 

 tion is approximately the same for Ss, SS, and 

 ss genotypes. 



9.64. Selection in Experimental Populations of 

 Drosophila melanogaster with Different Ge- 

 netic Backgrounds. Sergey Polivanov (New 

 York, U.S.A.). 



A study was made of the behavior of several 

 mutants of Drosophila melanogaster in experi- 

 mental populations, kept in laboratory popula- 

 tion cages. In all experiments the foundation 

 stocks contained mutant and wild type alleles 



164 



