Section 9 — Population Genetics 



The Chilean species, Drosophilu pavani, is 

 polymorphic for gene arrangements in its chro- 

 mosomes. Studies of the mechanisms which 

 contribute towards the maintenance of this poly- 

 morphism in the populations have shown that the 

 frequency of heterokaryotypes with respect to 

 some gene orders is significantly higher in 

 100-day-old flies than in larvae or in young 10- 

 day-old adults. This finding suggests that the 

 selective pressures which confer a higher fitness 

 to these heterokaryotypes may act both duiing 

 the preadult and the adult life stages. 



When mating activity was taken as a criterium 

 for the superiority of heterokaryotypes, it was 

 found that, in a very heterogeneous population, 

 the group of males which courted or copulated 

 within the first 30 min of contact with the female, 

 contained a significantly higher proportion of 

 heterozygotes for inversions than the group of 

 males which copulated after this period. As 

 mating activity may be important in fitness, these 

 findings could in part explain the maintenance 

 of balanced polymorphism in the populations. 



9.44. Sexual Isolation Between the "Sibling" Species 

 Drosophila pavani and Drosophila gaucha. 



Susi Koref-Santibanez (Santiago, Chile). 



Drosophila pavani and Drosophila gaucha are 

 two neotropical "sibling" species of the meso- 

 phragmatica group of the genus Drosophila. They 

 are mostly allopatric in their distribution, al- 

 though they have been found sympatrically in 

 certain regions. Hybrids have never been found 

 in nature, and in the laboratory, where they can 

 be produced abundantly, they are sterile. Chro- 

 mosomally the two species differ only in the gene 

 arrangement in their X chromosomes. 



The different levels of sexual isolation were 

 studied in order to try to establish the mechanisms 

 by means of which gene exchange between D. 

 gaucha and D. pavani is prevented. Thus, among 

 others, courtship behaviour, mating ability, 

 sperm viability and early embryonic develop- 

 ment were analyzed. Previous work of Fernan- 

 dez in our laboratory had shown that spermato- 

 genesis in the hybrids was apparently normal. 



Courtship behaviour in both species and in 

 their hybrids is very similar, although the hybrid 

 males are slightly more inactive. They copulate, 

 and hybrid females accept D. pavani, D. gaucha, 

 and hybrid males. Nevertheless, an embryonic 

 development is seen in the eggs of inseminated 

 females. In view of the above-mentioned data, 

 it seems that under laboratory conditions, the 

 main mechanism of hybrid sterility depends on 

 the lack of fusion of the male and female game- 



tes. To support this hypothesis there exists 

 experimental evidence of the rapid inactivation 

 of hybrid sperm within the female vagina. The 

 factors involved in this inactivation are now being 

 analyzed. 



9.45. On the Behavior of Lethals in Natural and in 

 Laboratory Populations. A. Brito da Cunha, 

 J. S. de Toledo, S. A. de Toledo, L. E. de 

 Magalhaes and C. Pavan (Sao Paulo, Brazil). 



To study the behavior of lethals in nature, 

 7 wild and 7 radiation induced lethals were 

 introduced in two isolated island populations 

 in 2 experiments. The lethals, which before had 

 an allelic frequency of 6 per 10,000 chromosomes, 

 reached, after the introduction, a mean frequency 

 of 0.105 in one population and of 0.119 in the 

 other in the first experiment and of 0.184 and of 

 0.205 in the second. The introduced lethal fre- 

 quencies in the first experiments dropped to 

 0.012 in one population and to 0.001 in the other 

 after 8 generations, being practically eliminated 

 after 15 generations. In the second experiment 

 the mean frequencies dropped from 0.205 to 

 0.015 in one island and from 0.184 to 0.008 in 

 the other in 4 generations. 



Parallel experiments with the same lethals were 

 carried out in the laboratory using population 

 cages. In these experiments every lethal had an 

 initial frequency of 0.250. After 15 generations the 

 lethals had frequencies between 0.1 74 and 0.030, 

 6 out 16 lethals having frequencies equal or 

 above 0.100. 



There is a sharp contrast between the behavior 

 of the lethals in nature and in the laboratory. 

 In the laboratory the selection was very slow and 

 the lethals behaved as recessive. In nature the 

 elimination was not uniform. The lethals had 

 deleterious effects in the heterozygotes behaving 

 as incomplete recessives with effects which varied 

 with ecological factors. The period of drastic 

 selection was always correlated with the deterio- 

 ration of the environmental conditions. Radiation 

 induced and wild lethals behaved similarly. 



9.46. Dynamics of Lethal Genes in Natural Popula- 

 tions of D. willistoni. L. E. de Magalhaes, 

 A. B. da Cunha, J. S. de Toledo and C. 

 Pavan (Sao Paulo, Brazil). 



The frequencies of lethals released in the popu- 

 lations of the islands of S. Joao and Queimada 

 Pequena decreased in the 1st to the 4th genera- 

 tions according to the expectation due to eli- 

 mination through homozygosis and dropped 



158 



