Section 9 — Population Genet to 



of Drosophila melanogaster showed a constant 

 value up till April 1961 after which it declined 

 significantly (from 41 per cent to 31 per cent) 

 up to August 1961. Heritability estimates rose 

 once again to about 42 per cent from Septem- 

 ber 1961 to December 1962, while populations 

 of D. simulans showed arise, though not signifi- 

 cantly so, in the heritability estimates (28 to 

 35 percent) from October 1961 up to December 

 1962. 



Phenotypic, genetic and environmental monthly 

 correlations between wing and thorax length in 

 both species of Drosophila were analyzed and 

 will be discussed. 



9.73. Evolution of the Mean Ovariole Number in 

 Experimental Populations of Drosophila me- 

 lanogaster. Georges Teissier (Paris, France). 



The mean ovariole number of wild type Dro- 

 sophila melanogaster may vary by a factor of 

 two, according to the geographical origin of the 

 studied strains; it is conditioned by a polygenic 

 system, which can be studied by the usual tech- 

 niques. It seemed useful to observe the evolution 

 of populations maintained for several years in 

 cages of the type we have been using since 

 1932. Six of these populations have been meas- 

 ured since 1956, six others since 1958. More 

 than 500 counts of the mean ovariole number 

 m of samples of 50 females, hatched from eggs 

 placed on a superabundance of food, have been 

 made during the last four years. 



In three populations, constitued by crossing 

 the French strain B U (m = 29,45) with the 

 Japanese strain O T (/;/ == 15.67), the values 

 of m, which were originally 21.12 for the Fi, 

 25.75 for Bi and 18.16 for B 2 , oscillated during 

 the last three years respectively around 20.93 i 

 0.07; 21.52 ± 0.13 and 21.36 ± 0.08. Similar 

 values have been found for the three other popu- 

 lations, constructed from several different 

 French strains, such as B U, and therefore or- 

 iginally more complex. 



It appears from these facts, and others which 

 will be discussed, that there occurred in all our 

 populations over the years a selection which 

 adjusted the ovariole number to a value optimal 

 for the conditions of a strong competition for 

 food characteristic of the life of Drosophila in 

 population cages. Moreover this value is not 

 the same for all the types of experiments. 



9.74. An Experimental Study of Environmental 

 Influences on Population Structure in Droso- 

 phila. J. A. Thomson (Melbourne, Australia). 



Laboratory populations of Drosophila psetido- 

 obscura initially polymorphic for the Standard 

 (ST) and Chiricahua (CH) sequences of chro- 

 mosome III have been studied undei a variety 

 of experimental regimes involving differences in 

 temperature, humidity, illumination and food 

 composition. A variable amount of gene flow 

 between interconnected populations held under 

 different environmental conditions was permit- 

 ted. The degree of genetic differentiation achie- 

 ved during 30 generations by such island popula- 

 tions in relation to the imposed environmental 

 differences has been investigated by following 

 the frequency of CH, certain physiological 

 properties, sex-ratio and productivity, in addition 

 to comparison of the accumulated recessive le- 

 thals of chromosome III and the additive ge- 

 netic variance of the sub-populations. Evidence 

 has been obtained that increased resistance to 

 desiccation occasioned by larger body size is a 

 major factor contributing to the adaptive 

 significance of the ST/CH polymorphism in at 

 least certain of these populations. 



9.75. Cycl ic Variation of Bristle Number with Parental 

 Age in Drosophila melanogaster. J. M. Wat- 

 tiaux and M. J. Heuts (Louvain, Belgium). 



Emergent flies from successive daily egg 

 batches, laid by single pairs from pupal hatching 

 till death, showed, in 8 out of 40 cases a periodi- 

 cal variation in mean abdominal bristle number. 

 In all these cases Kendall's correlogram takes 

 the form of a sinusoidal function with a constant 

 period whose value lays between 6 and 13 days 

 and whose maxima and minima differ by 3 to 5 

 bristles, according to the sibships. In the most 

 favourable case 5 periods of 6 days each are 

 evident. Linear trends are not discernible. 



Density influences can be ruled out as a cause 

 of the fluctuations on experimental evidence. 

 Separate treatment of males and females yields 

 highly significant correlations between daily 

 values. The heritability coefficients (h 2 ) calculated 

 from parent males and from parent females are 

 ± 0.67, viz. ± 0.35, which are considerably 

 higher than those found for sternopleural chae- 

 tae (Scossiroli, 1954). 



Cyclical segregation of oligogenes in Dr. 

 melanogaster have been described (Heuts, 

 1956). Pertinent questions as to the nature of 

 the mechanisms involved and as to the apparent 

 restricted occurrence of cycles in quantitative or 

 qualitative characters of offspring with relation 

 to parental age will be discussed in forthcoming 

 publications (Genet ica). 



168 



