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J. Maynard Smith 



of females because it accelerates processes which occur in any 

 case in ovariless females, or is it a process which would by 

 itself ultimately result in death, even if other ageing processes 

 could be arrested? We are again faced by a choice between a 

 single and a multiple theory. As yet we have not been able 

 to find a way of deciding between them, but we hope we may 

 be able to do so by studying ageing in genetically different 

 strains on varying diets, since in this way we have other 

 means of altering both the rate of ageing and of egg-laying. 



Table V 



The longevities of inbred and of outbred flies 

 IN days at 20° c. 



We may now turn to the genetics of ageing in D. subobscura. 

 Our interest in ageing originated with the discovery (Clarke 

 and Maynard Smith, 1955) that the hybrids between inbred 

 lines live for longer, and are less variable in lifespan, than their 

 inbred parents. These findings have been confirmed by later 

 work (Table V), although we were perhaps fortunate that the 

 particular pair of inbred lines originally available for study 

 showed the effect in a particularly striking manner. But later 

 work has shown that, in addition to genetic variance due to 

 "heterosis" or "overdominance", much of the genetic 

 variance of longevity is due to genes with sex-limited effects, 

 i.e. to genes with different effects on the longevity of males and 



