MECHANISMS OF LIFE-SPAN SHORTENING 237 



More direct evidence of the non-genetic nature of the natural ageing process 

 which occurs in vivo, in the soma of any given individual rather than of a 

 clone, is provided by observations made by Clark and Rubin (1961) on a 

 species of the parasitic wasp Habrobracon. Tliis particular species readily 

 produces both haploid and diploid males, as weU as females, which are all 

 diploid. It was observed that although the median life-span of the diploid 

 males was only two-thirds that of the females, when both were fed alike on 

 honey, nevertheless the median life-span of the diploid males was just the 

 same as that of the haploid males. In other words there was a difference 

 correlated with sex, as is found for so many developmental characters, but 

 none correlated with ploidy per se. Yet on any theory of genetic change as a 

 basis of ageing that could apply to these organisms, the haploids should age 

 far faster than the diploids. Moreover, a verification of this expectation was 

 provided when the wasps were irradiated at various stages of their life-cycle, 

 for in this case (if we rule out the immediate deaths induced by irradiation of 

 very early embryos) the life-span was reduced by a much greater fraction in 

 the haploid than in the diploid males, but by just the same fraction in the 

 females as in the diploid males. 



Some 5 years ago FaiUa (1957, Failla and McClement, 1957) put forward 

 the view that natural ageing, as well as its radiation-induced imperfect 

 simulacrum, was an effect of the accumulation of point-mutations. He 

 admitted, however, that quantitatively, the two processes failed to agree by 

 a factor of 24 (the natural ageing being that much too fast), on the basis of 

 the Russells' data (1952, 1954 et seq.) on spontaneous and induced mutation 

 rates in mouse germ cells, Failla then proposed (1960) that the data might be 

 reconciled by the ad hoc assumption of drastically different mutation rates 

 in early stages from those that had been observed, which pertamed to the 

 period of life after the age of 2 months. It could also be assumed that the ratio 

 of spontaneous to induced mutation rates may be very different in somatic 

 cells than in the germ track. However, taking into account our knowledge of 

 the high dominance of the great majority of normal genes it can readily be 

 calculated that neither natural, nor induced, point mutations can be occurring 

 at anything like the rates that would be necessary to cause natural senescence 

 or induced life-span shortening, respectively. 



SzUard's proposal (1959) that whole chromosomes or regions of them 

 become inactivated in situ, even in non-dividing cells, by "hits" some of which 

 are caused by radiation and others in other ways, assumes a process for which 

 no evidence has ever been adduced. One would expect such evidence to have 

 made itself known if the process were as frequent as would have to be 

 postulated. Although such a process seems imlikely, in the light of present 

 knowledge concerning the genetic material, it is true that it would give results 

 for radiation-induced life-span shortening that resembled in some significant 



