General Discussion 241 



here into the nature of heterosis, but I think it is fair to point out that 

 while Dr. Maynard-Smith has shown (,/. Genet., 1950, 52, 152) that in 

 this particuhir case an orthodox Mendehan explanation, a chromosomal 

 explanation, is necessary, vigour is a v ery heterogenous thing, and there 

 are quite a number of instances in which an apparent hybrid vigour can 

 be induced by non-chromosomal means, even, I believe, by cross- 

 suckling or by transplantation of foetuses from one animal to another. 



There is only one remaining point that I should like to make, and that 

 is that we are working with stocks that have been in the laboratory for 

 some time, and the wild Drosophila, which are not "wild-type" but are 

 actually caught in the wild, have often been noticed to have a great deal 

 longer life-span than our domestic Drosophila. The longest-lived one 

 which Dr. Maynard-Smith has observed had a life-span after catching — 

 it may have lived already for some days before that — which suggested 

 that it would live for about as long as those vigorous hybrids. Of 

 course, we have no evidence as yet that heterosis, hybridization, can 

 produce a greater life-span than is found in the wild animal. I just want 

 to stress the dangers into which we run if we do ageing experiments on 

 inbred lines which are showing the degree of stability and the degree of 

 inbreeding depression which we found in our Drosophila. 



Brull: I am glad that you have stressed the genetical influence, 

 which is so important in ageing. If you can say that in small animals 

 genetical influence is so gTcat, you could go further in mankind and say 

 that genetics in mankind not only influence duration of life but also the 

 form of ageing. In this field much can be done and should be done. If I 

 may make a suggestion for future research and for future meetings, it 

 would be to have meetings of clinicians who have studied the special 

 field of heredity in man, and cross clinicians with geneticists. 



Comfort: And thereby obtain some hybrid vigour? 



Lansing: I think Dr. Comfort has raised a very interesting point in 

 connection with hybrid vigour, and I'm very much impressed with the 

 studies on Drosophila that attempted to repeat the w^ork that we did 

 with rotifers and did not succeed. I wonder if there is a fundamental 

 difference in our material? We of course have flirted with the possibility 

 that there are differences. The rotifer ceases all cell division in the 

 embryo, the newly-hatched rotifer has all the cells it is going to have, 

 they merely get larger. If I'm not mistaken, in Drosophila division in 

 the ovarj^ continues, and so we may have there a very fundamental 

 difference between these stocks of material. The rotifer eggs have time 

 to age in the ovary, they age along with tjie mother, while in Drosophila 

 young eggs are available in the ovary of the older mother. 



Comfort: And of course here we are only dealing with the part of the 

 life of Drosophila that follows eclosion, when it has completed its develop- 

 ment and has ceased somatic growth, apart from in the ovary. I don't 

 know^ to what extent somatic mitosis continues in Drosophila. An 

 entomologist might be able to tell us; but we are only dealing here with 

 imaginal life. 



Timbridge: Would you now like to say anything about your definition 

 of ageing, Prof. Franklin? 



AGEING 17 



