The Biology of Senescence 



4-5 Choice of Material for Experimental Study of Age Effects 



Research on the senescence of man and most large mammals 

 necessarily involves work on genetically diverse populations. 

 Where closed laboratory stocks are used, or the subject is a 

 'geneticaP domestic animal such as the mouse or Drosophila, 

 genetic precautions are essential in ageing experiments, especi- 

 ally if comparisons are to be made between the life-spans of 

 different groups or different generations. The size of the effect 

 which can be produced in such stocks by heterosis has already 

 been mentioned. The presence or absence of uniformity in 

 the experimental population is also particularly important in 

 research involving life- tables, since in many inbred lines the form 

 of the life-table depends entirely on one cause of death which 

 is not typical of the species, or even the phylum. 



In non-genetical experiments (nutrition, biochemistry, growth- 

 rate and so on) the choice lies between inbred, hybrid and random- 

 bred material. Inbred lines commonly have a life-span which 

 is rather low for the species, and this may be advantage- 

 ous. Their vigour is often low, though inbreeding depression 

 is more evident in some species than others. Inbred lines are 

 often chosen by non-genetical workers for bioassay, in the 

 belief that they have the advantage of uniformity. This, how- 

 ever, is not so. Griineberg (1954) has stressed two important 

 characters of such lines: they cannot be relied upon to remain 

 constant in their heritable properties with the passage of time, 

 and may diverge rapidly when split into separate colonies; 

 and they do not constitute phenotypically uniform material, but 

 may, on the contrary, be strikingly more variable than F x 

 hybrids between strains, and even than random-bred material 

 (McLaren and Michie, 1954). 



Hybrid material, bred in each generation by crossing inbred 

 lines, suitably chosen, has a number of important advantages 

 for general work upon ageing. In such crosses the life-span 

 approaches the maximum for the species under the experi- 

 mental conditions. Vigour is high, so that 'background' losses 

 due to temperature change, infection, operative mortality and 

 accident is much reduced, and variation between individuals is 

 minimal. This uniformity is itself probably a reflection of 



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