Análisis de elementos finitos

The majority of studies reviewed here concern Drosophila, which is invaluable in life history studies. We can compare data from phenotypic

manipulations and quantitative genetic studies to build up a wealth of valuable information on Drosophila life histories (Partridge and Barton, 1993). However,

there is always the problem that once we have evidence for a phenomenon in

Drosophila we can only speculate on its significance in other species. The evolution and mechanisms of ageing have been examined in Drosophila, but there is little need to study a wide range of species for this purpose. It would be valuable, however, to gain information regarding species of importance,

especially humans (Rose, 1988).

Selection for long life in rodents would be useful for examining ageing in mammals and therefore humans, but would be expensive. Artificial selection, either directly upon life span or indirectly upon late life fertility, may not be the best solution. Studies in Drosophila have been started with prior knowledge of the species' life history, for example of genetic correlation patterns (Rose, 1988). Such information is not held for rodents and more is needed before embarking on such a project. Minor changes in the environment can have large

consequences for the outcome of selection, which has been shown in

Drosophila (Luckinbill and Clare, 1985; Johnson, 1988). Rigorous

standardisation procedures are needed to make results reliable. However, there might not be such a problem in mammals as in Drosophila because their life histories are less flexible (Rose, 1988). Artificial mutagenesis and construction of cross bred lines could be used to create mice with increased longevity (Johnson, 1988), but induced mutations invariably come with large deleterious effects which would confound the study of longevity (Rose, 1988). In the case of hybridisation, longer lived hybrids would only exist in the generation, after which the hybrid genotype would break down (Rose, 1988). Selection for longer lived rodents would be very useful for studying the genetic and physiological mechanisms of ageing in mammals, with special reference to humans. However, the problems will be far from easy to solve.

Life span manipulations have been carried out upon species other than

Drosophila, including Tribolium (Mertz, 1975), the bean weevil, Acanthoscelides

(Tucic etal., 1996) and the nematode, Caenorhabditis elegans (Johnson and Wood, 1982; Johnson, 1987; Friedman and Johnson, 1988; Johnson, 1990; Kenyon etal., 1993). This highly inbred hermaphrodite has a very high proportion of homozygous loci in both laboratory and natural environments (Johnson and Wood, 1982), removing the problem of heterosis effects occurring when different strains are crossed, and allowing the study of single gene effects on life span (Johnson and Wood, 1982). Recombinant inbred lines used by

Johnson and Wood (1982) showed a range of life spans between 10 and 31 days. The lack of genetic variation in such populations means that

Caenorhabditis is not suitable for artificial selection studies such as those carried out on Drosophila, but other types of genetic manipulation have been possible. For example, mutants of the age-1 gene showed a 40% extension of life span at 20°C, whereas age-1 hermaphrodites had a 75% reduction in fertility (Freidman and Johnson, 1988). Senescence in this species does not seem to affect the length of the reproductive period; short lived recombinant inbred lines had a rapid mortality rate but did not have a significantly shorter reproductive period (Johnson, 1987). Despite this, the results of Freidman and Johnson (1988) suggest that the level of reproduction does affect life span, as would be predicted by life history theory, even if the picture is not as clear as in

Drosophila. In a later study, Kenyon et al. (1993) found that daf-2 mutants of 0. elegans live twice as long as wild type individuals. The daf-2 gene governs the developmental step taken to form the long lived dauer hermaphrodite, used to endure environmental hardship. The mutants produced as many progeny as wild type adults, but in twice the time (Kenyon et al., 1993). This also fits the idea of a trade off between life span and reproductive effort. The ability of daf-2 to double life span when in mutant form is the result of its regulatory effect on many genes further along the developmental pathway, and can be regarded as a polygenic effect (Partridge and Harvey, 1993). It has since been found that

daf-2 and age-1 appear to extend life span along similar pathways, because both are reliant on the function of the same two genes downstream (Dorman et al., 1995). This suggests that the mutant age-1 also acts as a regulator for many other genes to extend life span.

Mating costs remain unclear in 0. elegans; one study has shown that spermatogenesis is the sole cost of reproduction, and that only males

experience this (Van Voorhies, 1992), whereas another study has evidence that hermaphrodites alone experience a cost of copulation, and that gamete

production is costly to neither males nor hermaphrodites (Gems and Riddle, 1996). Kenyon etal. (1993) ablated the germ cells of wild type hermaphrodites, and found no life span extension compared to control animals which supports the findings of Gems and Riddle (1996) that spermatogenesis is not costly. If copulation is indeed costly to hermaphrodites, then the extension of life span at the expense of fertility, as seen in the daf-2 and age-1 mutants, does support

the hypothesis that survival and fertility are traded off against one another. Therefore, this provides tentative support for the antagonistic pleiotropy theory of ageing in a species other than Drosophila.