12,408.25 2.1.3 Conexión eléctrica

Life history theory is the study of development characteristics that have a large effect on organism fitness. These traits naturally vary between species, but there are common themes. The seven characteristics typically considered are size at birth; growth patterns; age and size at maturity; number, size and sex ratio of offspring; reproductive investments; mortality investments and length of life (Stearns, 1992).

The early gerentology selection experiments using Drosophila, were aimed at unpicking the evolutionary history of ageing. They were looking for life history characteristics that seemed to be traded for long life during selection, since such tradeoffs would provide solid evidence for the antagonistic pleiotropy theory of ageing, over that of simple mutation accumulation. Long-lived organisms would be expected to show fitness tradeoffs for their longevity, in particular in early life fitness characteristics. Late life fitness characters tend to show very high variability, even within longer lived populations, further indicating that these early life traits may be primarily responsible for any effects on ageing (Rose, 1985).

Indeed, selecting to modify or delay early fitness has yielded the expected results, the ease by which lifespan is extended by selecting on delayed fecundity has been discussed already in Chapter 2. Other possibilities, selecting for lower adult body weight for instance, have successfully extended lifespan in D. melanogaster. In this case the lifespan extension may be due to the correlation between a higher adult body weight and higher early fecundity thus extending lifespan by the same mechanism; this further underlines the difficulty in unpicking the effects of individual life history characteristics on lifespan (Hillesheim and Stearns, 1992).

19.1

Fecundity

By far the most studied life history characteristic in longevity selection is lifetime fertility, so traits related to number of offspring and reproductive investment. When selecting for delayed

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they are discussed here because of their relevance to the accompanying lifespan extension. The initial pilot selection by Rose and Charlesworth (1981b) noted a highly significant (p<0.01) decrease in early egg laying, rate of laying and a significant increase in the age at last laying in the long-lived flies. This was corroborated by the accompanying sib-analysis which found a strong negative correlation between longevity and laying rate, and a strong positive correlation between longevity and age at last laying event (Rose and Charlesworth, 1981a). The follow up selection (Rose, 1984) further agreed with these observations. The long-lived selected lines had lower initial fecundity and higher later fecundity, although as mentioned the results get more erratic the older the flies get. Interestingly, these experiments show that longevity can be associated with a simple changing of reproductive schedule to be more spread out over a longer reproductive life, rather than a

depression in overall fecundity as might have been expected. Contemporary selection experiments tended to agree with these data. The long-lived selected lines of Luckinbill, et al. (1984) also showed a lower initial fecundity, laying fewer eggs than the controls, but maintained a younger egg layer profile for longer, and saw a spike of egg production at an advanced age. This was attributed to higher late-life amino acid incorporation of the selected lines, suggesting a slowed development and a metabolic peak later in life (Pretzlaff and Arking, 1989). Finally, neither set of selected lines of Partridge and Fowler (1992) saw a decrease in early fecundity, however they both saw an increased level of fecundity over their lifespan relative to the controls.

The recurrent patterns of fecundity are not surprising for this kind of selection experiment. The selection pressure is acting on improving late-life fitness characteristics, and coupled with the improved physiological health of the selected flies, they would be expected to have a significantly higher fecundity at these ages. Likewise the depressed early fecundity is expected, since the controls they are compared to are effectively selected for an increase in early fecundity, and the selection experiments where this characteristic was not seen (Partridge and Fowler, 1992; Zwaan et al., 1995) used methodologies designed to avoid it. Interestingly, given the impression that slowed

development in the selected lines is responsible for the lifespan extension and fecundity changes, direct selection on time to sexual maturity saw no changes in fecundity and only a weak lifespan extension in females, with none in males (Promislow and Bugbee, 2000). This supports the

conclusions of Pretzlaff and Arking (1989) that the increased amino acid turnover they observed in their long-lived flies was the cause of, and not a result of, the observed changes in fecundity.

19.2

Development

Larval development can be characterised in numerous ways, but typically focuses on the viability of eggs and larvae to reach adulthood, larval competitive ability and the time taken to pass through the

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various larval stages and reach adulthood. Viability is interesting from an ageing perspective,

because viability of offspring is one of many factors that declines with age (Kern et al., 2001) and yet it appears to be inversely correlated with longevity in many selected strains of Drosophila. While the lines of Rose (1984) showed no change in viability, those of Luckinbill et al. (1984) and an

independently generated replicate of this selection carried out later by the same lab (Buck et al., 2000) saw a decline in viability tightly correlated with longevity. Strangely, these lines have also shown the opposite when tested some years later, with the selected lines showing a relative increase in pre-adult viability (Chippindale et al., 1994). The lines of Partridge and Fowler (1992) agree with the initial results, showing a lower viability in the long-lived lines, both when the pure lines were tested and when they were hybridised within treatment to alleviate inbreeding depression. This effect was somewhat dependent on larval density, but consistent in most cases (Caroline Roper, Pignatelli and Partridge, 1993). Finally, when selecting directly on longevity, there has been no observed effect on viability (Zwaan, Bijlsma and Hoekstra, 1995). Given this method is designed to avoid selecting on early fitness characteristics as much as possible, it is not surprising that there is no viability effect. Although viability can be related to longevity, it is not necessary for its evolution.

Because long-lived adults show patterns of slower development, it may be expected that they would also have slower pre-adult development, this however is not the case. In several experiments, and after correcting for inbreeding depression where necessary, larval development time has shown no difference or been significantly shorter in selected lines compared to a base stock or randomly selected control (Caroline Roper, Pignatelli and Partridge, 1993; Zwaan, Bijlsma and Hoekstra, 1995; Buck et al., 2000). In cases where there has been an increase in development time associated with longevity, (Partridge and Fowler, 1992; Chippindale et al., 1994) there have been inconsistencies in the relation between development time and viability, with one study correlating longer

development with decreased viability, and the other with increased viability. Furthermore, differences in how the early life stages are handled in different selection experiments make these characteristics very difficult to compare.

Another characteristic common to many long-lived organisms, especially those whose longevity is dependent on a single mutation, is a reduced adult body size. This has been observed in Drosophila (David J. Clancy et al., 2001; Kapahi et al., 2004a) and mice (Flurkey et al., 2001) and runs counter to the observation in the wild that larger animals tend to live longer (Speakman, 2005). It is unlikely in these cases that the long-lived phenotype is a result of reduced body size, rather than a decreased metabolism caused by nutrient-sensing manipulations has led to a decreased body size and extended lifespan. In mammals especially, unhealthy ageing is associated with a decrease in body

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weight and this is shown to be more rapid in animals with mutations that lead to premature ageing (Trifunovic et al., 2004). In the same vein, longevity selected flies have sometimes been shown to have an increased body size (Partridge and Fowler, 1992), which could be a result of their increased development time, providing them with a more robust adult body at the expense of decreased early life viability and fitness.