the mortality rate with increasing age (Finch, 1990). The pattern of age-specific mortality in many species (including humans) can be described by the Gompertz equation, m(t) = Aeat, where m(t) is the mortality rate at time ', A is the initial mortality rate (IMR) and a is the Gompertz exponential function (Finch, 1990). Population cohorts may be extinguished as the result of mortality that is related or unrelated to aging; therefore, differences in lifespan per se do not necessarily reflect differences in the rate of aging. Limitations of lifespan by aging, as opposed to some other extrinsic cause of mortality (e.g., predation or disease), may be established by testing for the occurrence of an exponential increase in mortality rate with increasing age (Wachter and Finch, 1997). From plots of ln(mortality rate) against age, the IMR and the Gompertz exponential function can be calculated. The latter may be expressed in a form that is easier to grasp: the mortality rate doubling time (MRDT). These mortality parameters for free-living S. rat'i females and parasitic S. rat'i females are described below.

Age-specific mortality in S. ratti free-living adult females

The survival of free-living S. ratti females under optimized S. ratti culture conditions are shown in Figure 20.2B. This shows that its maximum lifespan is 4.5 ± 0.8 days. In our studies of free-living S. ratti we were at pains to distinguish whether the very short lifespan resulted from rapid aging, or from some form of pathology distinct from aging. To this end, we compared age changes in S. ratti with those in C. elegans. The latter was cultured either monoxenically (i.e., with E. coli 0P50 alone), or under the same conditions as S. ratti (i.e. with SR 0P50 and low-level contamination with other microbes). (Figure 20.2B).

The mean lifespan of C. elegans hermaphrodites under either culture condition (S. ratti conditions: 7.7 ± 0.1 days; monoxenic: 11.2 ± 0.1 days) was substantially and significantly longer than that of S. ratti (3.0 ± 0.1 days) (Gardner et al., 2004; Figure 20.2B). Unsurprisingly, C. elegans lifespan was greater in monoxenic culture conditions than in S. ratti optimized culture conditions, presumably due to the presence of contaminating microbes in the S. ratti culture conditions (Figure 20.2B).

In all cases, an age-specific mortality rate acceleration was observed (Figure 20.2C), consistent with the occurence of senescence (i.e., intrinsic aging). For S. ratti the MRDT (±SE) was lower and the IMR (±SE) higher (0.8 ± 0.1 days; 0.025 ± 0.002 days"1, respectively) than in C. elegans (in either culture condition). For C. elegans under the two culture conditions, there was no difference in the MRDT (S. ratti-conditions: 1.4 ± 0.2 days; monoxenic conditions: 2.0 ± 0.3 days), but the IMR for C. elegans kept under S. ratti culture conditions was almost twice that of monoxenically cultured populations (S. ratti conditions: 0.0036 ± 0.0003 days"1; monoxenic: 0.0018 ± 0.0002 days"1). Overall, these findings imply that S. ratti is shorter lived than C. elegans because (a) it is more frail, as reflected by higher IMR, and (b) it ages more quickly, as reflected by a lower MRDT.

Age-specific mortality in S. ratti parasitic females In S. ratti parasitic females, maximum lifespan is 403 days (Gardner, Gems and Viney, submitted; Figure 20.2D), which compares to 5 days in the free-living morph (Figure 20.2D).

The maximum lifespan of S. ratti parasitic females is 403 days, which is some 80 times greater than the 5-day lifespan of the free-living females. The parasitic females underwent an exponential increase in mortality rate with age, thereby showing that senescence was occuring. The IMR of the parasitic females was 0.0056 days"1, which is approximately 25% of that of the free-living females. The MRDT of the parasitic females was 22.7 days, which is some 30 times longer than that of the free-living females. Overall, this comparison of the parasitic and free-living females implies that free-living morphs are shorter-lived due to their increased frailty (as reflected by the greater IMR) and a faster rate of aging (as reflected by lower MRDT). S. ratti is the first parasitic nematode in which the occurrence of demographic senescence has been demonstrated in the laboratory.

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