One remaining oddity of S. ratti is that the free-living adults are really very short-lived, even by the standards of short-lived free-living nematode species. There are several possible explanations for this. First, the free-living adult phase of the life cycle is facultative (Viney, 1996). Hence when it does not occur, this may weaken selection on lifespan in the same manner as high extrinsic mortality. Second, it might result from antagonistic pleiotropy between the effects of genes on fitness in the free-living and parasitic forms. Aging may reflect action of alleles that increase early-life fitness (e.g., by increasing reproductive output) but have deleterious late-life effects (Williams, 1957). Such antagonistic pleiotropy is supported by experimental investigation (Kirkwood and Austad, 2000; Partridge and Gems, 2002). Hence, in the case of S. ratti, the short life of the free-living adults may result from the greater fecundity of the parasitic female, which may favor pleiotropic alleles that increase parasitic fecundity but reduce lifespan of free-living adults.
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