Comparative Aspects

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While we argue here that the honeybee is an emerging model organism in aging research, it has already attained the status of a model organism in several other biological disciplines and it serves as the model social insect. Social evolution has generated a wide variety of social systems with unique selection pressures and adaptations that provide many opportunities for testing ultimate theories of aging, as well as study the proximate causes of naturally evolved aging differentials. Sociality with overlapping generations, cooperative brood care, and (reproductive) division of labor, as in the honeybee, has evolved multiple times in insects, with termites, ants, wasps, and bees as prominent representatives (for other groups see Choe and Crespi, 1997).

The multiple, independent evolution of sociality offers the prospects of analyzing phylogenetic trends in a comparative context, such as the analysis of the evolution of lifespan itself (Keller and Genoud, 1997). We can study in detail the phylogenetic correlation between the degree of social evolution (such as societal complexity, colony size, or morphological queen-worker dimorphism) and the lifespan extension in reproductives. Furthermore, any lifespan-extending mechanism identified in honeybee queens can be tested for its generality and its natural evolutionary history reconstructed by studying it in related, less socially advanced species (Parker et al.,

2004). The broad taxonomic array of social insects with widely differing social systems and biology provides many opportunities for comparative research. Particularly valuable ''wide-scale'' comparisons would seem to be between flying and nonflying, predatory and herbivorous, or monogynous (groups with a single reproductive queen) and polygynous (multiple reproductive queens) groups as these life history variables seem to have some impact on longevity (Finch, 1990; Keller and Genoud, 1997). If we are to understand how social evolution has affected lifespan, it is not sufficient to compare solitary and highly social species but to study intermediates and follow plausible trends in social evolution. This can lead to insights into the ultimate factors that lead to long-lived reproductives and social evolution, as well as the reinforcement between the two (Carey, 2001; Carey and Judge, 2001). All four species of honeybees are highly social, but in their family (the Apidae) solitary species and numerous intermediates exist (Michener, 2000). One such intermediate are bumblebees whose colonies are annual and all individuals except for the newly produced (and mated) queens die at the end of the season. This life history should select for very different longevity patterns at the individual level, with an even more pronounced seasonal plasticity than in the honeybee. Another interesting feature of bumblebees is the large size variation among workers that leads to a division of labor that is not age-dependent. Therefore, we have life-long behavioral specialists with differences in external mortality and consequently predicted differences in endogenous aging rates.

Like bumblebees, some ant species also exhibit division of labor that is related to functional size divergence (Holldobler and Wilson, 1990) and cannot be regarded as an adaptation to aging (Tofilski, 2002). However, this has been used to confirm that a higher extrinsic mortality of outside workers has led to the evolution of a shorter intrinsic lifespan, relative to workers that work inside the protected colony (Chapuisat and Keller, 2002). Other ant species follow the honeybee model of age-dependent division of labor, probably with similar mortality patterns. In general, lifespans in ants are much longer than in honeybees. This makes the evolution of their lifespan extension more extreme, and significant research efforts are geared towards elucidating mechanisms of their great queen longevity (Hartmann and Heinze, 2003; Parker et al., 2004; Schrempf et al., 2005), which will provide interesting comparisons to the results from honeybees. However, the much longer lifespan also makes the practical aspects of aging studies in ants more difficult than studies in the honeybee.

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