Comparative Studies

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Ants as a group are extremely diverse, and this diversity lends itself to many types of comparative studies across species and across populations. There are also exploitable differences across caste and sex within a single species. Some of these represent reversals of typical life span correlations with size and longevity as observed in humans and birds. For example, there are two sizes of workers in the weaver ant, Oecophylla smaragdina. The major (large) workers perform the dangerous tasks outside the colony and have shorter life spans than the minor (small) workers who remain within the highly protected nest. But this difference in life span remains intact even in a protected laboratory environment (Chapuisat and Keller, 2002). It would be interesting to investigate if there are metabolic differences between these two sizes of worker ants.

The phenomena of monogynous/polygynous colonies have evolved independently many times across a wide range of ants and other social insect taxa. There is evidence that, within the same species, queens of monogynous populations live longer than queens of polygynous populations as predicted by evolutionary theory (Keller and Genoud, 1997). Solenopsis invicta is an excellent system to investigate life-span variation within a single species but across populations with different social structure as it has both polygynous and monogynous populations. In addition, the genetic basis for this difference in queen number is already known (Krieger and Ross, 2002).

One possibility for cross-species comparisons is the many cases of social parasites. Socially parasitic ants lack a worker caste and invade a host colony where their reproductive brood are raised by the host colony workers. In the genus Pogonomyrex, there are socially parasitic ants who live only 1-3 years and have recently evolved from a shared common ancestor of their longer lived hosts (P. barbatus and P. rugous (Johnson et al., 1996; Parker and Rissing, 2002). They represent an opportunity to investigate the evolution to a shorter life span once the relevant processes are discovered.

Reproductive rate correlates with life span such that highly fertile individuals have shorter life spans than less fertile members of the same species (Williams, 1957; Charlesworth, 1980; Partridge and Gems, 2002). Yet this fecundity/longevity tradeoff is reversed for queens and workers. The queen of a large social insect nest must lay eggs at a rapid rate to maintain the number of sterile workers in the colony, and yet she has an extremely long life span (Rueppell et al., 2004). This reversal of the fecundity/longevity tradeoff has been shown true even without the morphological and physiological differences which are present between queen and worker ants. In the Ponerine ant Platytyrea punctata, all workers are capable of producing diploid workers, yet the reproductive workers live significantly longer than their non-reproductive counterparts (Hartmann and Heinze, 2003).

Given that castes share the same genome, the differences in life span must be based on differential gene expression at some point(s) in their life history. This provides the foundation for testing gene expression differences already associated with aging in model systems. If the mechanisms hypothesized for life span extension in model systems are truly general, then there should be evidence of these same mechanisms being used in queen ants. One such mechanism, the resistance to oxidative stress conferred by superoxide dismutase

(SOD), was recently tested in a comparison of cytoplasmic SOD activity and expression levels across all castes (Parker et al., 2004a). The study found that a high level of cytoplasmic SOD does not correlate with life span as for previous work in Drosophila (Orr and Sohal, 1994; Sohal et al., 1995; Hari et al., 1998; Sun and Tower, 1999; Arking et al., 2002; Spencer et al., 2003) and is in agreement with previous comparative studies (Perez-Campo et. al, 1998; Barja, 2002).

These small, highly fecund and extraordinarily long-lived queens must overcome all of the physiological problems of maintaining mitochondria, proteins which are degraded, damaged and/or incorrectly folded, as well as accumulative damage to DNA and membranes that are associated with long life. Queens in particular must do all of these things an order of magnitude longer than workers, and while reproducing at a high enough rate to sustain a colony. All of these individual molecular processes represent potentially fruitful lines of inquiry that could reveal novel aging resistance mechanisms.

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