Our understanding of aging in certain model species such as the nematode Caenorhabditis elegans, the fruitfly Drosophila melanogaster, and the mouse is well advanced
(Longo and Finch, 2003). However, there are still relatively few animal species for which there are even basic descriptions of how they age. Among nematodes, patterns of aging are highly diverse, and lifespans can vary over more than a 1,000 fold range (Gems, 2001). Strongyloides ratti, the subject of this chapter, is a nematode that has previously been used as a model organism in parasitological studies (Grove, 1989). As a biogerontological model, it is attractive because of its unique level of phenotypic plasticity in aging and the genetics of its life cycle.
Much has been learned about the genetics of lifespan by the use of mutational studies in model organisms. In C. elegans, increases in lifespan of up to 7-fold have been observed in long-lived mutants (Houthoofd et al., 2004). While substantial, such differences are relatively small, compared with evolved differences in lifespan between different animal species. The genetic basis of such evolved differences remains unknown. It is also unclear whether similar genes and mechanisms underlie both the altered aging in model organisms as uncovered by the analysis of mutants and the evolved differences in aging between different species. These issues may be explored via a comparative approach and through insights gained into the biology of aging from the existing diversity in evolved patterns of aging in the animal kingdom. Thus, it is worthwhile to study the basic phenomenology of aging in species with interesting life histories, such as S. ratti.
Many animal species show phenotypic plasticity in aging, of which there are two major types. The first is the relatively limited response to a changing environment by a given stage in the life cycle. For example, dietary restriction extends lifespan in many species, including rodents (McCay et al., 1935), Drosophila (Nusbaum and Rose, 1999), and C. elegans (Lakowski and Hekimi, 1998). A more dramatic form of phenotypic plasticity in aging is associated with the development of different morphs within a given life cycle. For example, in C. elegans there is a facultative diapausal form of the third-stage larva, known as the dauer larva. This long-lived stress-resistant dispersal form develops when food is scarce and population density high (Golden and Riddle, 1984) and can survive for up to 3 months (Klass and Hirsh, 1976). This compares to a two- or three-week lifespan in C. elegans adults (Klass, 1977). Striking differences in lifespan between adult morphs also occur in social insects (Finch, 1990), where honey bee queens survive for up to 40-times longer than genetically identical summer worker bees. Understanding the evolutionary and physiological determinants of such plasticity of aging and lifespan can provide insight into the biology of aging.
In this chapter we review what is known about S. ratti aging. We describe optimization of culture conditions for aging studies, environmental factors that affect lifespan, and age changes in behavior, pathology and mortality that accompany aging in this unusual nema-tode. In the concluding section, we consider the question of how the phenotypic plasticity in S. ratti may have evolved.
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