CONICET | Buscador de Institutos y Recursos Humanos

There are different views on senescence as a process. In its most general conception it represents the change in the biology of an organism as it ages. However this process may be viewed either at the physiological or at the demographic level. In the former sense senescence deals with changes affecting cells and tissues of the organism and their function and its effect on the organism as a whole (somatic senescence). In the demographic sense (actuarial senescence) the emphasis is in the population´s survival decrease as a function of age (Promislow 1991, Tatar et al., 1993); this very general definition does not necessary imply somatic senescence (a physiological deterioration) because the organism may suffer an increased age-specific mortality rate because of an increased reproductive effort (Roff, 2002); the decrease in the reproductive performance with age may be termed reproductive senescence. Williams (1957), based on evolutionary arguments, claims that natural selection will frequently maximize vigor in youth at the expense of vigor later on and thereby he identifies senescence as a declination in vigor during adult life, using the term vigor as associated with a reproductive probability distribution. Here we are interested in this second approach to senescence, and we adhere to the definition of Rose (1991, cited in Roff 2002): "a persistent decline in the age-specific fitness components of an organism due to internal physiological deterioration". According to Charlesworth & Partridge (1997) there are two main theories trying to explain the senescence process: (1) natural selection is less effective at reducing the frequency of later-acting mutations in populations, and so ageing is expected to evolve, and this is known as the "mutation accumulation" theory of ageing; (2) mutations that increase fitness at younger ages (perhaps because they increase fertility) but at the expense of decreasing fitness at later ages (perhaps because they increase the death rate) can be incorporated into a population because natural selection will act more strongly on the earlier, beneficial effect. This is the reasoning behind the "antagonistic pleiotropy" or "trade-off" theory of ageing (Williams, 1957). Abrams & Ludwig (1995), based on an extension of the "disposable soma" model (Kirkwood & Holliday, 1979), provide an explicit realization of the trade-off idea which postulates a conflict between the allocation of resources to reproduction and to the repair of somatic damage. A reduction in damage repair at a given age is assumed to cause an elevated death rate at all subsequent ages. Given a functional relationship between repair allocation and reproductive rate at a given age, the age-specific pattern of allocation to repair versus reproduction that maximizes life-time reproductive success can be determined, yielding a prediction of the age-specific pattern of mortality for the optimal life history (Charlesworth & Partridge, 1997). Longevity and senescence patterns in mammals and birds are very variable according to the life-history of these organisms (Gaillard et al., 2004), and variation within and between phyla can be expected. Despite phylogenetic similarity is reciprocal to taxonomic level of relatedness (Cheverud et al., 1985) it is also possible that species phylogenetically related show different senescence and/or longevity patterns. So a comparative approach focused on the frequency of senescence in closely related species may contribute to our knowledge of the senescence process. Comparative studies in insects are scarce as compared to mammals and birds; within the insects, demographic analyses of senescence have been carried out mainly for the Diptera (Styer et al., 2007; Curtsinger et al., 1992; Carey et al., 1992, 2005; Fukui et al., 1993), and the Coleoptera (Tatar et al., 1993), and very few studies have considered hemimetabolous species (Dingle, 1966, Chaves et al., 2004a,b, Rabinovich et al., 2010). In this chapter we investigate the frequency of senescence in a closely related species group of insects: the Triatominae (Hemiptera: Reduviidae). We compiled from the bibliography and resorted to personal data to have phylogeny and life history traits of 27 species reared under laboratory controlled and comparable conditions, and analyzed mortality and fecundity through several death and reproductive parameters. In particular we investigated: (a) species patterns of mortality with respect to age (from cohort studies that followed all individuals from the egg stage until the death of the last individual), (b) the relationship between mortality and different life-history traits (size, reproductive allocation), and (c) the relationship between mortality and environmental factors. Being the 27 selected triatomine species close relatives (they belonged to only five different genera), for our comparative study we included in the analysis a correction for the possible effect of the degree of phylogenetic relatedness. The advantage of working on laboratory data is that we can estimate intrinsic mortality and fecundity rates without confounding effects resulting from extrinsic factors acting on mortality and fecundity (predation, accidental deaths, starvation, etc.). Even if work with triatomines has the advantage that under natural conditions all life stages occur in a single type of environment and have similar biological requirements, there is always the disadvantage that laboratory data does not reflect natural condition: insects are fed ad libitum, and predators, parasites and pathogens are kept out, so that it does not constitute the best of conditions to detect trade-offs between reproductive effort and mortality. However, we think (in agreement with Mueller et al., 2005) that the identification of which aspects of the environment matter in the evolution of trade-offs can only be obtained by performing experiments in which these environmental variables are carefully manipulated; additionally, if even under such stable and near optimal conditions we are able to detect trade-offs, then our conclusions become much more robust. Although life history traits such as fecundity, juvenile and adult survival, fasting capacity, developmental time, mortality patterns, and life span have been estimated under controlled conditions in the laboratory for a variety of triatomine species (about 500 scientific articles have been written on these aspects since 1910), very few studies have considered recent evolutionary ecology concepts (although see Menu et al., 2010) to shed some light on the trade-off aspects of life history traits. Understanding the mortality pattern in this group of insects is important both for academic and human health reasons. In the former sense we will provide elements to contribute to the theory of senescence and we discuss our results within of this theoretical background; in particular we will analyze the senescence pattern looking into the relationship between reproductive effort and mortality. In the latter sense, our analyses will provide information about a group of insects that are the vectors of Chagas disease, and represent a health threat estimated in 28 million people, living mostly in Latin America (see WHO, 2007).

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