Comments On Physiology

The appearance of male and female flies is described elsewhere; see the Recommended Resources section. Like the mammals for which it often serves as a proxy, Drosophila has a nervous system, hormonal system, and circulatory system. However it lacks several systems present in mammals, and many shared systems function somewhat differently. First, as a poikilotherm, temperature has a direct effect on metabolic rate, and lifespan of the flies can be significantly extended when they are kept at lower temperatures (see below). Because the adult fly consists mostly of postmitotic cells, issues such as replicative senescence, cell proliferation in aging, and cancer development have not been considered in flies; however, these factors may be important in higher organisms that have many mitotic cells.

In the mammalian insulin signaling pathway, functions of growth and metabolism in mammals are regulated by different receptors, the IGF-1 and insulin receptors. In contrast, in Drosophila, diverse physiological functions (including growth, metabolism, and reproduction) are regulated by a single hormonal receptor in the insulin signaling pathway (Tatar, 2004). The insulin-signaling pathway influences the levels of other insect hormones including juvenile hormone and the sterol ecdysone, which themselves may affect aging (Garofalo, 2002).

The respiration of Drosophila is mediated by simple diffusion; oxygen uptake is achieved by diffusion through the trachea (Weis-Fogh, 1964). In flight muscles, mitochondria are located almost adjacent to tracheole. Flight muscle mitochondria in insects have an exceptionally high oxidative metabolism capacity capable of supporting around 100-fold increases in oxygen consumption on initiation of flight (Sacktor, 1974). Recently, mitochondria have been at the center of attention in aging research. First, mitochondria are implicated as a major endogenous source of reactive oxygen species that can damage surrounding molecules; second, many experimental manipulations that extend lifespan are also known to alter glucose and lipid metabolism, and logically mitochondria are thought to play an important role. Finally, mitochondria are thought to be responsible for apoptosis, at least in part. Thus, together with the fact that mitochondria undergo significant morphological alterations with age (Sohal, 1976), highly active insect flight muscle mitochondria could be an interesting research subject.

The fat body, the organ that has been implicated as playing a role in regulating lifespan (Hwangbo et al., 2004; Giannakou et al., 2004), is equivalent to liver and white adipose tissue in mammals. It stores fat and glycogen, and is also responsible for immune function by producing antimicrobial peptides upon infection. In contrast to mammals and many other vertebrates, insects have only an innate immune system.

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