Figure. 4.15 Survival of control (C) and desiccation-resistant (D) Drosophila melanogaster during desiccation stress is strongly related to haemolymph volume. Filled circles are means for five D populations and open circles are means for five C populations (± SE). Haemolymph volume (ml) was measured by a blotting technique in 10 flies per population. Source: Folk et al. (2001).
for this 'canteen' of extra water is the haemolymph. Desiccation resistance is strongly correlated with haemolymph volume (Fig. 4.15), which increases significantly from a mean of 0.078 ml in control flies to 0.323 ml in desiccation-resistant flies (Folk et al. 2001). Moreover, the increased carbohydrate content which is related to survival during desiccation consists not only of glycogen, but also haemolymph trehalose. Trehalose is known to play a role in protecting against desiccation stress (Ring and Danks 1998).
Drosophila is an excellent model system for comparing the results of laboratory and natural selection for resistance to desiccation. Cactophilic Drosophila have invaded deserts on multiple occasions, and these desert species lose water less rapidly than non-desert species and are more tolerant of dehydration but, surprisingly, water contents do not differ in Drosophila species from a variety of habitats (Gibbs and Matzkin 2001). This is in marked contrast to the difference in water content between laboratory-selected D and C flies, but the lack of difference between them in dehydration tolerance (Gibbs et al. 1997). The mechanistic basis of resistance thus seems to differ in the laboratory and the field, probably because of differences in selection regimes. Laboratory selection is carried out at constant temperature, in a simple habitat which offers fewer behavioural options. Sex differences are apparent during selection and there are costs involved, such as slower development times in desiccation-resistant Drosophila (Chippindale et al. 1998). Desert Drosophila inhabit necrotic tissues in columnar cacti, with microclimates which offer hygric but not thermal benefits (Gibbs et al. 2003b). Perhaps desert Drosophila do not accumulate water because the additional load may compromise flight performance when it is necessary to fly to the next cactus, whereas D flies in the laboratory remain relatively inactive (Gibbs 1999).
Another advantage of studying water balance in Drosophila is that the evolutionary history of the genus is well studied and detailed phylogenetic information is available for use in comparative studies. Gibbs and colleagues (Gibbs and Matzkin 2001; Gibbs et al. 2003a) have now examined the evolution of water balance in 30 species from mesic and xeric environments. After phylogenetic relationships were incorporated into the analysis, cacto-philic Drosophila differed from their mesic congeners only in rates of water loss: their greater tolerance of dehydration appears to be an ancestral trait. Thus water conservation is the primary means of increasing survival during desiccation stress. The most recent evidence (Gibbs et al. 2003a) suggests that reduced water loss is achieved by varying respiratory parameters: compared to mesic species, cactophilic Drosophila are less active, have lower metabolic rates, and are more likely to exhibit a pattern of cyclic CO2 release with possible implications for water savings.
Can the evolution of desiccation resistance be reversed? Relaxation of directional selection in Drosophila led to a decline in desiccation resistance after 100 generations, along with flight duration (a correlated character also linked to glycogen reserves) (Graves et al. 1992). Reverse selection has recently been examined over a much longer (geological) time-scale in non-drosophilids—a group of sub-Antarctic weevils (Curculionidae) occurring on two Southern Ocean islands (Chown and Klok 2003). Phylogenetic analysis shows a sharp reduction in body size as angiosperms
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