Variations in body melanization impact desiccation resistance inDrosophila immigransfrom Western Himalayas

Abstract

Ectothermic species face problems of water balance under colder and drier climatic conditions in montane localities. We investigated five ecophysiological traits (body melanization, desiccation resistance, rate of water loss or gain and body size) in eight populations of Drosophila immigrans from an altitudinal gradient (600-2226 m) in the Western Himalayas. The traits showed bell-shaped variability patterns characteristic of quantitative traits. For body melanization, we observed high heritability (0.65) on the basis of parent-offspring regression. A comparison of highland versus foothill populations showed significant divergence for all the traits except body size. Darker flies from the highlands exhibited higher desiccation resistance but reduced rate of water loss or gain as compared with lighter flies from the foothills, which showed lower desiccation resistance and higher rates of water loss as well as gain. Lack of differences in the amount of epicuticular lipids cannot account for differential reduction in cuticular water loss in altitudinal populations. However, within-as well as between-population differences in body melanization can account for changes in desiccation resistance and reduction in cuticular water loss. Analyses of highland versus lowland populations as well as in assorted darker and lighter flies from a highland population have shown differences in haemolymph and dehydration tolerance. For the mechanistic basis of desiccation resistance, our results on wild populations of Drosophila immigrans are not in agreement with those reported for laboratory-selected desiccation-resistant strains in Drosophila melanogaster. Thus, ecophysiological mechanisms could be different under laboratory versus field selection.

Water loss in tree weta (Hemideina): adaptation to the montane environment and a test of the melanisation–desiccation resistance hypothesis

“…Under this resistance hypothesis, melanic individuals have a selective advantage in more desiccating environments, like montane environments, than less or non-melanic individuals. Melanic Drosophila morphs were found in more-desiccating environments (higher altitudes) than non-melanic morphs (Parkash et al, 2008b(Parkash et al, , 2009, whereas darker abdominal pigmentation positively correlated strongly with altitude in sub-Saharan D. melanogaster populations (Pool and Aquadro, 2007). However, the altitudinal distribution of melanic H. maori differs from this pattern.…”

Direct and correlated responses to laboratory selection for body melanisation inD. melanogaster: support for melanism- desiccation resistance hypothesis.

“…For adaptations to drier habitats, the function of cuticular lipids in reducing cuticular water loss is well known in different insect taxa from deserts (Hadley, 1994). However, some studies have shown the role of melanisation in reducing cuticular water loss in Drosophila species (Parkash et al, 2008b;Parkash et al, 2008c). Associations between pigmentation and desiccation resistance were initially proposed by Kalmus (Kalmus, 1941).…”

“…Changes in body melanisation have been shown to affect cuticular permeability in some Drosophila species (Parkash et al, 2008b;Parkash et al, 2008c). According to the melanisation-desiccation hypothesis, darker flies of D. melanogaster are abundant in cooler uplands while lighter flies are predominant in foothills (Parkash et al, 2008c).…”

“…Changes in body melanisation have been shown to affect cuticular permeability in some Drosophila species (Parkash et al, 2008b;Parkash et al, 2008c). According to the melanisation-desiccation hypothesis, darker flies of D. melanogaster are abundant in cooler uplands while lighter flies are predominant in foothills (Parkash et al, 2008c). The laboratory-selected desiccation-resistant and -sensitive strains of D. melanogaster have shown similar amounts of cuticular lipid mass (Gibbs et al, 1997), and there is also a lack of differences in the amount of cuticular lipid mass in northern versus southern populations of D. melanogaster (Parkash et al, 2008a).…”

“…Further, there is lack of differences in melting temperature (T m ), composition and/or amount of surface lipids in laboratory selected desiccation-resistant and -sensitive strains of D. melanogaster (Gibbs et al, 1997). Within species, Indian populations of D. immigrans differ in water-loss rate, but not in surface lipid amounts (Parkash et al, 2008c). In a laboratory selection experiment, populations selected for desiccation resistance lost water ~50% less rapidly than unselected controls, but the two groups exhibited minor differences in lipid composition and T m (Gibbs et al, 1997).…”