Body melanization and its adaptive role in thermoregulation and tolerance against desiccating conditions in drosophilids

Abstract

Melanism seems to have evolved independently through diverse mechanisms in various taxa and different ecological factors could be responsible for selective responses. Increased body melanization at higher altitudes as well as latitudes is generally considered to be adaptive for thermoregulation. Physiological traits such as body melanization and desiccation resistance have been investigated independently in diverse insect taxa at three levels: within populations, between populations and among species. A substantial number of Drosophila studies have reported clinal variations in both these traits along latitude. A possible link between these traits had remained unexplored in wild and laboratory populations of ectothermic insect taxa, including drosophilids, to date. Simultaneous analysis of these traits in assorted darker and lighter phenotypes in each population in the present study showed parallel changes for body melanization and desiccation resistance. The mechanistic basis of evolving desiccation resistance was explained on the basis of differential rates of water loss per hour in darker versus lighter phenotypes in six populations of Drosophila melanogaster from adjacent localities differing substantially in altitude all along the Indian subcontinent. Data on cuticular impermeability suggest a possible role of melanization in desiccation tolerance. However, substantial gaps remain in extending these results to other insect taxa and further exploring the physiological and molecular changes involved in melanization for conferring desiccation resistance.

Impact of body melanisation on contrasting levels of desiccation resistance in a circumtropical and a generalist Drosophila species

“…Laboratory selected dark and light strains in D. melanogaster might differ significantly in their desiccation resistance level but this aspect has not been analyzed so far. However, a positive correlation between body melanisation and desiccation resistance is evident from the analysis of yellow body color mutant strain of D. melanogaster (Kalmus 1941); and on the basis of assorted darker and lighter flies in populations of different Drosophila species (Rajpurohit et al 2008). In D. melanogaster, we found significant differences in desiccation resistance levels between assorted dark and light flies from a given population (Parkash et al 2008a, d).…”

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

“…Such a mismatch between field and laboratory selection may be due to selection acting on some other associated trait that may impact resistance to starvation and desiccation in different ways. There is ample empirical support for the thermal melanism hypothesis (Watt, 1969;David et al, 1985;David et al, 1990;Jong et al, 1996;Majerus, 1998;Rajpurohit et al, 2008). Several studies have demonstrated a direct influence of melanisation on body temperature by increasing solar absorption under cool conditions (Jong et al, 1996;Ottenheim et al, 1999;Ellers and Boggs, 2004).…”

Two genomic regions together cause dark abdominal pigmentation in Drosophila tenebrosa

“…In no Drosophila species except where pigmentation is sex-specific is the ecological force that selected for the dark phenotype known. Dark pigmentation of some Drosophila species tends to associate with higher altitudes, where the coloration may help the flies regulate temperature or prevent desiccation through the control of cuticular water loss (Gibert et al, 1998;Brisson et al, 2005;Pool and Aquadro, 2007;Rajpurohit et al, 2008). However, based on climate data, the geographic range of D. tenebrosa is not generally colder or drier compared with the boreal forests of the northern North America, where D. suboccidentalis and several other lightly colored and closely related quinaria group species occur.…”