An Experimental Evolution Test of the Relationship between Melanism and Desiccation Survival in Insects

Abstract

We used experimental evolution to test the ‘melanism-desiccation’ hypothesis, which proposes that dark cuticle in several Drosophila species is an adaptation for increased desiccation tolerance. We selected for dark and light body pigmentation in replicated populations of D. melanogaster and assayed several traits related to water balance. We also scored pigmentation and desiccation tolerance in populations selected for desiccation survival. Populations in both selection regimes showed large differences in the traits directly under selection. However, after over 40 generations of pigmentation selection, dark-selected populations were not more desiccation-tolerant than light-selected and control populations, nor did we find significant changes in mass or carbohydrate amounts that could affect desiccation resistance. Body pigmentation of desiccation-selected populations did not differ from control populations after over 140 generations of selection, although selected populations lost water less rapidly. Our results do not support an important role for melanization in Drosophila water balance.

Spatiotemporal dynamics and genome‐wide association analysis of desiccation tolerance in Drosophila melanogaster

“…Here, we observed a significant but shallow cline in desiccation tolerance at normal assay temperatures of 25°C (Figure ). While body size may play a role in desiccation tolerance, the relationship between variation in body size and desiccation tolerance among D. melanogaster populations appears complex (Rajpurohit et al., ).…”

“…However, the genetic architecture of desiccation tolerance remains unresolved. In Drosophila melanogaster, tolerance evolves rapidly in response to artificial selection in the laboratory (Bradley, Williams, & Rose, 1999;Gefen, Marlon, & Gibbs, 2006;Gibbs, Chippindale, & Rose, 1997;Telonis-Scott, Sgro, Hoffmann, & Griffin, 2016) and appears associated with reduced water loss rates (Rajpurohit, Peterson, Orr, Marlon, & Gibbs, 2016;Telonis-Scott, Guthridge, & Hoffmann, 2006). Selection for increased tolerance results in differential gene expression (Sorensen, Nielsen, & Loeschcke, 2007) and significant allele frequency changes across hundreds of genes (Telonis-Scott, Gane, DeGaris, Sgro, & Hoffmann, 2012;Telonis-Scott et al, 2006), indicating a complex genetic architecture (Foley & Telonis-Scott, 2011;Matzkin et al, 2009;Rajpurohit, Oliveira, et al, 2013).…”

Darker eggs resist more to desiccation: the case of melanin inAedes,AnophelesandCulexmosquito vectors

“…Another hypothesis is that melanin might be hydrophobic and thus hamper water flux through the cuticle, as recently suggested (Rajpurohit et al, 2016). Although both melanin precursors (DHICA and DHI, Figure 1B) are hydrophilic compounds, the molecular structure of melanin polymers varies depending on the biochemical conditions of polymerization and, therefore, "melanin" is a diffuse term for a rather diverse group of complex pigments (Prota, 1992; Ito et al, 2011; d'Ischia et al, 2013; Shamim et al, 2014; Arakane et al, 2016).…”

“…In fact, exists in the literature descriptions of melanin being both water-soluble (Mostert et al, 2010) and water-insoluble (Shamim et al, 2014). Thus the D. melanogaster darker-selected populations might not have a higher desiccation resistance (Rajpurohit et al, 2016) due to the production of "hydrophilic melanins" in this specific situation.…”

“…On the other hand, populations of D. melanogaster artificially selected for increased pigmentation does not resist desiccation more than control flies (Rajpurohit et al, 2016). This apparent incoherence might be due to other factors, since the reduction in the rate of water loss by the cuticle is one out of the three aspects of the desiccation resistance (see below).…”