Local adaptation for body color in Drosophila americana

Abstract

Pigmentation is one of the most variable traits within and between Drosophila species. Much of this diversity appears to be adaptive, with environmental factors often invoked as selective forces. Here, we describe the geographic structure of pigmentation in Drosophila americana and evaluate the hypothesis that it is a locally adapted trait. Body pigmentation was quantified using digital images and spectrometry in up to 10 flies from each of 93 isofemale lines collected from 17 locations across the United States and found to correlate most strongly with longitude. Sequence variation at putatively neutral loci showed no evidence of population structure and was inconsistent with an isolation-by-distance model, suggesting that the pigmentation cline exists despite extensive gene flow throughout the species range, and is most likely the product of natural selection. In all other Drosophila species examined to date, dark pigmentation is associated with arid habitats; however, in D. americana, the darkest flies were collected from the most humid regions. To investigate this relationship further, we examined desiccation resistance attributable to an allele that darkens pigmentation in D. americana. We found no significant effect of pigmentation on desiccation resistance in this experiment, suggesting that pigmentation and desiccation resistance are not unequivocally linked in all Drosophila species.

Local adaptation for body color in Drosophila americana: commentary on Wittkopp et al.

“…It is clear from our analyses of Wittkopp et al's (2011) data that the TMH may provide the major explanation for color variation in D. americana and requires closer scrutiny. Nevertheless, and in agreement with Wittkopp et al (2011), it seems that the desiccation resistance hypothesis is an unlikely explanation in this species. Studies should consider both hypotheses simultaneously when attempting to explain pigment variation of ectotherms in relation to climate.…”

“…has been associated with dry environments (for example, Parkash et al, 2010). This relationship represents the main hypothesis tested by Wittkopp et al (2011), and the authors conclude that pigmentation and desiccation resistance are not unequivocally linked among the populations investigated. However, other climatic explanations that are not addressed in the study may explain these color patterns.…”

“…Recently, the TMH has received support at several spatial scales in reptiles (Clusella-Trullas et al, 2008, 2009) and has also been demonstrated in many insect species (for example, Brakefield and Willmer, 1985;reviewed in Clusella-Trullas et al, 2007). Wittkopp et al (2011) showed that longitude was a better explanatory variable than environmental relative humidity. However, latitude and longitude are typically used as proxies of climate (for example, temperature and humidity) and site (topography) variation.…”

“…Using the data from Wittkopp et al (2011), we reanalyzed their largest dataset (Table 1, dataset A, p 6), seeking the most likely climatic explanation for the observed geographic variation in pigment intensity (with light-colored populations having higher values of pigment intensity than darker populations). We used an information-theoretic model selection approach (Burnham and Anderson, 2002) to address both the TMH and the desiccation resistance hypothesis as potential explanations.…”

Two genomic regions together cause dark abdominal pigmentation in Drosophila tenebrosa

“…While several other species in the quinaria group are also found in these mountains, D. tenebrosa is the only species that is found primarily at high elevations. In other Drosophila species, dark pigmentation has been correlated with increased UV resistance and tolerance to desiccation and cold temperature, all of which are expected to be greater at higher elevation (reviewed by True (2003) and Wittkopp and Beldade (2009), but see Wittkopp et al (2011)). Thus, it is possible that the dark pigmentation found in D. tenebrosa is a recently evolved adaptive trait.…”