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

Melanic pigmentation and light preference within and between two Drosophila species

Environmental adaptation and species divergence often involve suites of co-evolving traits. Pigmentation in insects presents a variable, adaptive, and well-characterized class of phenotypes for which correlations with multiple other traits have been demonstrated. In Drosophila, the pigmentation genes ebony and tan have pleiotropic effects on flies' response to light, creating the potential for correlated evolution of pigmentation and vision. Here, we investigate differences in light preference within and between two sister species, Drosophila americana and D. novamexicana, which differ in pigmentation in part because of evolution at ebony and tan and occupy environments that differ in many variables including solar radiation. We hypothesized that lighter pigmentation would be correlated with a greater preference for environmental light and tested this hypothesis using a habitat choice experiment. In a first set of experiments, using males of D. novamexicana line N14 and D. americana line A00, the light-bodied D. novamexicana was found slightly but significantly more often than D. americana in the light habitat. A second experiment, which included additional lines and females as well as males, failed to find any significant difference between D. novamexicana-N14 and D. americana-A00. Additionally, the other dark line of D. americana (A04) was found in the light habitat more often than the light-bodied D. novamexicana-N14, in contrast to our predictions. However, the lightest line of D. americana, A01, was found substantially and significantly more often in the light habitat than the two darker lines of D. americana, thus providing partial support for our hypothesis. Finally, across all four lines, females were found more often in the light habitat than their more darkly pigmented male counterparts. Additional replication is needed to corroborate these findings and evaluate conflicting results, with the consistent effect of sex within and between species providing an especially intriguing avenue for further research.

Genetic architecture of a body colour cline in Drosophila americana

Phenotypic variation within a species is often structured geographically in clines. In Drosophila americana, a longitudinal cline for body colour exists within North America that appears to be due to local adaptation. The tan and ebony genes have been hypothesized to contribute to this cline, with alleles of both genes that lighten body colour found in D. americana. These alleles are similar in sequence and function to the allele fixed in D. americana's more lightly pigmented sister species, Drosophila novamexicana. Here, we examine the frequency and geographic distribution of these D. novamexicana-like alleles in D. americana. Among alleles from over 100 strains of D. americana isolated from 21 geographic locations, we failed to identify additional alleles of tan or ebony with as much sequence similarity to D. novamexicana as the D. novamexicana-like alleles previously described. However, using genetic analysis of 51 D. americana strains derived from 20 geographic locations, we identified one new allele of ebony and one new allele of tan segregating in D. americana that are functionally equivalent to the D. novamexicana allele. An additional 5 alleles of tan also showed marginal evidence of functional similarity. Given the rarity of these alleles, however, we conclude that they are unlikely to be driving the pigmentation cline. Indeed, phenotypic distributions of the 51 backcross populations analysed indicate a more complex genetic architecture, with diversity in the number and effects of loci altering pigmentation observed both within and among populations of D. americana. This genetic heterogeneity poses a challenge to association studies and genomic scans for clinal variation, but might be common in natural populations.

Multiple loci linked to inversions are associated with eye size variation in species of the Drosophila virilis phylad

The size and shape of organs is tightly controlled to achieve optimal function. Natural morphological variations often represent functional adaptations to an ever-changing environment. For instance, variation in head morphology is pervasive in insects and the underlying molecular basis is starting to be revealed in the Drosophila genus for species of the melanogaster group. However, it remains unclear whether similar diversifications are governed by similar or different molecular mechanisms over longer timescales. To address this issue, we used species of the virilis phylad because they have been diverging from D. melanogaster for at least 40 million years. Our comprehensive morphological survey revealed remarkable differences in eye size and head shape among these species with D. novamexicana having the smallest eyes and southern D. americana populations having the largest eyes. We show that the genetic architecture underlying eye size variation is complex with multiple associated genetic variants located on most chromosomes. Our genome wide association study (GWAS) strongly suggests that some of the putative causative variants are associated with the presence of inversions. Indeed, northern populations of D. americana share derived inversions with D. novamexicana and they show smaller eyes compared to southern ones. Intriguingly, we observed a significant enrichment of genes involved in eye development on the 4th chromosome after intersecting chromosomal regions associated with phenotypic differences with those showing high differentiation among D. americana populations. We propose that variants associated with chromosomal inversions contribute to both intra- and interspecific variation in eye size among species of the virilis phylad.

Desiccation resistance and pigmentation variation reflects bioclimatic differences in the Drosophila americana species complex

Background

Disentangling the selective factors shaping adaptive trait variation is an important but challenging task. Many studies—especially in Drosophila—have documented trait variation along latitudinal or altitudinal clines, but frequently lack resolution about specific environmental gradients that could be causal selective agents, and often do not investigate covariation between traits simultaneously. Here we examined variation in multiple macroecological factors across geographic space and their associations with variation in three physiological traits (desiccation resistance, UV resistance, and pigmentation) at both population and species scales, to address the role of abiotic environment in shaping trait variation.

Results

Using environmental data from collection locations of three North American Drosophila species—D. americana americana, D. americana texana and D. novamexicana—we identified two primary axes of macroecological variation; these differentiated species habitats and were strongly loaded for precipitation and moisture variables. In nine focal populations (three per species) assayed for each trait, we detected significant species-level variation for both desiccation resistance and pigmentation, but not for UV resistance. Species-level trait variation was consistent with differential natural selection imposed by variation in habitat water availability, although patterns of variation differed between desiccation resistance and pigmentation, and we found little evidence for pleiotropy between traits.

Conclusions

Our multi-faceted approach enabled us to identify potential agents of natural selection and examine how they might influence the evolution of multiple traits at different evolutionary scales. Our findings highlight that environmental factors influence functional trait variation in ways that can be complex, and point to the importance of studies that examine these relationships at both population- and species-levels.

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

Montane insects are at a higher risk of desiccation than their lowland counterparts and are expected to have evolved reduced water loss. Hemideina spp. (tree weta; Orthoptera: Anostostomatidae) have both lowland (Hemideina femorata, Hemideina crassidens and Hemideina thoracica) and montane (Hemideina maori and Hemideina ricta) species. H. maori has both melanic and yellow morphs. We use these weta to test two hypotheses: that montane insects lose water more slowly than lowland species, and that cuticular water loss rates are lower in darker insects than lighter morphs, because of incorporation of melanin in the cuticle. We used flow-through respirometry to compare water loss rates among Hemideina species and found that montane weta have reduced cuticular water loss by 45%, reduced respiratory water loss by 55% and reduced the molar ratio of V̇H2 O:V̇CO2  by 64% compared with lowland species. Within H. maori, cuticular water loss was reduced by 46% when compared with yellow morphs. Removal of cuticular hydrocarbons significantly increased total water loss in both melanic and yellow morphs, highlighting the role that cuticular hydrocarbons play in limiting water loss; however, the dark morph still lost water more slowly after removal of cuticular hydrocarbons (57% less), supporting the melanisation-desiccation resistance hypothesis.

The Genetic Basis of Pigmentation Differences Within and Between Drosophila Species

In Drosophila, as well as in many other plants and animals, pigmentation is highly variable both within and between species. This variability, combined with powerful genetic and transgenic tools as well as knowledge of how pigment patterns are formed biochemically and developmentally, has made Drosophila pigmentation a premier system for investigating the genetic and molecular mechanisms responsible for phenotypic evolution. In this chapter, we review and synthesize findings from a rapidly growing body of case studies examining the genetic basis of pigmentation differences in the abdomen, thorax, wings, and pupal cases within and between Drosophila species. A core set of genes, including genes required for pigment synthesis (eg, yellow, ebony, tan, Dat) as well as developmental regulators of these genes (eg, bab1, bab2, omb, Dll, and wg), emerge as the primary sources of this variation, with most genes having been shown to contribute to pigmentation differences both within and between species. In cases where specific genetic changes contributing to pigmentation divergence were identified in these genes, the changes were always located in noncoding sequences and affected cis-regulatory activity. We conclude this chapter by discussing these and other lessons learned from evolutionary genetic studies of Drosophila pigmentation and identify topics we think should be the focus of future work with this model system.

A single gene causes an interspecific difference in pigmentation in Drosophila

The genetic basis of species differences remains understudied. Studies in insects have contributed significantly to our understanding of morphological evolution. Pigmentation traits in particular have received a great deal of attention and several genes in the insect pigmentation pathway have been implicated in inter- and intraspecific differences. Nonetheless, much remains unknown about many of the genes in this pathway and their potential role in understudied taxa. Here we genetically analyze the puparium color difference between members of the virilis group of Drosophila. The puparium of Drosophila virilis is black, while those of D. americana, D. novamexicana, and D. lummei are brown. We used a series of backcross hybrid populations between D. americana and D. virilis to map the genomic interval responsible for the difference between this species pair. First, we show that the pupal case color difference is caused by a single Mendelizing factor, which we ultimately map to an ∼11-kb region on chromosome 5. The mapped interval includes only the first exon and regulatory region(s) of the dopamine N-acetyltransferase gene (Dat). This gene encodes an enzyme that is known to play a part in the insect pigmentation pathway. Second, we show that this gene is highly expressed at the onset of pupation in light brown taxa (D. americana and D. novamexicana) relative to D. virilis, but not in the dark brown D. lummei. Finally, we examine the role of Dat in adult pigmentation between D. americana (heavily melanized) and D. novamexicana (lightly melanized) and find no discernible effect of this gene in adults. Our results demonstrate that a single gene is entirely or almost entirely responsible for a morphological difference between species.

Pigmentation in Drosophila melanogaster reaches its maximum in Ethiopia and correlates most strongly with ultra-violet radiation in sub-Saharan Africa

Background

Pigmentation has a long history of investigation in evolutionary biology. In Drosophila melanogaster, latitudinal and altitudinal clines have been found but their underlying causes remain unclear. Moreover, most studies were conducted on cosmopolitan populations which have a relatively low level of genetic structure and diversity compared to sub-Saharan African populations. We investigated: 1) the correlation between pigmentation traits within and between the thorax and the fourth abdominal segment, and 2) their associations with different geographical and ecological variables, using 710 lines belonging to 30 sub-Saharan and cosmopolitan populations.

Results

Pigmentation clines substantially differed between sub-Saharan and cosmopolitan populations. While positive correlations with latitude have previously been described in Europe, India and Australia, in agreement with Bogert's rule or the thermal melanism hypothesis, we found a significant negative correlation in Africa. This correlation persisted even after correction for altitude, which in its turn showed a positive correlation with pigmentation independently from latitude. More importantly, we found that thoracic pigmentation reaches its maximal values in this species in high-altitude populations of Ethiopia (1,600-3,100 m). Ethiopian flies have a diffuse wide thoracic trident making the mesonotum and the head almost black, a phenotype that is absent from all other sub-Saharan or cosmopolitan populations including high-altitude flies from Peru (~3,400 m). Ecological analyses indicated that the variable most predictive of pigmentation in Africa, especially for the thorax, was ultra-violet (UV) intensity, consistent with the so-called Gloger's rule invoking a role of melanin in UV protection.

Conclusion

Our data suggest that different environmental factors may shape clinal variation in tropical and temperate regions, and may lead to the evolution of different degrees of melanism in different high altitude populations in the tropics.

Pigmentation and behavior: potential association through pleiotropic genes in Drosophila

The molecular basis of pigmentation variation within and among Drosophila species is largely attributed to genes in melanin biosynthesis pathway, which involves dopamine metabolism. Most of the genetic changes underlying pigmentation variations reported to date are changes at the expression levels of the structural genes in the pathway. Within D. melanogaster, changes in cis-regulatory regions of a gene, ebony, are responsible for the naturally occurring variation of the body pigmentation intensity. This gene is also known to be expressed in glia, and many visual and behavioral abnormalities of its mutants have been reported. This implies that the gene has pleiotropic functions in the nervous systems. In this review, current knowledge on pigmentation variation and melanin biosynthesis pathway are summarized, with some focus on pleiotropic features of ebony and other genes in the pathway. A potential association between pigmentation and behavior through such pleiotropic genes is discussed in light of cis-regulatory structure and pleiotropic mutations.

The influence of abdominal pigmentation on desiccation and ultraviolet resistance in two species of Drosophila

Drosophila yakuba and D. santomea are sister species that differ in their levels of abdominal pigmentation; D. yakuba shows heavily pigmented posterior abdominal segments in both sexes, whereas D. santomea lacks dark pigment anywhere on its body. Using naturally collected lines, we demonstrate the existence of altitudinal variation in abdominal pigmentation in D. yakuba but not in D. santomea. We use the variation in pigmentation within D. yakuba and two body-color mutants in D. yakuba to elucidate selective advantage of differences in pigmentation. Our results indicate that although differences in abdominal pigmentation have no effect on desiccation resistance, lighter pigmentation confers ultraviolet radiation resistance in this pair of species.