Phototoxic effect of UVR on wild type, ebony and yellow mutants of Drosophila melanogaster: Life Span, fertility, courtship and biochemical aspects

No evidence for the melanin desiccation hypothesis in a larval Lepidopteran

Water regulation is an important physiological challenge for insects due to their small body sizes and large surface area to volume ratios. Adaptations for decreasing cuticular water loss, the largest avenue of loss, are especially important. The melanin desiccation hypothesis states that melanin molecules in the cuticle may help prevent water loss, thus offering protection from desiccation. This hypothesis has much empirical support in Drosophila species, but remains mostly untested in other taxa, including Lepidoptera. Because melanin has many other important functions in insects, its potential role in desiccation prevention is not always clear. In this study we investigated the role of melanin in desiccation prevention in the white-lined Sphinx moth, Hyles lineata (Lepidoptera, Sphingidae), which shows high plasticity in the degree of melanin pigmentation during the late larval instars. We took advantage of this plasticity and used density treatments to induce a wide range of cuticular melanization; solitary conditions induced low melanin pigmentation while crowded conditions induced high melanin pigmentation. We tested whether more melanic larvae from the crowded treatment were better protected from desiccation in three relevant responses: i) total water loss over a desiccation period, ii) change in hemolymph osmolality over a desiccation period, and iii) evaporation rate of water through the cuticle. We did not find support for the melanin desiccation hypothesis in this species. Although treatment influenced total water loss, this effect did not occur via degree of melanization. Interestingly, this implies that crowding, which was used to induce high melanin phenotypes, may have other physiological effects that influence water regulation. There were no differences between treatments in cuticular evaporative water loss or change in hemolymph osmolality. However, we conclude that osmolality may not sufficiently reflect water loss in this case. This study emphasizes the context dependency of melanin's role in desiccation prevention and the importance of considering how it may vary across taxa. In lepidopteran larvae that are constantly feeding phytophagous insects with soft cuticles, melanin may not be necessary for preventing cuticular water loss.

Climate and body size have differential roles on melanism evolution across workers in a worldwide ant genus

One of the main aspects associated with the diversity in animal colour is the variation in melanization levels. In ectotherms, melanism can be advantageous in aiding thermoregulation through heat absorption. Darker bodies may also serve as a shield from harmful UV-B radiation. Melanism may also confer protection against parasites and predators through improving immunity responses and camouflage in regions with high precipitation, with complex and shaded vegetations and greater diversity of pathogens and parasites. We studied melanism evolution in the globally distributed ant genus Pheidole under the pressures of temperature, UV-B radiation and precipitation, while considering the effects of body size and nest habit, traits that are commonly overlooked. More importantly, we account for worker caste polymorphism, which is marked by distinct roles and behaviours. We revealed for the first time distinct evolutionary trajectories for each worker subcaste. As expected, major workers from species inhabiting locations with lower temperatures and higher precipitation tend to be more melanised. Curiously, we show a slight trend where minor workers of larger species also tend to have darker bodies when inhabiting regions with higher precipitation. Lastly, we did not find evidence for the effects of UV-B radiation and nest habit in the lightness variation of workers. Our paper explores the evolution of ant melanization considering a marked ant worker polymorphism and a wide range of ecological factors. We discuss our findings under the light of the Thermal Melanism Hypothesis, the Photoprotection Hypothesis and the Gloger's Rule.

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.

Darker ants dominate the canopy: Testing macroecological hypotheses for patterns in colour along a microclimatic gradient

Gradients in cuticle lightness of ectotherms have been demonstrated across latitudes and elevations. Three key hypotheses have been used to explain these macroecological patterns: the thermal melanism hypothesis (TMH), the melanism‐desiccation hypothesis (MDH) and the photo‐protection hypothesis (PPH). Yet the broad abiotic measures, such as temperature, humidity and UV‐B radiation, typically used to detect these ecogeographical patterns, are a poor indication of the microenvironment experienced by small, cursorial ectotherms like ants.

We tested whether these macroecological hypotheses explaining cuticle lightness held at habitat and microclimatic level by using a vertical gradient within a tropical rainforest.

We sampled 222 ant species in lowland, tropical rainforest across four vertical strata: subterranean, ground, understory and canopy. We recorded cuticle lightness, abundance and estimated body size for each species and calculated an assemblage‐weighted mean for cuticle lightness and body size for each vertical stratum. Abiotic variables (air temperature, vapour pressure deficit and UV‐B radiation) were recorded for each vertical stratum.

We found that cuticle lightness of ant assemblages was vertically stratified: ant assemblages in the canopy and understory were twice as dark as assemblages in ground and subterranean strata. Cuticle lightness was not correlated with body size, and there was no support for the TMH. Rather, we attribute this cline in cuticle lightness to a combination of the MDH and the PPH.

Our findings indicate that broad macroecological patterns can be detected at much smaller spatial scales and that microclimatic gradients can shape trait variation, specifically the cuticle lightness of ants. These results suggest that any changes to microclimate that occur due to land‐use change or climate warming could drive selection of ants based on cuticle colour, altering assemblage structure and potentially ecosystem functioning.

Stage-Specific and Seasonal Induction of the Overwintering Morph of Spotted Wing Drosophila (Diptera: Drosophilidae)

Drosophila suzukii Matsumura (Diptera: Drosophilidae) is currently a major pest management challenge in berry and cherry production. This species has a winter morph phenotype with longer wings and increased melanization associated with survival in colder conditions. Measurements of wing morphology in Michigan D. suzukii collected during 2016 and 2017 showed that induction of this morph began in September and increased into December, correlated with decreasing temperature and day length. Importantly, we found that wing length increases along a continuous scale and there is overlap between the two morph types. We tested whether temperature or photoperiod elicited this phenotypic change using a factorial design with each preadult lifestage held at 10 or 25°C and 16:8 or 8:16 L:D. Our results support temperature as the main driver of transition to the winter morph for all immature stages. Comparing the reproductive capacity of winter morph flies in cold conditions and when previously acclimated to warm conditions, flies with the acclimation experience laid comparable numbers of eggs as the summer morphs at 25°C, indicating that winter morphs can reproduce after surviving cold periods. These results highlight the ability of D. suzukii to adapt to changing temperature conditions, allowing it to survive cold and also exploit warmer periods to build populations when conditions allow.

Life History Trade-offs within the Context of Mitochondrial Hormesis

Evolutionary biologists have been interested in the negative interactions among life history traits for nearly a century, but the mechanisms that would create this negative interaction remain poorly understood. One variable that has emerged as a likely link between reproductive effort and longevity is oxidative stress. Specifically, it has been proposed that reproduction generates free radicals that cause oxidative stress and, in turn, oxidative stress damages cellular components and accelerates senescence. We propose that there is limited support for the hypothesis because reactive oxygen species (ROS), the free radicals implicated in oxidative damage, are not consistently harmful. With this review, we define the hormetic response of mitochondria to ROS, termed mitochondrial hormesis, and describe how to test for a mitohormetic response. We interpret existing data using our model and propose that experimental manipulations will further improve our knowledge of this response. Finally, we postulate how the mitohormetic response curve applies to variation in animal performance and longevity.

Seasonal cues induce phenotypic plasticity of Drosophila suzukii to enhance winter survival

BACKGROUND

As global climate change and exponential human population growth intensifies pressure on agricultural systems, the need to effectively manage invasive insect pests is becoming increasingly important to global food security. Drosophila suzukii is an invasive pest that drastically expanded its global range in a very short time since 2008, spreading to most areas in North America and many countries in Europe and South America. Preliminary ecological modeling predicted a more restricted distribution and, for this reason, the invasion of D. suzukii to northern temperate regions is especially unexpected. Investigating D. suzukii phenology and seasonal adaptations can lead to a better understanding of the mechanisms through which insects express phenotypic plasticity, which likely enables invasive species to successfully colonize a wide range of environments.

RESULTS

We describe seasonal phenotypic plasticity in field populations of D. suzukii. Specifically, we observed a trend of higher proportions of flies with the winter morph phenotype, characterized by darker pigmentation and longer wing length, as summer progresses to winter. A laboratory-simulated winter photoperiod and temperature (12:12 L:D and 10 °C) were sufficient to induce the winter morph phenotype in D. suzukii. This winter morph is associated with increased survival at 1 °C when compared to the summer morph, thus explaining the ability of D. suzukii to survive cold winters. We then used RNA sequencing to identify gene expression differences underlying seasonal differences in D. suzukii physiology. Winter morph gene expression is consistent with known mechanisms of cold-hardening such as adjustments to ion transport and up-regulation of carbohydrate metabolism. In addition, transcripts involved in oogenesis and DNA replication were down-regulated in the winter morph, providing the first molecular evidence of a reproductive diapause in D. suzukii.

CONCLUSIONS

To date, D. suzukii cold resistance studies suggest that this species cannot overwinter in northern locations, e.g. Canada, even though they are established pests in these regions. Combining physiological investigations with RNA sequencing, we present potential mechanisms by which D. suzukii can overwinter in these regions. This work may contribute to more accurate population models that incorporate seasonal variation in physiological parameters, leading to development of better management strategies.

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.

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.

EPR Studies of DOPA-Melanin Complexes with Netilmicin and Cu(II) at Temperatures in the Range of 105-300 K

The application of electron paramagnetic resonance (EPR) spectroscopy in pharmacy of melanin complexes with netilmicin and Cu(II) was presented. The continuous microwave saturation of EPR spectra of DOPA-melanin and the complexes was performed. EPR spectra were measured on an X-band (9.3 GHz) spectrometer at temperatures in the range of 105-300 K. Paramagnetic copper ions decrease the intensity of the EPR lines of melanin's free radicals. It was found that fast spin-lattice relaxation characterizes DOPA-melanin-Cu(II) complexes. Slow spin-lattice relaxation processes exist in melanin's paramagnetic centers of DOPA-melanin and DOPA-melanin-netilmicin, [DOPA-melanin-netilmicin]-Cu(II), [DOPA-melanin-Cu(II)]-netilmicin complexes. Spin-lattice relaxation processes are faster at higher temperatures. The homogeneous broadening of EPR lines for melanin complexes was observed. The practical consequences of differences between paramagnetic properties of melanin complexes with netilmicin and the complexes with Cu(II) were discussed.

yellow0, a marker for low body weight in Drosophila melanogaster

Marker-assisted selection (MAS) is an important modern breeding technique, but it has been found that the effect of the markers for quantitative trait loci (QTL) is inconsistent, leading in some cases to MAS failure and raising doubts about its effectiveness. Here the model organism Drosophila melanogaster was employed to study whether an effective marker could be found and applied to MAS. We crossed the stock carrying the y (0) marker (a recessive mutation allele of the yellow gene on the X chromosome) with three other stocks carrying corresponding wild-type markers in an F2 design, and found that the y (0) marker was in significant association with low body weight (P<0.001). This association was consistent across different backgrounds and the marker effects in female and male were approximately 0.95 sigma (P) (phenotypic standard deviation) and 0.68 sigma (P), respectively. We next introgressed a fragment via the y (0) marker into a wild stock background over 20 generations of marker-assisted introgression (MAI), and constructed the introgression stock y (0)(OR)20 in which body weight decreased by 13% and 7%, in female and male, respectively, compared to the wild stock (P<0.0001). This indicated that there must be a single QTL for low body weight that is tightly linked to the y (0) marker. We then shortened the introgressed fragment to less than 1.5 cM by a deeper MAI using the y (0) marker and the white marker. This narrower fragment also resulted in a similar decrease in body weight to that induced by y (0)(OR)20, indicating that the QTL for low body weight is located within this less-than-1.5 cM interval. Molecular characteristics of the y (0) marker by PCR amplification and Southern blotting revealed that yellow gene was deficient in the y (0) stock, leading to disappearance of melanin from the cuticle and probably influencing the developmental process. The above results confirmed the existence of effective QTL markers applicable to MAS breeding schemes, and their potential application in breeding new stocks.