Heat induced male sterility in Drosophila melanogaster: adaptive genetic variations among geographic populations and role of the Y chromosome

Temperature-based extrinsic reproductive isolation in two species of Drosophila

Drosophila santomea and D. yakuba are sister species that live on the African volcanic island of São Tomé, where they are ecologically isolated: D. yakuba inhabits low-altitude open and semiopen habitats while D. santomea lives in higher-elevation rain and mist forest. To determine whether this spatial isolation reflected differential preference for and tolerance of temperature, we estimated fitness components of both species at different temperatures as well as their behavioral preference for certain temperatures. At higher temperatures, especially 28 degrees C, D. santomea was markedly inferior to D. yakuba in larval survival, egg hatchability, and longevity. Moreover, D. santomea females, unlike those of D. yakuba, become almost completely sterile after exposure to a temperature of 28 degrees C, and conspecific males become semisterile. Drosophila santomea adults prefer temperatures 2-3 degrees C lower than do adults of D. yakuba. Drosophila santomea, then, is poorly adapted to high temperature, partially explaining its restriction to cool, high habitats, which leads to extrinsic premating isolation and immigrant inviability. Rudimentary genetic analysis of the interspecific difference in egg hatchability and larval survival showed that these differences are due largely to cytoplasmic effects and to autosomal genes, with sex chromosomes playing little or no role.

Phenotypic plasticity of abdomen pigmentation in two geographic populations of Drosophila melanogaster: male-female comparison and sexual dimorphism

In Drosophila melanogaster male, the last abdominal tergites (A5-A6) are completely dark due to a strong internal constraint while, in female, all abdominal tergites (A2-A7) are phenotypically variable and highly plastic. Male A2-A4 are quite similar to those of female, but their plasticity was never investigated. In this paper, we compared the phenotypic plasticity of A2-A4 in both sexes in order to know if the major dimorphism (SD) expressed in male A5-A6 also extended toward the more anterior segments. We also compared two geographic populations living under very different climates in order to know if adaptive differences, previously observed in females also existed in males. With an isofemale line design, pigmentation variation according to growth temperature was investigated in the two populations from France and India. Male and female data were compared and sexual dimorphism (SD) analyzed in various ways. Reaction norms were quite similar in both sexes for A2 and A3, but clearly different for A4. Considering the total pigmentation (A2 + A3 + A4) males were darker than females at low temperatures and either identical to them (France) or lighter (India) above 25 degrees C. SD (male-female difference) was genetically variable among lines and significantly different among segments. Reaction norms of SD exhibited an overall decrease with temperature and also a significant difference among populations, suggesting a local adaptation of SD to thermal conditions. The three plastic segments in male (A2-A4) seem to react adaptively to the thermal environment more efficiently than the same segments in female, in agreement with the thermal budget hypothesis. To our knowledge, it is the first time that a SD trait exhibits an adaptive difference between geographic populations.

[Male sterility at high and low temperatures in Drosophila]

It is well known that in Mammals, spermatogenesis requires a temperature lower than that of the body. In Ectotherms, for example in Insects, male sterility/ fertility according to environmental conditions also remains a neglected field. In Drosophila melanogaster, a complete male sterility after development at 30 degrees C was described in 1971. A similar phenomenon, observed at low temperature, was described two years later. Recent comparative investigations have shown that what was found in D. melanogaster was also valid in other species. In each case, it is possible to define a range of temperatures compatible with a complete development. According to the investigated species, however, this range is very variable, for example 6-26 degrees C or 16-32 degrees C. In each case, the occurrence of sterile males is observed before the lethality threshold is reached. Such a phenomenon is probably important for understanding the geographic distributions of species. The cosmopolitan D. melanogaster lives under very different climates and exhibits corresponding adaptations. In countries with a very hot summer, such as India or the African Sahel, male sterility appears only at 31 degrees C. Crosses between a temperate population from France and a heat-resistant Indian population revealed that a large part of the genetic difference was carried by the Y chromosome. Such a result is surprising since the Y chromosome harbors only a very small number of genes. In conclusion, drosophilid species, during their evolution, were able to adapt to very different climates and the thermal sterility thresholds have changed, following these adaptations. But we still lack an evolutionary hypothesis for explaining why sterile males are, in all cases, produced at extreme, low or high temperatures.

Polymorphic Y chromosomes harbor cryptic variation with manifold functional consequences

The paucity of polymorphisms in single-copy genes on the Y chromosome of Drosophila contrasts with data indicating that this chromosome has polymorphic phenotypic effects on sex ratio, temperature sensitivity, behavior, and fitness. We show that the Y chromosome of D. melanogaster harbors substantial genetic diversity in the form of polymorphisms for genetic elements that differentially affect the expression of hundreds of X-linked and autosomal genes. The affected genes are more highly expressed in males, more meagerly expressed in females, and more highly divergent between species. Functionally, they affect microtubule stability, lipid and mitochondrial metabolism, and the thermal sensitivity of spermatogenesis. Our findings provide a mechanism for adaptive phenotypic variation associated with the Y chromosome.

Life history consequences of temperature transients in Drosophila melanogaster

The physiological and life history consequences of chronic temperatures are well studied in ectotherms. However, little is known about the consequences of short-term exposure to unusually high or low temperatures, as would occur during a weather front. What are the immediate life-history effects of such thermal transients? Can ectotherms recover quickly or do they suffer carry-over effects that persist after weather returns to normal? We measured the impact of thermal transients on egg and progeny production of Drosophila melanogaster Meigen from Washington State. We reared flies at 25°C and then transferred 3- to 5-day old adults to one of three transient treatments (1 or 3 days at 18°C, 1 day at 29°C) before returning them to 25°C. We monitored daily egg production and egg-to-adult viability before (as a control), during, and after the transient as well as fecundity and viability of flies held at constant 18°, 25° and 29°C. This population appears particularly heat tolerant as neither constant nor transient exposure to 29°C (usually a stressful temperature for this species) affected female fecundity or the viability of her progeny. However, a 1- or 3-day exposure to 18°C reduced female fecundity by 75–90% relative to controls, and eggs laid during the 3-day exposure had greatly reduced viability. When returned to 25°C after transient exposure to 18°C, females immediately matched the fecundity and viability of females maintained constantly at 25°C. Therefore, these flies do not suffer negative carry-over effects from these moderate thermal transients. Surprisingly, fitness (intrinsic rate of population growth) was not depressed by transient temperature exposure. However, the severity and especially the timing of the transient will probably determine the likelihood of carry-over effects as well as its effect on fitness.

Consequences of Heat Hardening on a Field Fitness Component in Drosophila Depend on Environmental Temperature

Heat hardening increases thermal resistance to more extreme temperatures in the laboratory. However, heat hardening also has negative consequences, and the net benefit of hardening has not been evaluated in the field. We tested short-term heat hardening effects on the likelihood of Drosophila melanogaster to be caught at different temperatures at baits in field sites without natural resources. We predicted that hardened flies should be more frequently caught at the baits at high but not low temperatures. Under cool conditions, flies hardened at 36 degrees C, and to a lesser extent at 34 degrees C, were less frequently caught at baits than nonhardened flies a few hours after release, indicating a negative effect of hardening. In later captures, negative effects tended to disappear, particularly in males. Under warm conditions, there was an overall balance of negative and positive effects, though with a different temporal resolution. Under very hot conditions, when capture rates were low, there was a large benefit of hardening at 36 degrees C and 34 degrees C but not 33 degrees C. Finally, based on climatic records, the overall benefit of hardening in D. melanogaster is discussed as an evolved response to high temperatures occasionally experienced by organisms at some locations.

Thermal evolution of pre-adult life history traits, geometric size and shape, and developmental stability in Drosophila subobscura

Replicated lines of Drosophila subobscura originating from a large outbred stock collected at the estimated Chilean epicentre (Puerto Montt) of the original New World invasion were allowed to evolve under controlled conditions of larval crowding for 3.5 years at three temperature levels (13, 18 and 22 degrees C). Several pre-adult life history traits (development time, survival and competitive ability), adult life history related traits (wing size, wing shape and wing-aspect ratio), and wing size and shape asymmetries were measured at the three temperatures. Cold-adapted (13 degrees C) populations evolved longer development times and showed lower survival at the highest developmental temperature. No divergence for wing size was detected following adaptation to temperature extremes (13 and 22 degrees C), in agreement with earlier observations, but wing shape changes were obvious as a result of both thermal adaptation and development at different temperatures. However, the evolutionary trends observed for the wing-aspect ratio were inconsistent with an adaptive hypothesis. There was some indication that wing shape asymmetry has evolutionarily increased in warm-adapted populations, which suggests that there is additive genetic variation for fluctuating asymmetry and that it can evolve under rapid environmental changes caused by thermal stress. Overall, our results cast strong doubts on the hypothesis that body size itself is the target of selection, and suggest that pre-adult life history traits are more closely related to thermal adaptation.

Heat-shock promoters: targets for evolution by P transposable elements in Drosophila

Transposable elements are potent agents of genomic change during evolution, but require access to chromatin for insertion—and not all genes provide equivalent access. To test whether the regulatory features of heat-shock genes render their proximal promoters especially susceptible to the insertion of transposable elements in nature, we conducted an unbiased screen of the proximal promoters of 18 heat-shock genes in 48 natural populations of Drosophila. More than 200 distinctive transposable elements had inserted into these promoters; greater than 96% are P elements. By contrast, few or no P element insertions segregate in natural populations in a “negative control” set of proximal promoters lacking the distinctive regulatory features of heat-shock genes. P element transpositions into these same genes during laboratory mutagenesis recapitulate these findings. The natural P element insertions cluster in specific sites in the promoters, with up to eight populations exhibiting P element insertions at the same position; laboratory insertions are into similar sites. By contrast, a “positive control” set of promoters resembling heat-shock promoters in regulatory features harbors few P element insertions in nature, but many insertions after experimental transposition in the laboratory. We conclude that the distinctive regulatory features that typify heat-shock genes (in Drosophila) are especially prone to mutagenesis via P elements in nature. Thus in nature, P elements create significant and distinctive variation in heat-shock genes, upon which evolutionary processes may act.

Transposable elements can be a major source of evolutionary change. Their insertion can directly affect the genes into, or next to, which they insert. To insert, however, they must first gain access to the host gene. The authors reasoned that, because the DNA in the promoters (i.e., regulatory regions) of heat-shock genes is unusually accessible, these genes might harbor many transposable elements. With a technique that can detect any insertion into a gene, they discovered more than 200 distinctive transposable elements in the promoter regions of heat-shock genes in fruit flies from the wild—but few or none in the promoter regions of more typical genes. Surprisingly, out of the one hundred kinds of transposable elements in fruit flies, almost all were P elements. P elements are remarkable because they invaded the fruit fly genome only during the last century. These findings imply that the combination of accessible DNA and the recent invasion of P elements have left a distinctive imprint on the promoters of heat-shock genes.

Quantitative trait loci for thermotolerance phenotypes in Drosophila melanogaster

For insects, temperature is a major environmental variable that can influence an individual's behavioral activities and fitness. Drosophila melanogaster is a cosmopolitan species that has had great success in adapting to and colonizing diverse thermal niches. This adaptation and colonization has resulted in complex patterns of genetic variation in thermotolerance phenotypes in nature. Although extensive work has been conducted documenting patterns of genetic variation, substantially less is known about the genomic regions or genes that underlie this ecologically and evolutionarily important genetic variation. To begin to understand and identify the genes controlling thermotolerance phenotypes, we have used a mapping population of recombinant inbred (RI) lines to map quantitative trait loci (QTL) that affect variation in both heat- and cold-stress resistance. The mapping population was derived from a cross between two lines of D. melanogaster (Oregon-R and 2b) that were not selected for thermotolerance phenotypes, but exhibit significant genetic divergence for both phenotypes. Using a design in which each RI line was backcrossed to both parental lines, we mapped seven QTL affecting thermotolerance on the second and third chromosomes. Three of the QTL influence cold-stress resistance and four affect heat-stress resistance. Most of the QTL were trait or sex specific, suggesting that overlapping but generally unique genetic architectures underlie resistance to low- and high-temperature extremes. Each QTL explained between 5 and 14% of the genetic variance among lines, and degrees of dominance ranged from completely additive to partial dominance. Potential thermotolerance candidate loci contained within our QTL regions are identified and discussed.

Climatic adaptation of Drosophila buzzatii populations in southeast Australia

Variation in 19 traits possibly relevant for thermal adaptation was studied in 11 populations of Drosophila buzzatii collected in southeast Australia. Using stepwise multiple regression, the variation was compared to variation in geographic coordinates and to a set of climatic variables estimated for each collection site. For 13 of the traits, a significant part of the variation was explained by climatic variables and/or geographic coordinates, suggesting directional selection for adaptation to the environment in the majority of traits studied. In 10 of the traits, both geographic coordinates and climatic variables explained significant proportions of the variation, with R2 ranging from 0.075 to 0.58. Although larvae, pupae and adults of D. buzzatii share a common habitat, the measured traits were not correlated across life stages and gender. Also, there seemed to be special conditions in marginal populations near species borders, giving rise to nonlinear relations with latitude. Climate apparently does influence the adaptive evolution of the traits studied, but they also are affected by other factors that vary with latitude, longitude and distance to coast. These results highlight the complex challenges imposed by the environment on the adaptive process.

Male sterility at extreme temperatures: a significant but neglected phenomenon for understanding Drosophila climatic adaptations

The thermal range for viability is quite variable among Drosophila species and it has long been known that these variations are correlated with geographic distribution: temperate species are on average more cold tolerant but more heat sensitive than tropical species. At both ends of their viability range, sterile males have been observed in all species investigated so far. This symmetrical phenomenon restricts the temperature limits within which permanent cultures can be kept in the laboratory. Thermal heat sterility thresholds are very variable across species from 23 degrees C in heat sensitive species up to 31 degrees C in heat tolerant species. In Drosophila melanogaster, genetic variations are observed among geographic populations. Tropical populations are more tolerant to heat induced sterility and recover more rapidly than temperate ones. A genetic analysis revealed that about 50% of the difference observed between natural populations was due to the Y chromosome. Natural populations have not reached a selection limit, however: thermal tolerance was still increased by keeping strains at a high temperature, close to the sterility threshold. On the low temperature side, a symmetrical reverse phenomenon seems to exist: temperate populations are more tolerant to cold than tropical ones. Compared to Mammals, drosophilids exhibit two major differences: first, male sterility occurs not only at high temperature, but also at a low temperature; second, sterility thresholds are not evolutionarily constrained, but highly variable. Altogether, significant and sometimes major genetic variations have been observed between species, between geographic races of the same species, and even between strains kept in the laboratory under different thermal regimes. In each case, it is easily argued that the observed variations correspond to adaptations to climatic conditions, and that male sterility is a significant component of fitness and a target of natural selection.

Historical biogeography of Drosophila simulans based on Y-chromosomal sequences

Y-chromosomal sequences have been used for phylogeographic studies in humans and other mammals, but so far have been ignored as a source of historical information in Drosophila and other insects with X/Y sex determination. Here, we present the first phylogeographic study of Drosophila simulans based on the Y chromosome. Geographic distribution of Y-chromosomal haplotypes suggests a high degree of population subdivision within Africa, as well as between the African and cosmopolitan groups of populations. Consistent with earlier studies based on autosomal and X-linked loci, our results suggest that D. simulans originated in Madagascar or East Africa, and that the South and West African populations of this species are derived.