Heat-hardening effects on mating success at high temperature in Drosophila melanogaster.

Reproduction is strongly influenced by environmental temperature in insects. At high temperature, mating success could be influenced not only by basal (non-inducible) thermotolerance but also by inducible plastic responses. Here, mating success at high temperature was tested in flies carrying contrasting genotypes of heat resistance in Drosophila melanogaster. The possible heat-hardening effect was tested. Mating success did not differ between heat-resistant and heat-sensitive genotypes when tested both at high (33 °C) and benign (25 °C) temperature, independently of the heat-hardening status. Importantly, heat-hardening pre-treatment increased in a 70% the number of matings at 33 °C in a mass-mating experiment. Further, mating latency at 33 °C was shorter with heat hardening than without it in single-pair assays Heat-hardening had previously been showed to improve short-term thermotolerance in many organisms including Drosophila, and the present results show that heat hardening also improve mating success at elevated temperature. Previous exposures to a mild heat stress improve short-term mating success as a plastic response of ecological relevance. Such heat-hardening effects on mating success should be relevant for predicting potential evolutionary responses to any possible current scenery of global warming, as well as in sterile insect release programs for pest control in elevated temperature environments.

Patterns of longevity and fecundity at two temperatures in a set of heat-selected recombinant inbred lines of Drosophila melanogaster.

Quantitative trait loci (QTL) were mapped for longevity and fecundity at two temperatures, 20 and 30 °C, in two sets of recombinant inbred lines (RIL) highly differing in thermotolerance. Early fecundity (EF) and longevity showed a negative association between temperatures. For instance, longevity was higher and fecundity was lower in the RIL panel showing higher life span at 30 °C. One X-linked QTL (7B3-12E) co-localized for longevity and EF at 20 °C, with one QTL allele showing a positive additive effect on longevity and a negative effect on EF. The across-RIL genetic correlation between longevity and EF was not significant within each temperature, and most QTL that affect life span have no effect on EF at each temperature. EF and longevity can mostly be genetically uncoupled in the thermotolerance-divergent RIL within each temperature as opposed to between temperatures. QTL were mostly temperature specific, although some trait-specific QTL showed possible antagonistic effects between temperatures.

Heat-stress survival in the pre-adult stage of the life cycle in an intercontinental set of recombinant inbred lines of Drosophila melanogaster.

In insects, pre-adult stages of the life cycle are exposed to variation in temperature that may differ from that in adults. However, the genetic basis for adaptation to environmental temperature could be similar between the pre-adult and the adult stages of the life cycle. Here, we tested quantitative trait loci (QTL) for heat-stress survival in larvae of Drosophila melanogaster, with and without a mild-heat-stress pre-treatment. Two sets of recombinant inbred lines derived from lines artificially selected for high and low levels of knockdown resistance to high temperature in young flies were used as the mapping population. There was no apparent increase in heat-shock survival between heat-pretreated and non-pretreated larvae. There was a positive correlation between the two experimental conditions of heat-shock survival (with and without a heat pre-treatment) except for males from one set of lines. Several QTL were identified involving all three major chromosomes. Most QTL for larval thermotolerance overlapped with thermotolerance QTL identified in previous studies for adults, indicating that heat-stress resistance is not genetically independent between life cycle stages because of either linkage or pleiotropy. The sign of the effects of some QTL alleles differed both between the sexes and between life stages.

X-linked QTL for knockdown resistance to high temperature in Drosophila melanogaster.

Knockdown Resistance to High Temperature (KRHT) is an adaptive trait of thermotolerance in insects. An interval mapping was performed on chromosome X of Drosophila melanogaster to search for quantitative trait loci (QTL) affecting KRHT. A backcross population was obtained from two lines that dramatically differ for KRHT. Microsatellites were used as markers. Composite interval mapping identified a large-effect QTL in the region of band 10 where putative candidate genes map. To further test for this QTL a set of recombinant (but non-inbred) lines was obtained from backcrosses between the parental lines used for the interval mapping. Recombinant line analysis confirmed that one QTL is targeted by band 10. We identify and discuss candidate loci contained within our QTL region.

Altitudinal patterns for longevity, fecundity and senescence in Drosophila buzzatii.

We tested for variation in longevity, senescence rate and early fecundity of Drosophila buzzatii along an elevational transect in Argentina, using laboratory-reared flies in laboratory tests performed to avoid extrinsic mortality. At 25 degrees C, females from lowland populations lived longer and had a lower demographic rate of senescence than females from highland populations. Minimal instead of maximal temperature at the sites of origin of population best predicted this cline. A very different pattern was found at higher test temperature. At 29.5 degrees C, longevity of males increased with altitude of origin of population. No clinal trend was apparent for longevity of females at 29.5 degrees C. There was evidence for a trade-off between early fecundity and longevity at non-stressful temperature (25 degrees C) along the altitudinal gradient. This trait association is consistent with evolutionary theories of aging. Population-by-temperature and sex-by-temperature interactions indicate that senescence patterns are expressed in environment specific ways.

Altitudinal variation for stress resistance traits and thermal adaptation in adult Drosophila buzzatii from the New World.

Multiple stress resistance traits were investigated in the cactophilic fly Drosophila buzzatii. Adults from seven populations derived from North-Western Argentina were compared with respect to traits relevant for thermal stress resistance and for resistance to other forms of environmental stress. The populations were collected along an altitudinal gradient spanning more than 2000 m in height, showing large climatic differences. The results suggest that knock-down resistance to heat stress, desiccation resistance and Hsp70 expression at a relatively severe stressful temperature best reflect thermal adaptation in this species. Furthermore, cold resistance seemed to be of less importance than heat resistance, at least for the adult life stage, in these populations. Clinal variation in thermal resistance traits over short geographical distances suggests relatively strong adaptive differentiation of the populations. This study provides the first evidence for altitudinal differentiation in stress-related traits, and suggests that Hsp70 expression level can be related to altitudinal clines of heat-stress resistance.

Quantitative trait loci affecting knockdown resistance to high temperature in Drosophila melanogaster.

Knockdown resistance to high temperature is an ecologically important trait in small insects. A composite interval mapping was performed on the two major autosomes of Drosophila melanogaster to search for quantitative trait loci (QTL) affecting knockdown resistance to high temperature (KRHT). Two dramatically divergent lines from geographically different thermal environments were artificially selected on KRHT. These lines were crossed to produce two backcross (BC) populations. Each BC was analysed for 200 males with 18 marker loci on chromosomes 2 and 3. Three X-linked markers were used to test for X-linked QTL in an exploratory way. The largest estimate of autosome additive effects was found in the pericentromeric region of chromosome 2, accounting for 19.26% (BC to the low line) and 29.15% (BC to the high line) of the phenotypic variance in BC populations, but it could represent multiple closely linked QTL. Complete dominance was apparent for three QTL on chromosome 3, where heat-shock genes are concentrated. Exploratory analysis of chromosome X indicated a substantial contribution of this chromosome to KRHT. The results show that a large-effect QTL with dominant gene action maps on the right arm of chromosome 3. Further, the results confirm that QTL for heat resistance are not limited to chromosome 3.

Chromosomal inversions effect body size and shape in different breeding resources in Drosophila buzzatii.

The cactophilic Drosophila buzzatii provides an excellent model for the study of reaction norms across discrete environments because it breeds on rotting tissues (rots) of very different cactus species. Here we test the possible effects of second chromosome inversions on body size and shape (wing loading) across suitable natural breeding substrates. Using homokaryotypic stocks derived from several lines homozygous for four naturally occurring chromosomal inversions, we show that arrangements significantly affect size-related traits and wing loading. In addition, karyotypes show differing effects, across natural breeding resources, for wing loading. The 2st and 2jz(3) arrangements decrease and the 2j arrangement increases wing loading. For thorax length and wing loading, karyotypic correlations across host plants are slightly lower in females than in males. These results support the hypothesis that these traits have a genetic basis associated with the inversion polymorphism.

Developmental time, body size and wing loading in Drosophila buzzatii from lowland and highland populations in Argentina.

Genotype-by-temperature interaction is a necessary condition for adaptive evolution of fitness traits as a response to temperature. Several fitness-related traits (developmental time, pre-adult survival, thorax and wing lengths, and wing loading) were measured in laboratory-reared D. buzzatii from four populations sampled at different altitudes in north-western Argentina: a lowland population (407 m a.s.l.), two populations from intermediate altitude (780 to 950 m a.s.l.), and a highland population (2380 m a.s.l.). Temperature is the main climatic difference between the collection sites: lowland but not highland populations are exposed to physiologically high temperatures during both spring and summer in nature. Three growth temperatures (20, 25 and 30 degrees C) were used to test for population-by-temperature interactions. Both developmental time and pre-adult survival exhibit highly significant population-by-temperature interaction. Pre-adult survival at 30 degrees C is significantly higher in lowland than in highland populations, but not so at lower growth temperatures (20 and 25 degrees C). Both wing length and wing loading show no population-by-temperature interaction, indicating that these traits are not the direct targets of thermal adaptation in nature. Wing loading is higher in highland than in lowland populations, suggesting that flight performance is subject to stronger selection in the highland population. This hypothesis is consistent with ecological observations in both types of populations. There is no obvious among-population relationship between developmental time and body size, even though both traits are related within populations in a well-known trade-off. Overall, thermal adaptation is evident for developmental time and pre-adult survival but not for size-related traits.

Negative genetic correlation between traits of the Drosophila head, and interspecific divergence in head shape.

For morphological traits that are negatively correlated, the genetic correlation (rg) between them might strongly influence patterns of morphological divergence and shape. Here, the pattern of divergence between two sibling species of cactophilic Drosophila, D. buzzatii and D. koepferae, is examined for two traits that are known to be negatively correlated in other Drosophila species: face width (FW) and width of both eyes (EW). Head width (HW, the sum of FW and EW, i. e. the total width of the head capsule) was also examined. Genetic and phenotypic correlations were estimated in the laboratory G2 generation of a sample of wild D. buzzatii derived from a population where D. koepferae is not present. Phenotypic correlations were also estimated in D. buzzatii and D. koepferae from another, very different, population where the species are sympatric. Consistent with studies in other Drosophila species, rg was negative and significant for the correlation between FW and EW, and positive (but nonsignificant at a matrix-wide P-value of 0.05) for the correlations of HW with both FW and EW. This well-defined correlation pattern was also consistent with the phenotypic correlations in both D. buzzatii and D. koepferae. No significant difference in these traits was detected between D. buzzatii populations, but head shape has diverged between D. buzzatii and D. koepferae. Specifically, the two negatively correlated traits, FW and EW, have evolved in opposite directions in these two species, with HW showing no significant interspecific difference. The overall picture of this divergence pattern shows a striking concordance with the present evidence of negative correlations between FW and EW, and is consistent with the notion of rg-related constraints on the pattern of interspecific differentiation.

Sexual selection related to developmental stability in Drosophila buzzatii.

Relationships among developmental stability, sexual selection, and body size were examined in the cactophilic fly Drosophila buzzatii. Developmental stability, as measured in terms of fluctuating asymmetry (FA) of wing length, was positively associated to male's copulatory success in a mass-mating experiment with wild-reared flies. Previous studies showed that body size is positively correlated with mating success and other fitness components in this species, but no phenotypic correlation between wing asymmetry and size was found in the present study. Thus, wing asymmetry does not indicate male quality in terms of phenotypic variation in body size. This study suggests that sexual selection against wing asymmetry may be mediated through unknown factors related to developmental stability rather than body size.

Genetic and phenotypic correlations among size-related traits, and heritability variation between body parts in Drosophila buzzatii.

Recent studies have shown that body size is a heritable trait phenotypically correlated with several fitness components in wild populations of the cactophilic fly Drosophila buzzatii. To obtain further information on size-related variation, heritabilities as well as genetic and phenotypic correlations among size-related traits of several body parts (head, thorax and wings) were estimated. The study was carried out on an Argentinean natural population in which size-related selection was previously detected. The genetic parameters were estimated using offspring-parent regressions (105 families) in the laboratory G2 generation of a sample of wild flies. The traits were also scored in Wild-Caught Flies (WCF). Laboratory-Reared Flies (LRF) were larger and less variable than WCF. Although heritability estimates were significant for all traits, heritabilities were higher for thorax-wing traits than for head traits. Phenotypic and genetic correlations were all positive. The highest genetic correlations were found between traits which are both functionally and developmentally related. Genetic and phenotypic correlations estimated in the lab show similar correlation pattern (r = 0.49; P = 0.02, Mantel's test). However, phenotypic correlations were found to be typically larger in WCF than in LRF. The genetic correlation matrix estimated in the relatively homogeneous lab environment is not simply a constant multiplicative factor of the phenotypic correlation matrix estimated in WCF.

Correlations among size-related traits affected by chromosome inversions in Drosophila buzzatii: the comparison within and across environments.

Genetic variation in correlations among size-related traits of head, thorax, and wings was examined in Drosophila buzzatii, by comparing the pattern of the Phenotypic Correlation Matrix (Rp) between inversion karyotypes of the second chromosome. No similarity in Rp was observed between some karyotypes in a natural population. The pattern of Rp in wild-reared heterokaryotypes, but not in homokaryotypes, was similar to the whole population represented by laboratory-reared flies. While phenotypic correlations in wild-reared flies were found to be larger than in laboratory-reared flies, similarity in the pattern of Rp was very high for one homokaryotype reared in both environments: the relatively homogeneous lab environment and the more variable field environment. While no such a similarity across environments was detected between different karyotypes, the pattern of Rp was similar for a same homokaryotype in different populations. Thus, the lack of karyotypic similarity in Rp is, at least partially, genetic. These results indicate that chromosomal inversions are factors affecting genetic correlations among traits known to be phenotypically correlated with adult fitness components in this species.

Correlations among size-related traits are affected by chromosome inversions in an adaptive polymorphism in Drosophila buzzatii.

Genetic variation in correlations among size-related traits of head, thorax and wings was examined in Drosophila buzzatii, by comparing the correlation pattern of the phenotypic correlation matrix (CP-Rp) between inversion karyotypes of the second chromosome. CP-Rp differed between some karyotypes in a natural population. CP-Rp in homokaryotypic classes of wild-reared flies, but not in heterokaryotypes, differed from the whole population represented by laboratory-reared flies. Similarity in CP-Rp was highly significant for a same homokaryotype in two populations. In one of them, the chromosome is polymorphic for four inversions. In the other population, one of the inversions is almost fixed. CP-Rp was significantly similar between these populations, illustrating that similarity of CP-Rp may even occur between populations which have greatly diverged in frequencies of some genotypes affecting correlation patterns. It is suggested that chromosomal inversions are factors affecting genetic correlations among traits known to be phenotypically correlated with fitness components.

An adaptive chromosomal polymorphism affecting size-related traits, and longevity selection in a natural population of Drosophila buzzatii.

Size-related phenotypic variation among second-chromosome karyotypes in Drosophila buzzatii was examined in an Argentinian natural population. For all measured traits (thorax and wing length; wing, head and face width), this inversion polymorphism exhibited a significant and (additive) linear contribution to the phenotypic variance in newly emerged wild flies. The results suggest that only overall body size, and not body shape, is affected, as no karyotypic variation was found for any trait when the effects of differences in within-karyotype size were removed with Burnaby's method. Likewise, in an experiment of longevity selection in the wild, variation in chromosomal frequencies was verified in the direction predicted on the basis of: (i) previous studies on longevity selection for body size in the wild and (ii) the pattern of chromosomal effects we observed on size. The direction of such selection is consistent with a pattern of antagonistic selection detected in previous studies on the inversion polymorphism.