Investigating latitudinal clines for life history and stress resistance traits in Drosophila simulans from eastern Australia

Latitudinal clines have been demonstrated for many quantitative traits in Drosophila and are assumed to be due to climatic selection. However, clinal studies are often performed in species of Drosophila that contain common cosmopolitan inversion polymorphisms that also show clinal patterns. These inversion polymorphisms may be responsible for much of the observed clinal variation. Here, we consider latitudinal clines for quantitative traits in Drosophila simulans from eastern Australia. Drosophila simulans does not contain cosmopolitan inversion polymorphisms, so allows the study of clinal selection on quantitative traits that are not confounded by associations with inversions. Body size showed a strong linear cline for both females and males. Starvation resistance exhibited a weak linear cline in females, whereas chill-coma recovery exhibited a significant nonlinear cline in females only. No clinal pattern was evident for development time, male chill-coma recovery, desiccation or heat resistance. We discuss these results with reference to the role inversion polymorphisms play in generating clines in quantitative traits of Drosophila.     

Altitudinal patterns for latitudinally varying traits and polymorphic markers in Drosophila melanogaster from eastern Australia

Altitudinal changes in traits and genetic markers can complement the studies on latitudinal patterns and provide evidence of natural selection because of climatic factors. In Drosophila melanogaster, latitudinal variation is well known but altitudinal patterns have rarely been investigated. Here, we examine five traits and five genetic markers on chromosome 3R in D. melanogaster collected at high and low altitudes from five latitudes along the eastern coast of Australia. Significant altitudinal differentiation was observed for cold tolerance, development time, ovariole number in unmated females, and the microsatellite marker DMU25686. Differences tended to match latitudinal patterns, in that trait values at high altitudes were also found at high latitudes, suggesting that factors linked to temperature are likely selective agents. Cold tolerance was closely associated with average temperature and other climatic factors, but no significant associations were detected for the other traits. Genes around DMU25686 represent good candidates for climatic adaptation.     

Polymorphism in the couch potato gene clines in eastern Australia but is not associated with ovarian dormancy in Drosophila melanogaster

Natural selection can generate parallel latitudinal clines in traits and gene frequencies across continents, but these have rarely been linked. An amino acid (isoleucine to lysine, or I462K) polymorphism of the couch potato (cpo) gene in Drosophila melanogaster is thought to control female reproductive diapause cline in North America (Schmidt et al. 2008, Proc Natl Acad Sci USA, 105, 16207-16211). Here, we show that under standard diapause-inducing conditions (12 °C and short photoperiod) (Saunders et al. 1989, Proc Natl Acad Sci USA, 86, 3748-3752), egg maturation in Australian flies is delayed, but not arrested at previtellogenic stages. At 12 °C, the phenotypic distribution in egg development was bimodal at stages 8 and 14 and showed a strong nonlinear pattern on the east coast of Australia, with incidence of egg maturation delay (ovarian dormancy) increasing both toward tropical and temperate climates. Furthermore, we found no evidence for an association between the cpo I462K polymorphism and ovarian dormancy at either 12 or 10 °C (when egg maturation was often delayed at stage 7). Owing to strong linkage disequilibrium, the latitudinal cline in cpo allele frequencies was no longer evident once variation in the In(3R)P inversion polymorphism was taken into account. Our results suggest that the standard diapause-inducing conditions (12 °C and short photoperiod) were not sufficient to cause the typical previtellogenic developmental arrest in Australian flies and that the cpo I462K polymorphism does not explain the observed delay in egg development. In conclusion, ovarian dormancy does not show a simple latitudinal cline, and the lack of cpo-dormancy association suggests a different genetic basis to reproductive dormancy in North America and Australia.     

Single-locus latitudinal clines and their relationship to temperate adaptation in metabolic genes and derived alleles in Drosophila melanogaster

We report a study in Drosophila melanogaster of latitudinal clines for 23 SNPs embedded in 13 genes (Pgi, Gapdh1, UGPase, Pglym78, Pglym87, Eno, Men, Gdh, Sod, Pgk, Mdh1, TreS, Treh) representing various metabolic enzymes. Our samples are from 10 populations spanning latitude from southern Florida to northern Vermont. Three new clines with latitude were detected. These are the amino acid polymorphisms in the NAD-dependent glutamate dehydrogenase (Gdh) and trehalase (Treh) genes, and a silent site polymorphism in the UDP-glucose pyrophosphorylase gene (UGPase). The result, when combined with the overall incidence and pattern of reports for six other genes (Adh, Gpdh, Pgm, G6pd, 6Pgd, Hex-C), presents a picture of latitudinal clines in metabolic genes prevalent around the branch point of competing pathways. For six of the seven amino acid polymorphisms showing significant latitudinal clines in North America, the derived allele is the one increasing with latitude, suggesting temperate adaptation. This is consistent with a model of an Afrotropical ancestral species adapting to temperate climates through selection favoring new mutations.     

Response to selection for rapid chill-coma recovery in Drosophila melanogaster: physiology and life-history traits

Resistance to low temperatures can vary markedly among invertebrate species and is directly related to their distribution. Despite the ecological importance of cold resistance this trait has rarely been studied genetically, mainly because low and variable fitness of offspring from cold-stressed mothers makes it difficult to undertake selection experiments and compare cold resistance of parents and offspring. One measure of cold resistance that varies geographically in Drosophila melanogaster and that is amenable to genetic analysis is chill-coma recovery. Three replicate lines of D. melanogaster were selected every second generation, for over 30 generations, for decreased recovery time following exposure to 0 degrees C. Correlated responses were scored to characterize underlying physiological traits and to investigate interactions with other traits. Lines responded rapidly to the intermittent selection regime with realized heritabilities varying from 33% to 46%. Selected lines showed decreased recovery time after exposure to a broad range of low temperatures and also had a lower mortality following a more severe cold shock, indicating that a general mechanism underlying cold resistance had been selected. The selection response was independent of plastic changes in cold resistance because the selected lines maintained their ability to harden (i.e. a short-term exposure to cool temperature resulted in decreased recovery time in subsequent chill-coma assays). Changes in cold resistance were not associated with changes in resistance to high temperature exposure, and selected lines showed no changes in wing size, development time or viability. However, there was a decrease in longevity in the selected lines due to an earlier onset of ageing. These results indicate that chill-coma recovery can be rapidly altered by selection, as long as selection is undertaken every second generation to avoid carry-over effects, and suggest that lower thermal limits can be shifted towards increased cold resistance independently of upper thermal limits and without tradeoffs in many life-history traits.     

The molecular genetics of clinal variation: a case study of ebony and thoracic trident pigmentation in Drosophila melanogaster from eastern Australia

Widespread pigmentation diversity coupled with a well‐defined genetic system of melanin synthesis and patterning in Drosophila provides an excellent opportunity to study phenotypes undergoing evolutionary change. Pigmentation variation is highly correlated with different ecological variables and is thought to reflect adaptations to different environments. Several studies have linked candidate genes from Drosophila melanogaster to intra‐population variation and interspecific morphological divergence, but less clearly to variation among populations forming pigmentation clines. We characterized a new thoracic trident pigmentation cline in D. melanogaster populations from eastern Australia, and applied a candidate gene approach to explain the majority of the geographically structured phenotypic variation. More melanized populations from higher latitudes tended to express less ebony than their tropical counterparts, and an independent artificial selection experiment confirmed this association. By partitioning temperature dependent effects, we showed that the genetic differences underlying clinal patterns for trident variation at 25 °C do not explain the patterns observed at 16 °C. Changes in thoracic trident pigmentation could be a common evolutionary response to climatically mediated environmental pressures. On the Australian east coast most of the changes appear to be associated with regulatory divergence of the ebony gene but this depends on temperature.     

The effect of developmental temperature on the genetic architecture underlying size and thermal clines in Drosophila melanogaster and D. simulans from the east coast of Australia

Body size and thermal tolerance clines in Drosophila melanogaster occur along the east coast of Australia. However the extent to which temperature affects the genetic architecture underlying the observed clinal divergence remains unknown. Clinal variation in these traits is associated with cosmopolitan chromosome inversions that cline in D. melanogaster. Whether this association influences the genetic architecture for these traits in D. melanogaster is unclear. Drosophila simulans shows linear clines in body size, but nonlinear clines in cold resistance. Clinally varying inversions are absent in D. simulans. Line-cross and clinal analyses were performed between tropical and temperate populations of D. melanogaster and D. simulans from the east coast of Australia to investigate whether clinal patterns and genetic effects contributing to clinal divergence in wing centroid size, thorax length, wing-to-thorax ratio, cold and heat resistance differed under different developmental temperatures (18 °C, 25 °C, and 29 °C). Developmental temperature influenced the genetic architecture in both species. Similarities between D. melanogaster and D. simulans suggest clinally varying inversion polymorphisms have little influence on the genetic architecture underlying clinal divergence in size in D. melanogaster. Differing genetic architectures across different temperatures highlight the need to consider different environments in future evolutionary and molecular studies of phenotypic divergence.     

Microspatial structure of Drosophila melanogaster populations in Brazzaville: evidence of natural selection acting on morphometrical traits

Two genetically distinct habitat races of Drosophila melanogaster coexist in Brazzaville (Congo). One is the typical field type of Afrotropical populations, the other mainly breeds in beer residues in breweries. These two populations differ in their ethanol tolerance, in their allelic frequencies at several enzyme and microsatellite loci and in the composition of their cuticular hydrocarbons. The brewery population is quite similar to European temperate populations with regard to all these traits. Previous investigations of two morphological traits (ovariole number and sternopleural bristle number) failed to detect any difference between the two habitat races. Here we investigated other morphological traits (wing and thorax length, thorax pigmentation and female abdomen pigmentation). The reaction norms of these traits according to growth temperature were compared in the two Afrotropical habitat races and in a French temperate population. As expected, the French population was very different from the field African population: as a general rule, the brewery population (Kronenbourg) was intermediate in several aspects between the other two. We conclude that the strong selective forces that maintain the genetic divergence between the two habitat races also act on morphometrical traits, and the possible selective mechanisms are discussed.     

Fertility and viability at the Sod locus in Drosophila melanogaster: non-additive and asymmetric selection

Experiments were designed to test in Drosophila melanogaster the effect of mating type at the Sod locus on fertility and viability. The experiments show that fertility is neither additive (or multiplicative) nor symmetric, i.e. that the fertility of a mating type cannot be predicted from the average fertility of the two genotypes involved in the mating. There is no significant male x female interaction with respect or progeny viability; but the interaction is significant for productivity, i.e. when fertility and viability are jointly taken into account. There is overdominance with respect to female fertility, but not with respect to male fertility or to viability. There also is alloprocoptic selection with respect to fertility and with respect to productivity, i.e. mating between like homozygotes are less fertile and productive than matings between dissimilar homozygotes. Selection at the Sod locus yields stable polymorphic equilibria, with the frequency of the F allele predicted at P = 0.641 or 0.695, respectively for low and high larval density.     

Long-term patterns of genomic P element content and P-M characteristics of Drosophila melanogaster in eastern Australia

A latitudinal cline in characteristics associated with the P DNA transposable element is well known in eastern Australian populations of Drosophila melanogaster. In order to survey the long-term patterns of P-M system characteristics and genomic P element content, we established 292 isofemale lines from 54 localities in 1996-1997 and evaluated them for gonadal dysgenesis (GD) sterility and the ratio of KP to full-size P elements (KP/FP ratio). The results were compared to those from collections made in 1983-1986 and 1991-1994. Over 10-14 years, 1) the cross A GD scores of the northern-middle populations declined dramatically; 2) the clinal pattern of the cross A* GD scores did not change; 3) the latitudinal pattern of the KP/FP ratio did not change. The results suggest that only a few P elements determine P-M characteristics and that there has been selection for genomes with fewer active P elements, but not for a great change in proportions of size classes.     

A comprehensive assessment of geographic variation in heat tolerance and hardening capacity in populations of Drosophila melanogaster from eastern Australia

We examined latitudinal variation in adult and larval heat tolerance in Drosophila melanogaster from eastern Australia. Adults were assessed using static and ramping assays. Basal and hardened static heat knockdown time showed significant linear clines; heat tolerance increased towards the tropics, particularly for hardened flies, suggesting that tropical populations have a greater hardening response. A similar pattern was evident for ramping heat knockdown time at 0.06 °C min^?1 increase. There was no cline for ramping heat knockdown temperature (CT_max) at 0.1 °C min^?1 increase. Acute (static) heat knockdown temperature increased towards temperate latitudes, probably reflecting a greater capacity of temperate flies to withstand sudden temperature increases during summer in temperate Australia. Larval viability showed a quadratic association with latitude under heat stress. Thus, patterns of heat resistance depend on assay methods. Genetic correlations in thermotolerance across life stages and evolutionary potential for critical thermal limits should be the focus of future studies.     

Opportunities for natural selection on DNA and protein at the Adh locus in Drosophila melanogaster

A study was made of environmental and genetic factors affecting the quantity and disposition of the alcohol dehydrogenase (ADH) protein in Drosophila melanogaster. It was found that the amount of enzyme per fly is greatly influenced by the environmental conditions in which it develops. A critical factor is the concentration of yeast in the medium. A high concentration of yeast can double the quantity of ADH. The yeast appears to act through the provision of protein, and the protein to act through the provision of threonine, which is already known to induce ADH in fungi. Various genetic factors affect the quantity of enzyme. Males have more ADH than females. Files homozygous for the Fast allele have more ADH than those homozygous for the slow allele, and the difference is greater in females than in males. One particular line (ve), homozygous for Slow, has approximately half the normal quantity of enzyme, and the quantity segregates with the electrophoretic allele. Lines differ in the relative amounts of ADH in the gut (including Malpighian tubules) and the fat body. In general it seems that slow lines have relatively more enzyme in the fat body. In a cross between ve and a line homozygous to Fast, the difference in tissue distribution segregated with the electrophoretic allele. It is argued, but not demonstrated, that the differences in quantity and tissue distribution are due to nucleotide substitutions in noncoding regions close to, or within, the structural gene. It seems likely that the observed environmental and genetic differences in the quantity and disposition of ADH will influence the relative selective values of the electrophoretic genotypes.     

Overwintering in Drosophila melanogaster: outdoor field cage experiments on clinal and laboratory selected populations help to elucidate traits under selection

Insects can adapt to temperate environments by increasing levels of resistance to cold conditions over winter and/or altering reproductive patterns to focus reproduction in favourable conditions. In temperate areas, Drosophila melanogaster persists over winter at the adult stage. A previous experiment, conducted with flies kept in outdoor population cages in the temperate winter, indicated that temperate populations produced more eggs than did tropical populations following an abrupt increase in reproduction in late winter. In contrast, the tropical populations produced more eggs prior to the increase. Both patterns resulted in a higher net number of surviving offspring for temperate populations. Here we again examine the clinal pattern in reproduction using outdoor cages, this time held under tropical winter conditions. In this environment, surprisingly, egg production was higher and on average earlier in populations originating from temperate areas. However, mortality rates also increased with latitude of origin, and the relationship of lifetime egg production to latitude should therefore be measured. To test the role of altered pattern of egg production per se in the reproductive advantage of temperate populations in the temperate winter, we tested the performance of laboratory lines selected for altered reproductive patterns, under temperate winter conditions. Lines selected for high early fecundity exhibited this characteristic in the field cages and lines selected for late reproduction exhibited a relatively high fecundity in spring. The timing of the abrupt increase in egg production was identical in these sets of lines and occurred at the same time in recently collected populations, suggesting evolutionary conservation of the switch. These findings suggest that changes in early and late reproduction per se determine adaptation to temperate winter conditions, and illustrate how laboratory selection lines can be used to understand traits underlying adaptive shifts in field performance.     

Perturbation of gene frequencies in a natural population of Drosophila melanogaster: evidence for selection at the Adh locus

The cellar population of Drosophila melanogaster at the Chateau Tahbilk Winery (Victoria, Australia) was perturbed for alcohol dehydrogenase (Adh) gene frequencies. Phenol oxidase (Phox) frequencies were also perturbed and monitored as a control. Subsequent gene frequency changes, together with information on population structure, indicated that selection acted on the chromosome regions of both loci. Adh gene frequencies returned to preperturbation levels in a predictable manner. A model in which the relative fitness of Adh phenotypes was determined by temperature-dependent specific activities of enzymes of Adh genotypes adequately accounts for the rate of gene frequency change at this locus. Thus temperature behaves as a selective agent in modulating Adh gene frequencies in this cellar environment.     

Morphological variation and preadult viability at one-generation inbreeding in Drosophila melanogaster: A family selection hypothesis

Wing length, thorax length, sterno-pleural and sternal chaetae numbers were determined in offspring of sib crosses in a Drosophila melanogaster population, in relation to egg-to-adult viability. It appears that high between- and within-family variabilities for wing and thorax length together with lack of developmental homeostasis are associated with a high rate of embryonic and larvo-pupal mortality. Effects on chaetae numbers are more controversial. Results suggest that families which are homozygous for particular gene combinations may be eliminated by natural selection. Maintenance of high levels of heterozygosity might result in populations where circumstances foster substantial inbreeding.