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S W McKechnie

Publications and source records attributed to S W McKechnie.

At least 19 recordsLinked to original sources
PubMed2006

Microsatellites reveal a lack of structure in Australian populations of the diamondback moth, Plutella xylostella (L.).

The diamondback moth, Plutella xylostella, is renowned for developing resistance to insecticides and causing significant economic damage to Brassica vegetable crops throughout the world. Yet despite its economic importance, little is known about the population structure and movement patterns of this pest both at local and regional scales. In Australia, the movement patterns and insecticide resistance status of P. xylostella infesting canola, vegetables, forage brassicas and weeds have fundamental implications for the management of this pest. Here we use six polymorphic microsatellite loci to investigate population structure and gene flow in Australian populations of P. xylostella. Samples of P. xylostella from New Zealand, Malaysia, Indonesia and Kenya were also scored at these loci. We found no evidence of population structure within Australia, with most populations having low inbreeding coefficients and in Hardy-Weinberg equilibrium. In addition, a sample from the North Island of New Zealand was indistinguishable from the Australian samples. However, large genetic differences were found between the Australia/New Zealand samples and samples from Kenya, Malaysia and Indonesia. There was no relationship between genetic distance and geographic distance among Australian and New Zealand samples. Two of the loci were found to have null alleles, the frequency of which was increased in the populations outside the Australia/New Zealand region. We discuss these results with reference to insecticide resistance management strategies for P. xylostella in Australia.

Analysis of Variance↗
PubMed2006

In search of clinal variation in the period and clock timing genes in Australian Drosophila melanogaster populations.

Clinal variation for repeat number in the Thr-Gly region of the period circadian timing gene in Drosophila melanogaster was described in Europe and has subsequently been used as evidence of thermal selection on period alleles. To test for clinal variation in this gene along the east coast of Australia, the period polymorphism was scored on flies from multiple samples collected repeatedly over a 5-year interval, along with variation at another circadian rhythm locus, clock. For period, there was no consistent evidence of clinal variation in the 17 and/or 20 repeat alleles, although when average allele length was examined a weak consistent clinal pattern was detected. For clock there was no evidence of clinal variation in the two most common alleles or in average repeat size. These data are inconsistent with the reported patterns in Europe and suggest that clinal variation in timing genes needs to be re-examined in this region.

Animals↗
PubMed2006

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.

Altitude↗
PubMed2005

A rapid shift in a classic clinal pattern in Drosophila reflecting climate change.

Geographical clines in genetic polymorphisms are widely used as evidence of climatic selection and are expected to shift with climate change. We show that the classic latitudinal cline in the alcohol dehydrogenase polymorphism of Drosophila melanogaster has shifted over 20 years in eastern coastal Australia. Southern high-latitude populations now have the genetic constitution of more northerly populations, equivalent to a shift of 4 degrees in latitude. A similar shift was detected for a genetically independent inversion polymorphism, whereas two other linked polymorphisms exhibiting weaker clinal patterns have remained relatively stable. These genetic changes are likely to reflect increasingly warmer and drier conditions and may serve as sensitive biomarkers for climate change.

Adaptation, Physiological↗
PubMed2003

Thermal tolerance trade-offs associated with the right arm of chromosome 3 and marked by the hsr-omega gene in Drosophila melanogaster.

Drosophila melanogaster occurs in diverse climatic regions and shows opposing clinal changes in resistance to heat and resistance to cold along a 3000 km latitudinal transect on the eastern coast of Australia. We report here on variation at a polymorphic 8 bp-indel site in the heat shock hsr-omega gene that maps to the right arm of chromosome 3. The frequency of the genetic element marked by the L form of the gene was strongly and positively associated with latitude along this transect, and latitudinal differences in L frequency were robustly associated with latitudinal differences in maximum temperature for the hottest month. On a genetic background mixed for genes from each end of the cline a set of 10 lines was derived, five of which were fixed for the L marker, the absence of In(3R)P and 12 kb of repeats at a second polymorphic site at the 3' end of hsr-omega, and five that were fixed for the S marker, In(3R)P and 15 kb of hsr-omega repeats. For two different measures of heat tolerance S lines outperformed L lines, and for two different measures of cold tolerance L lines outperformed S lines. These data suggest that an element on the right arm of chromosome 3, possibly In(3R)P, confers heat resistance but carries the trade-off of also conferring susceptibility to cold. This element occurs at high frequency near the equator. The alternate element on the other hand, at high frequency at temperate latitudes, confers cold resistance at the cost of heat susceptibility.

Acclimatization↗
PubMed1999

The Drosophila heat shock hsr-omega gene: an allele frequency cline detected by quantitative PCR.

The hsr-omega gene of Drosophila melanogaster produces RNA products both constitutively and at elevated levels in response to heat stress. A single-nucleotide difference in this gene that has been detected using denaturing gradient gel electrophoresis (DGGE) is responsible for an hsr-omegaa/b polymorphism, and selection experiments have indicated an association between the hsr-omegaa allele and susceptibility to heat stress. Since allele frequency estimates for population surveys using PCR and DGGE for single flies would be relatively time-consuming and expensive, we here develop a quantitative competitive-PCR method using mass-grind genomic DNA preparations for this purpose. Geographical and temporal variation of allele frequency at the hsr-omega locus in Australian populations of D. melanogaster are examined. Regular samples from a southern population through a summer season suggested stability of hsr-omegaa frequency. Field populations sampled from a approximately 2,250 km north-south transect along eastern Australia revealed a strong positive association between the frequency of hsr-omegaa and latitude, and marked spatial autocorrelation. Using appropriate analyses, strong association between population differences in hsr-omegaa frequencies and differences in temperature and rainfall measures, after controlling for latitudinal differences, support the idea that the cline in hsr-omegaa frequency may be attributable to some form of climatic selection.

Alleles↗
PubMed1998

Both allelic variation and expression of nuclear and cytoplasmic transcripts of Hsr-omega are closely associated with thermal phenotype in Drosophila.

Inducible heat shock genes are considered a major component of the molecular mechanisms that confer cellular protection against a variety of environmental stresses, in particular high temperature extremes. We have tested the association between expression of the heat shock RNA gene hsr-omega and thermoresistance by generating thermoresistant lines of Drosophila melanogaster after application of two distinct regimes of laboratory selection. One set of lines was selected for resistance to knockdown by heat stress and the other was similarly selected but before selection a mild heat exposure known to increase resistance (heat hardening) was applied. A cross between resistant and susceptible lines confirmed our earlier observation that increased thermal tolerance cosegregates with allelic variation in the hsr-omega gene. This cosegregating variation is attributed largely to two haplotype groups. Using quantitative reverse transcription-PCR, we find evidence for divergent phenotypic responses in the two selection regimes, involving both structural and regulatory changes in hsr-omega. Lines selected after hardening showed increased levels of the cytoplasmic transcript but decreased levels of the nuclear transcript. Lines selected without hardening showed decreased levels of the cytoplasmic transcript. The allelic frequency changes at hsr-omega could not by themselves account for the altered transcription patterns. Our results support the idea that the functional RNA molecules transcribed from hsr-omega are an important and polymorphic regulatory component of an insect thermoresistance phenotype.

Alleles↗
PubMed1996

Response of two heat shock genes to selection for knockdown heat resistance in Drosophila melanogaster.

To identify genes involved in stress resistance and heat hardening, replicate lines of Drosophila melanogaster were selected for increased resistance to knockdown by a 39 degrees heat stress. Two selective regimes were used, one with and one without prior hardening. Mean knockdown times were increased from approximately 5 min to > 20 min after 18 generations. Initial realized heritabilities were as high as 10% for lines selected without hardening, and crosses between lines indicated simple additive gene effects for the selected phenotypes. To survey allelic variation and correlated selection responses in two candidate stress genes, hsr-omega and hsp68, we applied denaturing gradient gel electrophoresis to amplified DNA sequences from small regions of these genes. After eight generations of selection, allele frequencies at both loci showed correlated responses for selection following hardening, but not without hardening. The hardening process itself was associated with a hsp68 frequency change in the opposite direction to that associated with selection that followed hardening. These stress loci are closely linked on chromosome III, and the hardening selection established a disequilibrium, suggesting an epistatic effect on resistance. The data indicate that molecular variation in both hsr-omega and.hsp68 contribute to natural heritable variation for hardened heat resistance.

Alcohol Dehydrogenase↗
PubMed1994

A model of the Drosophila melanogaster glycerol-3-phosphate oxidase-encoding gene.

We have sequenced the Drosophila melanogaster gene encoding the mitochondrial (mt) enzyme glycerol-3-phosphate oxidase (GPO; EC 1.1.99.5) that is involved in flight and alcohol metabolism. Available data suggests a simple model for this gene that includes four exons. Exon I contains a mt import signal, exon II, a transmembrane segment and an FAD-binding site, and exon IV, an iron-sulfur centre.

Alternative Splicing↗
PubMed1994

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

The cellular 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.

Alcohol Dehydrogenase↗
PubMed1993

Micro-evolution in a wine cellar population: an historical perspective.

The population of Drosophila melanogaster inside the wine cellar of Chateau Tahbilk of Victoria, Australia was found by McKenzie and Parsons (1974) to have undergone microevolution for greater alcohol tolerance when compared to the neighboring population outside the cellar. This triggered additional studies at Tahbilk, and at other wine cellars throughout the world. The contributions and interactions of researchers and the development of ideas on the ecology and genetics of this unique experimental system are traced. Although the ADH-F/ADH-S polymorphism was found to be maintained by selection in the Tahbilk populations, the selection is not significantly associated with alcohol tolerance. The environment inside the Tahbilk wine cellar is not as rich in ethanol as was originally anticipated, and selection that affects the alcohol dehydrogenase polymorphism may be more concerned with the relative efficiency with which ethanol is used as a nutrient by D. melanogaster. The synthesis and modification of lipids, particularly in membranes, appears to be important to alcohol tolerance. The studies of the Tahbilk population are at a crossroad. New experimental approaches promise to provide the keys to the selection that maintains the alcohol dehydrogenase polymorphism, and to factors that are important to alcohol tolerance and stress adaptation. From these research foundations at Tahbilk very significant contributions to our future understanding of the genetic processes of evolution can be made.

Alcohol Dehydrogenase↗
PubMed1993

Long-chain dietary fatty acids affect the capacity of Drosophila melanogaster to tolerate ethanol.

Four-day post-hatch larvae (mid-third instar) of Drosophila melanogaster were fed an intermediate diet with or without supplement of an individual fatty acid for 2 d and then transferred to a diet with a growth-limiting level of 0.94 mol/L ethanol (5.5%, v/v) or an ethanol-free diet. The ethanol stress decreased survival and larval development rate but increased the weight of surviving adult males. Dietary long-chain fatty acids altered the fatty acid composition of tissue lipids of larvae. When an unsaturated fatty acid was fed, except for 18:2(n-6), the tissue level of total unsaturated fatty acids was markedly increased. Both saturated and unsaturated 18-carbon fatty acids shortened larval development time. Linoleic acid [18:2(n-6)] and linolenic acid [18:3(n-3)] enhanced survival overall and, together with stearic acid (18:0), gave marked protection from ethanol stress in terms of survival. Correlation analysis across the different fatty acid diets indicated a strong positive association between tissue 18-carbon fatty acid levels and ethanol tolerance and between 18-carbon fatty acid levels and development rate. No major differences were observed in the effects of the fatty acids on the Canton-S and OD4 (Tahbilk) wild-type strains. Thus, the fatty acid content of D. melanogaster larvae is important for growth and survival in ethanol-rich habitats.

Animals↗
PubMed1993

The lepidopteran mitochondrial control region: structure and evolution.

For several species of lepidoptera, most of the approximately 350-bp mitochondrial control-region sequences were determined. Six of these species are in one genus, Jalmenus; are closely related; and are believed to have undergone recent rapid speciation. Recent speciation was supported by the observation of low interspecific sequence divergence. Thus, no useful phylogeny could be constructed for the genus. Despite a surprising conservation of control-region length, there was little conservation of primary sequences either among the three lepidopteran genera or between lepidoptera and Drosophila. Analysis of secondary structure indicated only one possible feature in common--inferred stem loops with higher-than-random folding energies--although the positions of the structures in different species were unrelated to regions of primary sequence similarity. We suggest that the conserved, short length of control regions is related to the observed lack of heteroplasmy in lepidopteran mitochondrial genomes. In addition, determination of flanking sequences for one Jalmenus species indicated (i) only weak support for the available model of insect 12S rRNA structure and (ii) that tRNA translocation is a frequent event in the evolution of insect mitochondrial genomes.

Animals↗
PubMed1992

Transposable elements as a factor in the aging of Drosophila melanogaster.

We have considered the hypothesis that transposable elements may contribute to the aging process through somatic mutation. We have presented evidence to suggest that at least two elements, Copia and 412, are capable of somatic activity in adult Drosophila tissue. A strain harboring a third transposable element, P, was produced that showed eye color mosaicism and reversion to wild phenotype (red eyes) as a result of somatic and germ line transposition. A high-fat diet, known to accelerate aging, increased the frequency of eye color mosaicism and red eyes. We induced life span shortening by artificially activating somatic transposition of P elements, and the extent of reduction in life span was similar in both sexes. These data are consistent with the notion that some aspects of the age phenotype may be caused by mutational activity of transposable elements in somatic tissues. The hypothesis is readily tested in other organisms, including humans. It offers new dimensions in the understanding and management of age-associated changes.

Aging↗
PubMed1992

Dynamics of cytoplasmic incompatibility and mtDNA variation in natural Drosophila simulans populations.

In Drosophila simulans a cytoplasmically transmitted microorganism causes reduced egg hatch when infected males mate with uninfected females. The infection is rapidly spreading northward in California. Data on a specific mtDNA restriction site length polymorphism show that changes in the frequency of mtDNA variants are associated with this spread. All infected flies possess the same mtDNA allele, whereas the uninfected flies are polymorphic. Given that both paternal inheritance of the infection and imperfect maternal transmission have been demonstrated, one might expect instead that both infected and uninfected flies would possess both mtDNA variants. Our data suggest that imperfect female transmission of the infection (and/or the loss of the infection among progeny) is more common in nature than paternal transmission. A simple model of intrapopulation dynamics, with empirically supported parameter values, adequately describes the joint frequencies of the mtDNA variants and incompatibility types.

Animals↗
PubMed1992

An association between ADH protein levels and polymorphic nucleotide variation in the Adh gene of Drosophila melanogaster.

Southern analysis of the Adh region of 212 Drosophila melanogaster lines collected from the Tahbilk winery revealed linkage disequilibrium between a 37-bp insertion [designated delta 2 by Kreitman (1983)] and the fast electrophoretic variant of alcohol dehydrogenase (ADH-F). Among these lines 34% contained the insert and encoded ADH-F, 33.5% encoded ADH-F and did not have the insert, and 32.5% encoded the slow electrophoretic variant of alcohol dehydrogenase (ADH-S). Strong linkage association between this insert and ADH-F is evident worldwide. Twenty-nine of the second chromosome lines were characterized for ADH protein quantity by using radial immunodiffusion. ADH quantity was estimated in both larvae and adult males raised in the presence and absence of alcohol supplement to each of two different food media. Analyses of variance indicated higher levels of ADH protein in larvae from lines with the insert (all ADH-F), compared with those without (both ADH-F and ADH-S), independent of either dietary alcohol or media type. No such difference in ADH quantity between insert- and noninsert-containing ADH-F lines was detected in adults, although the expected higher levels occurred in ADH-F lines compared with ADH-S lines. Given the high levels of linkage disequilibrium in the Adh region, these data suggest that either polymorphic nucleotide-site variants positively associated with delta 2 on the second chromosome or delta 2 itself increases larval levels of ADH protein.

Alcohol Dehydrogenase↗
PubMed1991

Micro-spatial population differentiation in activity of glycerol-3-phosphate oxidase (GPO) from mitochondria of Drosophila melanogaster.

Replicate mass-bred laboratory populations of D. melanogaster were derived from females collected in the Tahbilk winery cellar and from females collected outside but from within two kilometers of the cellar. When mitochondrial extracts from larvae were assayed for specific activity of glycerol-3-phosphate oxidase the cellar populations had levels only 50% of those from the outside area, confirming an earlier report of such a difference among isofemale lines derived from these same areas. This micro-spatial differentiation occurred when larvae were raised on a medium supplemented with both sucrose (5% w/v) and ethanol (4% v/v), known to effect high GPO activity, but was not detected when the larvae were raised on unsupplemented medium. A heritable basis for larval GPO activity variation was confirmed in a set of 32 isogenic second chromosome substitution lines and measured in a subset of 4 of these lines about 25 generations later. A reciprocal cross using two isogenic substitution lines with the highest and lowest activities suggested the difference was attributable to genes acting additively and that there were no maternal or paternal effects. The detection of a collection site difference in GPO enzyme activity in the isogenic lines suggests that polymorphic variation on the second chromosome is responsible for the differentiation at the winery. Variation in adult GPO activity did not show a dependence on the winery location from where the isogenic lines were derived nor was there an effect of line. Adult GPO activity was significantly higher than that detected in larval tissues and did not show a dependence on the sugar/ethanol level in the growth medium.

Animals↗