Effects of temperature on physiology and reproductive success of a montane leaf beetle: implications for persistence of native populations enduring climate change

Understanding how climate change impacts natural systems requires investigations of the effects of environmental variation on vulnerable species and documentation of how populations respond to change. The willow beetle Chrysomela aeneicollis is ideal for such studies. It lives in California's Sierra Nevada on the southern edge of its worldwide range. Beetles experience elevated air temperatures during summertime egg laying and larval development. Exposure to these temperatures causes physiological stress, which may reduce reproductive success and endanger populations. The glycolytic enzyme phosphoglucose isomerase (PGI) is a marker of temperature adaptation in C. aeneicollis. PGI allele frequency varies across a latitudinal gradient: allele 1 is common in Rock Creek (RC), which is cooler and to the north, and allele 4 is common in Big Pine Creek (BPC), which is warmer and to the south. In populations that are intermediate in geography and climate (e.g., Bishop Creek [BC]), PGI-4 frequency increases from north to south such that alleles 1 and 4 are in relatively equal frequency in southern BC. Over the past decade, Sierra Nevada beetle populations have colonized high elevations and have become extinct at lower elevations where they were once common. In BC, the magnitude of PGI allele frequency fluctuations among life-history stages is related to maximal air temperature, with the frequency of PGI-4 increasing after the hottest part of summer. To identify mechanisms that may cause shifts in PGI allele frequency, we measured metabolic rate and fecundity for beetles collected at BC. Metabolic rate of males and females was measured at 20 degrees and 36 degrees C using flow-through respirometry. To measure laboratory fecundity, mating pairs were acclimated for 4 h each afternoon at a control temperature (20 degrees C) or at mildly elevated temperatures (26 degrees or 32 degrees C) and number of eggs laid was counted daily for 24 d, after which tissue levels of 70-kD heat shock proteins (Hsp70) were determined. Previous studies had demonstrated differences in Hsp70 expression among PGI genotypes at these temperatures. To measure field fecundity, mating pairs from BC were transplanted to similar elevations in BPC, BC, and RC and were monitored in situ for 24 d. Metabolic rate was higher for PGI 4-4 genotypes than for PGI 1-4 or PGI 1-1 individuals at 36 degrees C but not at 20 degrees C. In contrast, laboratory fecundity was greatest for females possessing PGI-1, independent of acclimation temperature. At the end of the laboratory fecundity experiment, Hsp70 expression was positively related to fecundity, suggesting minimal reproductive cost of upregulation of heat shock proteins in response to mild heat stress. In the field, fecundity was highest for PGI 1-1 and PGI 1-4 individuals in RC and PGI 4-4 individuals in BPC and was similar for all genotypes in BC. Thus, fecundity in nature was greatest for the genotypes that were most common in each area. Taken together, data reported here suggest that hot, dry summers in the Sierra Nevada may result in an increase in frequency of the PGI-4 allele and shifts to higher elevations for C. aeneicollis populations.     

Mixed reproductive strategy in Tubifex tubifex (Oligochaeta, Tubificidae)?

Despite the wide use of the tubificid oligochaete Tubifex tubifex in eco-toxicological studies, the reproductive strategy adopted by the species, that can reasonably be assumed to be the key to its ecological success, is still not well investigated. This study reports on breeding experiments analysed by allozyme markers (Pgi, Pgm, and Idh), accompanied by a study of the sperm production in the species, with the aim to: (1) clarify the type of uniparental reproduction adopted by the species, (2) test the hypothesis that uniparental reproduction is reversible, (3) investigate the occurrence of biparental reproduction in laboratory cultures. Studies of parent-offspring comparison at polymorphic allozyme loci showed parthenogenetic reproduction and maintenance of asexuality in the parthenogenetic individuals. A cross-breeding test performed with couples made up of randomly assorted individuals, whose genotype combinations were suitable for discriminating between sexual and asexual reproduction, failed to show biparental reproduction in laboratory cultures: T. tubifex always reproduced parthenogenetically. Unexpectedly, spermiogenetic analysis indicated that both kinds of sperm produced by the species (eusperm and parasperm) were differentiated in individuals raised either in cohort cultures or in isolation (first, second, and third parthenogenetic generations), with a similar pattern of sperm production correlated to the sexual stage. Interestingly, there was no avoidance of mating in any of the collective cultures analysed. Concomitance between parthenogenetic reproduction and a "normal" male functionality, which is typical of a sexually reproducing species, could be justified by a mixed reproductive strategy or a pseudogamy process occurring in T. tubifex. However, several aspects of the reproductive behaviour of the species deserve further investigation.     

Absence of population genetic differentiation in the New Zealand greenshell mussel Perna canaliculus (Gmelin 1791) as assessed by allozyme variation

Genetic variation in the endemic New Zealand greenshell mussel, Perna canaliculus (Gmelin 1791), was examined using starch-gel electrophoresis at seven protein-coding loci (Idh; Acon-1; Acon-2; Gpd; Pgi; Pgm; Pgd) in 35 populations (N=1038 mussels). For all loci and all populations, Fisher's exact tests indicated highly significant departures from Hardy-Weinberg equilibrium (HWE), but this overall result was caused by significant heterozygote deficiencies at only two loci (Pgm and Pgd), and in only three northern populations (Kuaotunu, Te Haumi and Days Bay). Allelic and genotypic differentiation between population pairs at individual loci and across all loci were nonsignificant, and genotypic disequilibrium at each locus pair was also nonsignificant for all populations. Genetic variation in all populations was high (mean heterozygosity, 0.210+/-0.113), while Nei's D among populations was very low (0.002+/-0.002). Low population subdivision (θ=-0.001+/-0.002) and high levels of gene flow (Nm(p)=10.18; Nm(θ)=infinity) also indicated that the single panmictic unit model best explains population genetic homogeneity in P. canaliculus over a north-south distance >2000 km. Lack of genetic subdivision in this species is discussed in light of two previous allozyme studies, with differing results: one suggested that a north-south division exists between greenshell mussel stocks, and the other suggested that population structure in this species can be explained through isolation by distance model modified by local hydrology.     

Genetic variation and population structure of cod, Gadus morhua L., in some fjords in northern Norway

Biochemical genetic polymorphism in cod from several fjords in Troms, northern Norway was analysed. Gene frequencies at several polymorphic loci ( Hb, Pgi, Ldh, Pgm, Gpd and Idh ) are given. Significant variation was found both within and between fjords, and even between samples from the same locality sampled in different years, indicating a mosaic structure of the cod population in the area studied. The results of the haemoglobin variation are compared to similar results obtained more than 20 years ago in the same and adjacent areas. The biochemical genetic variation is discussed in relation to stability of gene frequencies, isolation mechanisms and migration patterns.     

Fitness-related traits of allozyme genotypes in Bromus hordeaceus L. (Poaceae) associated with field habitat and experimental flooding

Genetic variation at alcohol dehydrogenase and phosphoglucose isomerase loci in Bromus hordeaceus has in an earlier study been found to show substantial microgeographic spatial structuring. The present study reports differences in fitness related characters between the enzyme genotypes, both from a field study and a greenhouse experiment. The field study showed overall differences in seed set between allozyme genotypes and also that Pgi-1b genotypes differed in number of seeds set at different levels of herb cover in their habitat. In the greenhouse, dry, normal or flooded conditions were applied. Seeds from individuals with the Adh-1b-11 genotype matured faster in the dry and slower in the flooded treatments than did seeds from individuals with the Adh-1b-22 genotype. Individuals containing Pgi-1b-1f1f alleles and Adh-1b-11 alleles are more plastic than individuals with other allele combinations, meaning that allozyme variation could partly explain what could be seen as adaptive phenotypic plasticity. Mean seed weight was different between dry and flooding treatments for Pgi-1b genotypes. There were also direct effects of allozyme genotype on the probability of survival, total plant weight, weight of reproductive parts, seed weight, days to seed maturation and the percentage of reproductive parts out of the total plant weight.     

Genetic differentiation in Chilean hake Merluccius gayi gayi (Pisces: Merlucciidae)

The genetic structure of Chilean hake Merluccius gayi gayi, was analyzed using starch gel protein electrophoresis. Samples were collected from four localities along the coast off Chile. A total of 1500 specimens sampled from Coquimbo, San Antonio, Talcahuano and Puerto Montt were used in the study. Genetic information was obtained for six allozyme loci Pgi-1, Pgi-2, Pgm, Idh-1, Idh-2 and Aat. The results of the analysis showed no significant allelic differences among samples from various localities, with an average F_ST value of 0.007 for all loci and samples. A heterogeneity test for all loci and specimens from the four localities showed only two significant values. Thus, Chilean hake populations along the coast of Chile appear to be genetically homogeneous.     

Isoenzyme electrophoretic patterns in discus fish (Symphysodon aequifasciatus Pellegrin, 1904 and Symphysodon discus Heckel, 1840) from the Central Amazon

The discus is a very popular and expensive aquarium fish belonging to the family Cichlidae, genus Symphysodon, formed by three Amazon basin endemic species: Symphysodon aequifasciatus, S. discus and S. tarzoo. The taxonomic status of these fish is very controversial, with a paucity of molecular research on their population genetic structure and species identification. Information on molecular genetic markers, especially isoenzymes, in search of a better understanding of the population genetic structure and correct identification of fish species, has been receiving more attention when elaborating and implementing commercial fishery management programs. Aiming to contribute to a better understanding of the species taxonomic status, the present study describes the isoenzymatic patterns of 6 enzymes: esterase (Est - EC 3.1.1.1), lactate dehydrogenase (Ldh - EC 1.1.1.27), malate dehydrogenase (Mdh - EC 1.1.1.37), phosphoglucomutase (Pgm - EC 5.4.2.2), phosphoglucose isomerase (Pgi - EC 5.3.1.9), and super oxide dismutase (Sod - EC 1.15.1.1) extracted from skeletal muscle specimens and analyzed by starch gel electrophoresis. Monomorphic patterns, presumably controlled by 11 loci: Est-1, Est-2, Est-3, Ldh-1, Ldh-2, Mdh-1, Mdh-2, Pgi-1, Pgi-2, Pgm-1, and Sod-1 were fixed for the same alleles: Est-1(1), Est-2(1), Est-3(1), Ldh-1(1), Ldh-2(1), Mdh-1(1), Mdh-2(1), Pgi-1(1), Pgi-2(1), Pgm-1(1), and Sod-1(1), respectively, and detected in all 60 specimens examined (27 S. aequifasciatus from Manacapuru and 33 S. discus from Novo Air?o, Central Amazon). The failure in the present study to detect diagnostic loci, which could be very useful for differentiating S. aequifasciatus from S. discus species, and polymorphic loci, which could also be applied for possible identification and delimitation of their stocks, does not rule out the possibility of there existing in other isoenzyme gene loci to be analyzed in the future.     

Clinal variation for amino acid polymorphisms at the Pgm locus in Drosophila melanogaster

Clinal variation is common for enzymes in the glycolytic pathway for Drosophila melanogaster and is generally accepted as an adaptive response to different climates. Although the enzyme phosphoglucomutase (PGM) possesses several allozyme polymorphisms, it is unique in that it had been reported to show no clinal variation. Our recent DNA sequence investigation of Pgm found extensive cryptic amino acid polymorphism segregating with the allozyme alleles. In this study, we characterize the geographic variation of Pgm amino acid polymorphisms at the nucleotide level along a latitudinal cline in the eastern United States. A survey of 15 SNPs across the Pgm gene finds significant clinal differentiation for the allozyme polymorphisms as well as for many of the cryptic amino acid polymorphisms. A test of independence shows that pervasive linkage disequilibrium across this gene region can explain many of the amino acid clines. A single Pgm haplotype defined by two amino acid polymorphisms shows the strongest correlation with latitude and the steepest change in allele frequency across the cline. We propose that clinal selection at Pgm may in part explain the extensive amino acid polymorphism at this locus and is consistent with a multilocus response to selection in the glycolytic pathway.     

Role of α-phosphoglucomutase and phosphoglucose isomerase activities at the branching point between sugar catabolism and anabolism in Lactobacillus casei

Aims: To evaluate the role of α‐phosphoglucomutase (α‐Pgm) and phosphoglucose isomerase (Pgi) activities in growth rate, sugar‐phosphates, UDP‐sugars and lactate biosynthesis in Lactobacillus casei. Methods and Results: The pgm and pgi genes coding for α‐Pgm and Pgi activities in L. casei BL23, respectively, were identified, cloned and shown to be functional by homologous overexpression. In MRS fermentation medium with glucose, overexpression of pgm gene in L. casei resulted in a growth rate reduced to 75% and glucose‐6P levels reduced to 47%. By contrast, with lactose, the growth rate was raised to 119%. An increment of α‐Pgm activity had no significant effect on UDP‐sugar levels. Remarkably, Pgi overexpression in L. casei grown in lactose or galactose resulted in almost a double growth rate with respect to the control strain. The increased Pgi activity also resulted in glucose‐6P levels reduced to 25 and 59% of control strain cultured in glucose and lactose, respectively, and the fructose‐6P levels were increased to 128% on glucose. UDP‐glucose and UDP‐galactose levels were reduced to 66 and 55%, respectively, of control strain levels cultured in galactose. In addition, the lactate yield increased to 115% in the strain overproducing Pgi grown in galactose. Conclusions: The physiological amount of α‐Pgm and Pgi activities is limited for L. casei growth on lactose, and lactose and galactose, respectively, and that limitation was overcome by pgm and pgi gene overexpression. The increment of α‐Pgm and Pgi activities, respectively, resulted in modified levels of sugar‐phosphates, sugar‐nucleotides and lactate showing the modulation capacity of the carbon fluxes in L. casei at the level of the glycolytic intermediate glucose‐6P. Significance and Impact of the Study: Knowledge of the role of key enzymes in metabolic fluxes at the branching point between anabolic and catabolic pathways would allow a rational design of engineering strategies in L. casei.     

Phosphoglucose isomerase isozymes and allozymes of the brown trout, Salmo trutta L

1. In brown trout the Pgi-1 and Pgi-2 loci are predominantly expressed in white skeletal muscle; Pgi-3 being mainly expressed in most other tissues. 2. Total PGI activity determinations revealed that the allele formerly designated Pgi-2(65) is a null allele, Pgi-2(n). 3. Enzyme kinetic studies on the partially purified PGI homodimeric isozymes revealed that from 5 to 25 degrees C both PGI-1 and PGI-2 had significantly higher mean Km(F6P) values compared to PGI-3. 4. Distinct metabolic roles for the "muscle" (PGI-1, PGI-2) and "liver" (PGI-3) isozymes are proposed. 5. Significant Km (F6P) differences were found among the PGI-2 allozymes and among the PGI-3 allozymes.     

Molecular genetic markers provide no evidence for reproductive isolation among retreat building phenotypes of the net-spinning caddisfly Macrostemum carolina

Larvae of the stream-dwelling, filter-feeding caddisfly Macrostemum carolina construct silken catchnets within protective retreats. In the Savannah River, M. carolina individuals make three different retreats, each with a distinct water entrance hole: (i) at the end of a silken tube; (ii) with a approximately 180 degrees silken backstop; and (iii) flush with the top of the retreat. To resolve whether these different retreats represent alternative behavioural phenotypes within a single panmictic population or fixed phenotypes within three genetically distinct populations or species, we compared the allele frequencies at three polymorphic nuclear loci (allozyme electrophoresis for Gpi, Mpi and Pgm) and the mitochondrial DNA (mtDNA) haplotype frequencies among individuals displaying the three retreat morphs. We also calculated pairwise exact tests of population differentiation using the allozyme and mtDNA allele frequencies. No significant genetic differentiation was detected among caddisflies exhibiting the different retreat morphs. Therefore, these morphs apparently represent a single panmictic population in the Savannah River. Consequently, additional study is required to assess whether this retreat polymorphism is a phenotypically plastic trait under conditional control, or is mediated by alternative alleles at a Mendelian gene or genes (or a combination of the two).     

中华稻蝗四个种群的遗传分化

采用水平淀粉凝胶电泳技术研究中华稻蝗(Oxya chinensis)四个种群的12个基因座位,探讨其遗传分化.这四个种群分别采自内蒙古呼和浩特、山西代县、山西太原和陕西西安.Ck和Mdh-2在四个种群中均为单态,其余的基因座位至少在一个种群内有两个以上的等位基因;在Ldh和Mdh-1的等位基因频率呈现梯度分布趋势.多态基因座位百分率(P)和平均每个基因座位的等位基因数目(A)分别为58.3%-66.7%和2.2-2.8,平均杂合度为Ho=0.173-0.240.除Gpi,Hk-2,Idh,Ldh和Mdh-1在部分蝗虫种群符合Hardy-Weinberg平衡外,其余绝大多数基因座位的基因型频率显著偏离Hardy-Weinberg平衡.四个种群间FST平均值不存在显著差异(FST=0.0510,P＞0.05),结合高的Nei's遗传一致度(I＞0.97)可知,种群之间遗传分化不明显.我们认为人类的农业活动可能促进了种群间的基因交流,从而降低了分化程度[动物学报50(2)187-192,2004].     

Metabolism,swimming performance,and tissue biochemistry of high desert redband trout (Oncorhynchus mykiss ssp.): evidence for phenotypic differences in physiological function

Redband trout (Oncorhynchus mykiss ssp.) in southeastern Oregon inhabit high-elevation streams that exhibit extreme variability in seasonal flow and diel water temperature. Given the strong influence and potential limitations exerted by temperature on fish physiology, we were interested in how acute temperature change and thermal history influenced the physiological capabilities and biochemical characteristics of these trout. To this end, we studied wild redband trout inhabiting two streams with different thermal profiles by measuring (1) critical swimming speed (U(crit)) and oxygen consumption in the field at 12 degrees and 24 degrees C; (2) biochemical indices of energy metabolism in the heart, axial white skeletal muscle, and blood; and (3) temperature preference in a laboratory thermal gradient. Further, we also examined genetic and morphological characteristics of fish from these two streams. At 12 degrees C, maximum metabolic rate (Mo2max) and metabolic power were greater in Little Blitzen redband trout as compared with those from Bridge Creek (by 37% and 32%, respectively). Conversely, Bridge Creek and Little Blitzen trout had similar values for Mo2max and metabolic power at 24 degrees C. The U(crit) of Little Blitzen trout was similar at the two temperatures (61+/-3 vs. 57+/-4 cm s(-1)). However, the U(crit) for Bridge Creek trout increased from 62+/-3 cm s(-1) to 75+/-3 cm s(-1) when water temperature was raised from 12 degrees to 24 degrees C, and the U(crit) value at 24 degrees C was significantly greater than for Little Blitzen fish. Cost of transport was lower for Bridge Creek trout at both 12 degrees and 24 degrees C, indicating that these trout swim more efficiently than those from the Little Blitzen. Possible explanations for the greater metabolic power of Little Blitzen redband trout at 12 degrees C include increased relative ventricular mass (27%) and an elevation in epaxial white muscle citrate synthase activity (by 72%). Bridge Creek trout had 50% higher lactate dehydrogenase activity in white muscle and presumably a greater potential for anaerobic metabolism. Both populations exhibited a preferred temperature of approximately 13 degrees C and identical mitochondrial haplotypes and p53 gene allele frequencies. However, Bridge Creek trout had a more robust body form, with a relatively larger head and a deeper body and caudal peduncle. In summary, despite the short distance ( approximately 10 km) and genotypic similarity between study streams, our results indicate that phenotypic reorganization of anatomical characteristics, swimming ability at environmentally pertinent temperatures and white axial muscle ATP-producing pathways occurs in redband trout.     

华北2蝗区东亚飞蝗种群遗传结构的比较研究

利用水平淀粉凝胶电泳对采自天津北大港和河北黄骅两个相临蝗区的东亚飞蝗（Locusta migratoria manilensis)种群进行等位酶基因频率分析,比较了这两个种群的遗传结构,等位酶酶谱分析表明,19个基因座中4个基因座（Mdh-l,Pgm,Adk,G3pd)的等位基因频率变化很小,常见等位基因的频率均高于0．95,其他基因座有2－4个等位基因,但是两个种群的等位基因频率除两个基因座（Fbp,Got-2)外都很相似,多态位点的27个χ2检验表明,由于常见等位基因纯合子的高频率的和相应杂合子的缺乏,仅有北大港种群的2个基因座（Pgi,Got-1)符合Hardy-Weinberg平衡,在每个种群内的蝗虫存在明显的遗传变异,但在种群间遗传结构极为相似,多态位点的百分数P分别为73．7％和78．9％,每个基因座的平均等位基因数A为2．9和3．1,平均每个基因座的实际杂合度几乎相等（约为0．138）,F－统计量（FST＝0．053）也表明了两个种群间的遗传 一致性,遗传相似性系数（I）高达0．938,这些结果提示,这两个种群可能属于1 个大种群,在两个种群的一定位点上的遗传多态性和分化可能都与迁飞因素有关,因为东亚飞蝗的高度扩散能力有利于遗传结构的连续分布,高度的迁飞能力也导致个体暴露于各种不同的环境,而在种群水平上的遗传为异能增强种群在各种生态条件生存和繁殖能力,因此,迁飞有利于维持东亚飞蝗种群的遗传多态性的动态平衡。     

Depression of sublingual temperature by cold saliva.

Sublingual and oesophageal temperatures were compared at various air temperatures in 16 subjects. In warm air (25-44 degrees C) sublingual temperatures stabilized within plus or minus 0-45 degrees C of oesophageal temperatures, but in air at room temperature (18-24 degrees C) they were sometimes as much as 1-1 degrees C below and in cold air (5-10 degrees C) as much as 4-4 degrees C below oesophageal readings. The sublingual-oesophageal temperature difference in cold air was greatly reduced by keeping the face warm, but it was not reduced in two patients breathing through tracheostomies and thereby eliminating cold air flow from the nose and pharynx. Parotid saliva temperature was low and saliva flow high during exposure, and cold saliva seemed to be mainly responsible for the erratic depression of sublingual temperature in the cold. These results indicate hazards in the casual use of sublingual temperatures, and indicate that external heat may have to be supplied to enable them to give reliable clinical assessments of body temperature.     

Rapid cold hardening in the western flower thrips Frankliniella occidentalis

A rapid cold hardening process is reported in first instar larvae of Frankliniella occidentalis. When larvae are transferred directly from 20 degrees C to -11.5 degrees C for 2h there is 78% mortality, whereas exposure to 0 degrees C for 4h prior to transfer to -11.5 degrees C reduces mortality to 10%. The response can also be induced by exposure to 5 degrees C for 4h or by gradual cooling at rates between 0.1 and 0.5 degrees C min(-1.) The acquired cold tolerance is transient and is rapidly lost (after 1h at 20 degrees C). Rapid cold hardening extends survival times at -11.5 degrees C and depresses lethal temperatures in short (2h) exposures. Rearing at 15 degrees C (12L:12D), (a cold acclimation regime for F. occidentalis), does not protect against the cold shock induced by direct transfer to -11.5 degrees C (which rapid cold hardening does) but does extend survival time at -5 degrees C (i.e. increased chill tolerance) whilst rapid cold hardening does not. The rapid and longer term cold hardening responses in F. occidentalis therefore appear to have different underlying mechanisms.     

Diversifying selection underlies the origin of allozyme polymorphism at the phosphoglucose isomerase locus in Tigriopus californicus

The marine copepod Tigriopus californicus lives in intertidal rock pools along the Pacific coast, where it exhibits strong, temporally stable population genetic structure. Previous allozyme surveys have found high frequency private alleles among neighboring subpopulations, indicating that there is limited genetic exchange between populations. Here we evaluate the factors responsible for the diversification and maintenance of alleles at the phosphoglucose isomerase (Pgi) locus by evaluating patterns of nucleotide variation underlying previously identified allozyme polymorphism. Copepods were sampled from eleven sites throughout California and Baja California, revealing deep genetic structure among populations as well as genetic variability within populations. Evidence of recombination is limited to the sample from Pescadero and there is no support for linkage disequilibrium across the Pgi locus. Neutrality tests and codon-based models of substitution suggest the action of natural selection due to elevated non-synonymous substitutions at a small number of sites in Pgi. Two sites are identified as the charge-changing residues underlying allozyme polymorphisms in T. californicus. A reanalysis of allozyme variation at several focal populations, spanning a period of 26 years and over 200 generations, shows that Pgi alleles are maintained without notable frequency changes. Our data suggest that diversifying selection accounted for the origin of Pgi allozymes, while McDonald-Kreitman tests and the temporal stability of private allozyme alleles suggests that balancing selection may be involved in the maintenance of amino acid polymorphisms within populations.     

Genetic structure,allozyme-habitat associations and reproductive fitness in Gypsophila fastigiata (Caryophyllaceae)

Relationships between allozyme differentiation, habitat variation and individual reproductive success were examined in local populations of a perennial herb, Gypsophila fastigiata, on the Baltic island of Öland (Sweden). Relatively little (c. 2%) of the total allozyme diversity in this largely outcrossing species is explained by differentiation between sites tens of kilometres apart. The low level of geographic differentiation suggests that gene flow between sites is, or has recently been, extensive. Yet the component of allozyme diversity due to differentiation between plots (only tens of meters apart) within sites is 3 times larger than the between-site component of diversity. Allozyme variation, especially at the Pgi-2 locus, is significantly associated with habitat variation within sites. Different allele x habitat combinations for the Pgi-2 locus are associated with differences in individual reproductive fitness. Differential selection in different local habitats may thus contribute to the fine-scale structuring of genetic diversity within sites.     

Induction of rapid cold hardening by cooling at ecologically relevant rates in Drosophila melanogaster

Over a decade ago it was hypothesized that the rapid cold hardening process allows an organism's overall cold tolerance to track changes in environmental temperature, as would occur in nature during diurnal thermal cycles. Although a number of studies have since focused on characterizing the rapid cold hardening process and on elucidating the physiological mechanisms upon which it is based, the ecological relevance of this phenomenon has received little attention. We present evidence that in Drosophila melanogaster rapid cold hardening can be induced during cooling at rates which occur naturally, and that the protection afforded in such a manner benefits the organism at ecologically relevant temperatures. Drosophila melanogaster cooled at natural rates (0.05 and 0.1 degrees C min(-1)) exhibited significantly higher survival after one hour of exposure to -7 and -8 degrees C than did those directly transferred to these temperatures or those cooled at 0.5, or 1.0 degrees C min(-1). Protection accrued throughout the cooling process (e.g., flies cooled to 0 degrees C were more cold tolerant than those cooled to 11 degrees C). Whereas D. melanogaster cooled at 1.0 degrees C min(-1) had a critical thermal minimum (i.e., the temperature at which torpor occurred) of 6.5+/-0.6 degrees C, those cooled at an ecologically relevant rate of 0.1 degrees C min(-1) had a significantly lower value of 3.9+/-0.9 degrees C.