ADAPTATIONS TO TEMPERATE CLIMATES AND EVOLUTION OF OVERWINTERING STRATEGIES IN THE DROSOPHILA MELANOGASTER SPECIES GROUP

The Drosophila melanogaster species group is considered to have originated in the tropics and only recently invaded temperate habitats. The temperate species of this group that were studied here may be subdivided into the warm-temperate species (D. lutescens and D. rufa) and the cool-temperate species (species of the auraria complex). The warm-temperate species were more cold-hardy than were their tropical relatives (D. takahashii or D. melanogaster) at the larval and imaginal stages, and the cool-temperate species were more cold-hardy than the warm-temperate species, although only at the imaginal stage. However, these species showed little or no intraspecific variation in cold-hardiness, in spite of great variation in winter temperature within the species' ranges. It is assumed that cold-hardiness is one of the main factors restricting their distributions at high latitudes and that it is the key for evolution of the warm- and cool-temperate species from their subtropical or warm-temperate ancestors. Both warm- and cool-temperate species had photoperiodically controlled reproductive diapause. In the cool-temperate species, the development of cold-hardiness was affected by diapause, but diapause had little or no effect on cold-hardiness in the warm-temperate species. Critical daylengths and the diapause rates varied from species to species according to variation in their overwintering plans and also varied geographically in consequence of their adaptation to local climates. Species showed different responses to temperature in preimaginal and ovarian development. These differences are considered to reflect adaptation to different environmental temperatures.     

Diapause and cold tolerance in Asian species of the parasitoid Leptopilina (Hymenoptera: Figitidae)

Diapause and cold tolerance are essential for temperate insects to pass the winter, with the mechanisms controlling these two traits varying considerably among insects. In the present study, diapause and cold tolerance are compared among three Leptopilina species: Leptopilina japonica Novkovi? & Kimura, Leptopilina victoriae Nordlander and Leptopilina ryukyuensis Novkovi? & Kimura, all larval parasitoids of frugivorous drosophilid flies, with the aim of understanding their climatic adaptations. The first species is divided into the temperate (Leptopilina japonica japonica) and subtropical subspecies (Leptopilina japonica formosana), and the latter two species are distributed in the tropical and subtropical regions. The temperate subspecies of L. japonica enters prepupal diapause at low temperatures (15 or 18 °C), irrespective of photoperiod, and some individuals enter diapause when exposed to 0 °C for 1 or 2 day(s) or when placed at low humidity. Leptopilina victoriae also shows signs of diapause initiation at 15 °C, although L. ryukyuensis and L. j. formosana from the subtropical regions do not. Preimaginal viability at low temperature (13, 14 or 15 °C) is usually lower in L. victoriae from the tropical regions compared with L. japonica or L. ryukyuensis from the temperate or subtropical regions. Diapausing prepupae of the temperate subspecies appear to be cold tolerant. However, the cold tolerance of nondiapausing prepupae, pupae and adult females varies little among the tropical, subtropical and temperate species or subspecies, and adult males of the temperate subspecies of L. japonica are less cold tolerant than those of the tropical or subtropical species or subspecies. Cold tolerance may be unnecessary, except for diapausing individuals of the temperate species, because nondiapausing individuals appear in warmer seasons.     

Phylogenetic relationships and climatic adaptations in the Drosophila takahashii and montium species subgroups

We analyze phylogenetic relationships among temperate, subtropical highland, and subtropical lowland species of the Drosophila takahashii and montium species subgroups based on sequence data of COI and Gpdh genes and discuss the evolution of temperate species in these subgroups with reference to their climatic adaptations. In the takahashii subgroup, D. lutescens (the temperate species) branched off first in the tree based on the combined data set, but D. prostipennis (the subtropical highland species) branched off first in the trees based on single genes. Thus, phylogenetic relationships in this subgroup are still ambiguous. In the montium subgroup, the cool-temperate species are phylogenetically close to the warm-temperate species, and these cool- and warm-temperate species form a cluster with the subtropical highland species. This suggests that perhaps the cool-temperate species derived from the warm-temperate species and the warm-temperate species derived from the subtropical highland species. In comparison with the subtropical lowland species, the subtropical highland species may be better able to colonize temperate areas since, as in the temperate species, they have an ability to develop their ovaries at moderately low temperature. However, the subtropical highland species, as well as the subtropical lowland species, were much less cold tolerant than the temperate species. Therefore, considerable genetic reformation would be required for both the subtropical highland and the subtropical lowland species to adapt to temperate climates.     

Opposing clines for high and low temperature resistance in Drosophila melanogaster

In insects, species comparisons suggest a weak association between upper thermal limits and latitude in contrast to a stronger association for lower limits. To compare this to latitudinal patterns of thermal responses within species, we considered latitudinal variation in heat and cold resistance in Drosophila melanogaster. We found opposing clines in resistance to these temperature extremes in comparisons of 17–24 populations from coastal eastern Australia. Knockdown time following heat shock increased towards the tropics, whereas recovery time following cold shock decreased towards temperate latitudes. Mortality following cold shock also showed a clinal pattern. Clinal associations with latitude were linear and related to minimum temperatures in the coldest month (for cold resistance) and maximum temperatures in the warmest month (for heat resistance). This suggests that within species both high and low temperature responses can vary with latitude as a consequence of direct or indirect effects of selection.     

The influence of eclosion timing on winter survival and triacylglycerol accumulation in four temperate species of Drosophila

Abstract. Diapause incidence, winter survival and triacylglycerol content were studied in temperate species of Drosophila by outdoor rearing in Sapporo (a cool-temperate region), northern Japan. In strains of D.subauraria and D.triauraria (cool-temperate species) from northern Japan, diapause was induced in adults having eclosed after early September, but in those having eclosed after mid October in strains of D.rufa and D.lutescens (warm-temperate species) from southern Japan. A subtropical strain of D.triauraria did not enter diapause. In the strains of cool-temperate species from northern Japan, about 70% survived until spring when eclosed in mid autumn, whereas when eclosed earlier and later flies had a lower ability to overwinter. The warm-temperate species and a subtropical strain of D.triauraria also survived longer when eclosed in mid autumn than when eclosed later, but they were unable to survive until spring. The triacylglycerol content was higher in the cool-temperate species from northern Japan than in the warm-temperate species and a subtropical strain of D.trauraria. In each species the triacylglycerol content of adults was higher when they eclosed in early and mid than in late autumn. It is assumed that triacylglycerol plays an important role in overwintering of these Drosophila species and the low triacylglycerol level is the primary cause of the low overwintering capacity of individuals eclosing in late autumn.     

Can amphibians take the heat? Vulnerability to climate warming in subtropical and temperate larval amphibian communities

Predicting the biodiversity impacts of global warming implies that we know where and with what magnitude these impacts will be encountered. Amphibians are currently the most threatened vertebrates, mainly due to habitat loss and to emerging infectious diseases. Global warming may further exacerbate their decline in the near future, although the impact might vary geographically. We predicted that subtropical amphibians should be relatively susceptible to warming‐induced extinctions because their upper critical thermal limits (CT_max) might be only slightly higher than maximum pond temperatures (T_max). We tested this prediction by measuring CT_max and T_max for 47 larval amphibian species from two thermally distinct subtropical communities (the warm community of the Gran Chaco and the cool community of Atlantic Forest, northern Argentina), as well as from one European temperate community. Upper thermal tolerances of tadpoles were positively correlated (controlling for phylogeny) with maximum pond temperatures, although the slope was steeper in subtropical than in temperate species. CT_max values were lowest in temperate species and highest in the subtropical warm community, which paradoxically, had very low warming tolerance (CT_max–T_max) and therefore may be prone to future local extinction from acute thermal stress if rising pond T_max soon exceeds their CT_max. Canopy‐protected subtropical cool species have larger warming tolerance and thus should be less impacted by peak temperatures. Temperate species are relatively secure to warming impacts, except for late breeders with low thermal tolerance, which may be exposed to physiological thermal stress in the coming years.     

Heat- and cold-shock responses and temperature adaptations in subtropical and temperate species of Drosophila

Accumulation of Hsp70 mRNA was investigated with relation to heat and cold tolerance in adult males of three Drosophila species. The subtropical lowland species (D. watanabei) and the cool-temperate species (D. triauraria) were more tolerant to heat than the subtropical highland species (D. trapezifrons), and the cool-temperate species were much more tolerant to cold than the two subtropical species. Thus, heat and cold tolerance was related to temperature conditions in the habitats. The threshold temperatures for the induction of Hsp70 mRNA at heat and cold were higher in D. watanabei than in D. trapezifrons or D. triauraria, but were not different between the latter two species in spite of the difference in their heat and cold tolerance. In D. trapezifrons, exposures to 0 degrees C for 12h and 6 degrees C for 24h killed about 40% of individuals, but the former treatment induced Hsp70 mRNA while the latter one did not. Thus, the relation between the heat- and cold-shock responses and temperature tolerance was not rigid in the species studied. In D. triauraria, the threshold temperatures for the induction of Hsp70 mRNA at heat and cold were lower when reared at a lower temperature.     

Comparison of oxygen consumption in drosophilid flies from different climates

Oxygen consumption at rest was studied in drosophilid species from cool‐temperate, warm‐temperate and subtropical regions to assess whether adaptations to different climates are associated with changes in metabolic rates. In experiments at 23°C using 8‐day‐old males of 28 species, body mass was revealed to be a significant predictor of oxygen consumption. No significant relation was detected between mass‐adjusted oxygen consumption and latitudinal distribution or thermal tolerance by either conventional regression analysis or a phylogenetically based method. The effect of temperature on oxygen consumption was studied with experiments at 15, 18, 23 and 28°C using 8‐ and 24‐day‐old males of four species of each of the montium species subgroup and the subgenus Drosophila. In these experiments, it was confirmed that temperature was a significant predictor of mass‐adjusted oxygen consumption. In both lineages, mass‐adjusted oxygen consumption was not higher in cool‐temperate species than in subtropical species. Thus, adaptations to colder climates are not associated with elevation of metabolic rates in these drosophilid species. The results of the present study also indicate that oxygen consumption is not related to the capacity to walk quickly.     

Disparate patterns of thermal adaptation between life stages in temperate vs. tropical Drosophila melanogaster

Many terrestrial ectothermic species exhibit limited variation in upper thermal tolerance across latitude. However, these trends may not signify limited adaptive capacity to increase thermal tolerance in the face of climate change. Instead, thermal tolerance may be similar among populations because behavioural thermoregulation by mobile organisms or life stages may buffer natural selection for thermal tolerance. We compared thermal tolerance of adults and embryos among natural populations of Drosophila melanogaster from a broad range of thermal habitats around the globe to assess natural variation of thermal tolerance in mobile vs. immobile life stages. We found no variation among populations in adult thermal tolerance, but embryonic thermal tolerance was higher in tropical strains than in temperate strains. We further report that embryos live closer to their upper thermal limits than adults – that is, thermal safety margins are smaller for embryos than adults. F1 hybrid embryos from crosses between temperate and tropical populations had thermal tolerance that matched that of tropical embryos, suggesting the dominance of heat‐tolerant alleles. Together, our findings suggest that thermal selection has led to divergence in embryonic thermal tolerance but that selection for divergent thermal tolerance may be limited in adults. Further, our results suggest that thermal traits should be measured across life stages to better predict adaptive limits.     

Thermal tolerance and geographical range size in the Agabus brunneus group of European diving beetles (Coleoptera: Dytiscidae)

Aim Within clades, most taxa are rare, whilst few are common, a general pattern for which the causes remain poorly understood. Here we investigate the relationship between thermal performance (tolerance and acclimation ability) and the size of a species’ geographical range for an assemblage of four ecologically similar European diving beetles (the Agabus brunneus group) to examine whether thermal physiology relates to latitudinal range extent, and whether Brown’s hypothesis and the environmental variability hypothesis apply to these taxa. Location Europe. Methods In order to determine the species tolerances to either low or high temperatures we measured the lethal thermal limits of adults, previously acclimated at one of two temperatures, by means of thermal ramping experiments (± 1°C min^?1). These measures of upper and lower thermal tolerances (UTT and LTT respectively) were then used to estimate each species’ thermal tolerance range, as total thermal tolerance polygons and marginal UTT and LTT thermal polygons. Results Overall, widespread species have higher UTTs and lower LTTs than restricted ones. Mean upper lethal limits of the Agabus brunneus group (43 to 46°C), are similar to those of insects living at similar latitudes, whilst mean lower lethal limits (?6 to ?9°C) are relatively high, suggesting that this group is not particularly cold‐hardy compared with other mid‐temperate‐latitude insects. Widespread species possess the largest thermal tolerance ranges and have a relatively symmetrical tolerance to both high and low temperatures, when compared with range‐restricted relatives. Over the temperature range employed, adults did not acclimate to either high or low temperatures, contrasting with many insect groups, and suggesting that physiological plasticity has a limited role in shaping distribution. Main conclusions Absolute thermal niche appears to be a good predictor of latitudinal range, supporting both Brown’s hypothesis and the environmental variability hypothesis. Restricted‐range species may be more susceptible to the direct effect of climate change than widespread species, notwithstanding the possibility that even ‘thermally‐hardy’, widespread species may be influenced by the indirect effects of climate change such as reduction in habitat availability in Mediterranean areas.     

The occurrence and abundance of plants with extrafloral nectaries, the basis for antiherbivore defensive mutualisms, along a latitudinal gradient in east Asia

Abstract. The occurrence and abundance of indigenous plants with extrafloral nectaries was evaluated within local communities and regional floras along a north to south gradient from tundra in northeastern Russia (64–70°N) through temperate types in eastern Russia and Korea to subtropical vegetation in the Bonin Islands (26–27°N) south of Japan. Moving from tundra to subtropical vegetation, there is a pattern of increasing abundance of extrafloral bearing plants as a function of total plant cover (from 10.25 to 40.18%), number of species per sampled area (from 0.11 to 1.13/100 m), and proportion of species within regional floras (from 0.32 to 7.46%). There were some plants with extrafloral nectaries in all communities but their abundance varied greatly, c. 1–25% in the four northern latitudes and c. 7–70% in the subtropical region. Ants, the primary mutualists associated with plants bearing extrafloral nectaries, have a similar pattern of increasing abundance (species richness, nest density, and colony size) along the same north–south latitudinal gradient.     

Seedling Mortality and Herbivory Damage in Subtropical and Temperate Populations: Testing the Hypothesis of Higher Herbivore Pressure Toward the Tropics

Herbivory rates are generally thought to be higher in tropical than in temperate forests. Nevertheless, tests of this biogeographic prediction by comparing a single plant species across a tropical-temperate range are scarce. Here, we compare herbivore damage between subtropical and temperate populations of the evergreen tree Aextoxicon punctatum (Olivillo), distributed between 30° and 43° S along the Pacific margin of Chile. To assess the impact of herbivory on Olivillo seedlings, we set up 29 experimental plots, 1.5 × 3 m: 16 in forests of Fray Jorge National Park (subtropical latitude), and 13 in Guabún, Chiloé Island (temperate latitude). Half of each plot was fenced around with chicken wire, to exclude small mammals, and the other half was left unfenced. In each half of the plots we planted 16 seedlings of Olivillo in December 2003, with a total of 928 plants. Seedling survival, leaf production and herbivory by invertebrates were monitored over the next 16 mo. Small mammal herbivores killed ca 30 percent of seedlings in both sites. Nevertheless, invertebrate herbivory was greater in the temperate forest, thus contradicting the expected trend of increasing herbivore impact toward the tropics. Seedling growth was greater in subtropical forest suggesting better conditions for tree growth or that higher invertebrate herbivory depressed seedling growth in the temperate forest. Invertebrate herbivory increased toward temperate latitudes while small mammal herbivory was similar in both sites. We suggest that comparison of single species can be useful to test generalizations about latitudinal patterns and allow disentangling factors controlling herbivory patterns across communities.     

Density dependence in forests is stronger in tropical and subtropical climates among closely related species

Density dependence has long been considered an important mechanism for species coexistence in forests. Density‐dependent processes can be important mechanisms driving differences in species diversity across latitudes. Here we examined the decline in strength of density dependence with increasing latitude, and particularly how density dependence affected both conspecifics and heterospecifics. Conspecific individuals within a species were predominantly aggregated at the three different latitudes of the three study sites in China. The percentage of aggregated species declined with increasing spatial scale and growth stages, which confirmed the overall importance of density dependence. Compared with a latitudinal gradient, the intensity of aggregation in the most northerly temperate (Changbaishan) plot was significantly higher than that in the tropical (Bawangling) or subtropical (Heishiding) plots. This showed that the strength of density dependence among conspecific individuals at low latitudes was stronger than that for high latitudes. We found that the more closely related species were more spatially adjacent in the temperate plot, while the opposite was true in the tropical and subtropical plots at most scales. After calculating the recruitment probability of all species of mature trees, we found that 19 of the 32 species in the tropical plot and 7 of the 12 species in the subtropical plot were less likely to recruit near closely related species. In the northern temperate plot, only one species demonstrated this phenomenon. These results therefore suggest that latitudinal variation in the intensity of negative density‐dependent recruitment resulting from specialist natural enemies (the Janzen–Connell hypothesis) may contribute to the latitudinal gradient of diversity in trees. The strength of density dependence at low latitudes was stronger than at high latitudes, regardless of whether this dependence was measured between only conspecific individuals or between individuals of closely related species.     

Effects of photoperiodically induced reproductive diapause and cold hardening on the cold tolerance of Drosophila montana

Coping with seasonal and daily variation in environmental conditions requires that organisms are able to adjust their reproduction and stress tolerance according to environmental conditions. Females of Drosophila montana populations have adapted to survive over the dark and cold winters at high latitudes and altitudes by spending this season in photoperiodically controlled reproductive diapause and reproducing only in spring/summer. The present study showed that flies of a northern population of this species are quite tolerant of low temperatures and show high seasonal and short-term plasticity in this trait. Culturing the flies in short day length (nearly all females in reproductive diapause), as well as allowing the flies to get cold hardened before the cold treatment, increased the cold tolerance of both sexes both in chill coma recovery time test and in mortality assay. Chill coma recovery time test performed for the females of two additional D. montana populations cultured in a day length where about half of the females enter diapause, also showed that diapause can increase female cold tolerance even without a change in day length. Direct linkage between diapause and cold tolerance was found in only two strains representing a high-altitude population of the species, but the phenomenon will certainly be worth of studying in northern and southern populations of the species with larger data sets.     

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

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

Evolutionary trade-offs between thermal tolerance and locomotor and developmental performance in drosophilid flies

Cold-tolerant ectothermal animals are generally absent from warmer regions, suggesting that the acquisition of cold tolerance is associated with the loss of adaptation to warmer environments. In the present study, we compared thermal tolerance, walking speed (WS) and pupal development (PD) for 28 drosophilid species from cool-temperate, warm-temperate, and subtropical regions by the phylogenetically-based method and the conventional regression to understand trade-offs between these traits. A significant negative relationship was observed between cold tolerance and the rate of PD in both sexes and between cold tolerance and WS in the male in the phylogenetically-based analysis. Similar results were obtained in the conventional regression analysis, although the level of significance somewhat differed. Thus, cold tolerant species are assumed to have lost abilities to develop and walk fast. Subsequently, they may become more vulnerable to predators, parasitoids or infectious microorganisms and may have become extinct in warmer regions through enemy-mediated interactions and/or resource competition with cold susceptible species with faster development and quicker locomotion. In the present study, no significant relationship was observed between heat tolerance and WS or the rate of PD in the phylogenetically-based analysis, although heat tolerance was significantly related with the rate of PD in the conventional regression analysis. Thus, trade-offs associated with heat tolerance were not apparent.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 375–380.     

Thermal specialization across large geographical scales predicts the resilience of mangrove crab populations to global warming

The broad prediction that ectotherms will be more vulnerable to climate change in the tropics than in temperate regions includes assumptions about centre/edge population effects that can only be tested by within‐species comparisons across wide latitudinal gradients. Here, we investigated the thermal vulnerability of two mangrove crab species, comparing populations at the centre (Kenya) and edge (South Africa) of their distributions. At the same time, we investigated the role of respiratory mode (water‐ versus air‐breathing) in determining the thermal tolerance in amphibious organisms. To do this, we compared the vulnerability to acute temperature fluctuations of two sympatric species with two different lifestyle adaptations: the free living Perisesarma guttatum and the burrowing Uca urvillei, both pivotal to the ecosystem functioning of mangroves. The results revealed the air‐breathing U. urvillei to be a thermal generalist with much higher thermal tolerances than P. guttatum. Importantly, however, we found that, while U. urvillei showed little difference between edge and centre populations, P. guttatum showed adaptation to local conditions. Equatorial populations had elevated tolerances to acute heat stress and mechanisms of partial thermoregulation, which make them less vulnerable to global warming than temperate conspecifics. The results reveal both the importance of respiratory mode to thermal tolerance and the unexpected potential for low latitude populations/species to endure a warming climate. The results also contribute to a conceptual model on the latitudinal thermal tolerance of these key species. This highlights the need for an integrated population‐level approach to predict the consequences of climate change.     

Climate change,boundary increase and elongation of a pre‐existing cline: A case study in Drosophila ananassae

During the past two to three decades, Drosophila ananassae, a warm adapted tropical species, has invaded low to mid altitude localities in the western Himalayas. Due to its cold sensitivity, this species had never been recorded from higher latitudes as well as altitudes in India to the 1960s. A latitudinal cline in this desiccation‐sensitive species corresponds with southern humid tropical localities rather than northern drier subtropical localities. An extension of its cline into lowland to midland montane localities has resulted due to global climatic change as well as local thermal effects through anthropogenic impact. However, D. ananassae populations at species borders are characterized by lower genetic variability for body melanization as well as for desiccation resistance. There is a lack of thermal plastic effects for body melanization, and the observed extended cline might represent evolutionary (genetic) response due to selection pressure imposed by drier habitats. A comparison of fecundity, hatchability and viability at three growth temperatures (17, 20 and 25°C) showed significant reduction in trait values at 17°C in D. ananassae. Thus, its recent range expansion into northern montane localities might involve genetic effects on stress‐related traits and plastic effects on life history traits. We suggest that D. ananassae could serve as an indicator species for analyzing range expansion under changing climatic conditions.     

Diet of Anguilla mossambica (Teleostei,Anguillidae) elvers in the Sundays River,Eastern Cape,South Africa

The southern African crab Hymenosoma orbiculare was recently split into five distinct species, of which three are estuarine/coastal and have peripatric distributions that are linked to temperature-defined marine bioregions. This suggests that the species’ ranges may be limited by physiological adaptations to their thermal environment. We explored this hypothesis by rearing the larvae of the warm-temperate lineage of H. orbiculare and the warm-temperate/subtropical H. longicrure at a range of water temperatures, and found clear temperature-dependent differences in the duration of larval development. Our study contributes to the growing body of evidence that stresses the importance of adaptation to regional environmental conditions, rather than physical dispersal barriers on their own, in limiting the mixing of marine species between temperature-defined biogeographic regions.     

Energy storage during reproductive diapause in the Drosophila melanogaster species group

Summary Temperate species of the Drosophila melanogaster group enter reproductive diapause for overwintering in response to short daylength. During the prediapause period they accumulate triacylglycerols, but not glycogen, as energy resources. The capacity for storing triacylglycerols differs between species, and appears to be closely correlated with diapause and cold-hardiness; cool-temperate species, such as those of the auraria species complex, which enter a deep diapause and are highly cold-hardy, accumulate larger quantities of triacylglycerols than warm-temperate species, such as D. rufa and D. lutescens, which enter a weak diapause and are less cold-hardy. Among the cool-temperate spcies, D. subauraria occurs at a higher latitude and has the greatest capacity for accumulating triacylglycerols. A subtropical species, D. takahashii, which has no diapause in nature and is not cold-hardy, is unable to store the same quantities of triacylglycerols as temperate species.