Thermal acclimation in a complex life cycle: The effects of larval and adult thermal conditions on metabolic rate and heat resistance in Culex pipiens (Diptera: Culicidae)

It has now been well established that insects can respond to variation in their environment via acclimation, yet the extent of the response varies among populations and environmental characteristics. One under-investigated theme which may contribute to this variation concerns acclimation effects across the life cycle. The present study explores how acclimation in the larval stage of Culex pipiens affects thermal relations in the adult stage. Mosquitoes were reared in a full factorial design at 18 or 26 °C as larvae and adults, then critical thermal maxima (CT_max) and metabolic rate–temperature relationships (MR–T) were determined for all 4 treatments. CT_max was positively affected by both larval and adult acclimation treatments. MR–T slope was significantly affected only by adult treatment: warm acclimated adults had on average shallower slopes and higher y-intercepts than cool acclimated ones. These results demonstrate that larval acclimation effects can alter adult phenotypes in a species whose life cycle includes two drastically different environments, an aquatic and a terrestrial stage. Studying insects with complex life cycles, especially those with aquatic or subterranean larval stages, can provide valuable information on the effects of thermal variability and predictability on phenotypic plasticity.     

A moderate change in temperature induces changes in fatty acid composition of storage and membrane lipids in a soil arthropod

A moderate change in ambient temperature can lead to vital physiological and biochemical adjustments in ectotherms, one of which is a change in fatty acid composition. When temperature decreases, the composition of membrane lipids (phospholipid fatty acids) is expected to become more unsaturated to be able to maintain homeoviscosity. Although different in function, storage lipids (triacylglycerol fatty acids) are expected to respond to temperature changes in a similar way. Age-specific differences, however, could influence this temperature response between different life stages. Here, we investigate if fatty acid composition of membrane and storage lipids responds similarly to temperature changes for two different life stages of Orchesella cincta. Juveniles and adults were cold acclimated (15 °C → 5 °C) for 28 days and then re-acclimated (5 °C → 15 °C) for another 28 days. We found adult membranes had a more unsaturated fatty acid composition than juveniles. Membrane lipids became more unsaturated during cold acclimation, and a reversed response occurred during warm acclimation. Membrane lipids, however, showed no warm acclimation, possibly due to the moderate temperature change. The ability to adjust storage lipid composition to moderate changes in ambient temperature may be an underestimated fitness component of temperature adaptation because fluidity of storage lipids permits accessibility of enzymes to energy reserves.     

Effects of alternate reef states on coral reef fish habitat associations

The present study describes ontogenetic shifts in habitat use for 15 species of coral reef fish at Rangiroa Atoll, French Polynesia. The distribution of fish in different habitats at three ontogenetic stages (new settler, juvenile, and adult) was investigated in coral-dominated and algal-dominated sites at two reefs (fringing reef and inner reef of motu). Three main ontogenetic patterns in habitat use were identified: (1) species that did not change habitats between new settler and juvenile life stages (60% of species) or between juvenile and adult stages (55% of species—no ontogenetic shift); (2) species that changed habitats at different ontogenetic stages (for the transition “new settler to juvenile stage”: 15% of species; for the transition “juvenile to adult stage”: 20% of species); and (3) species that increased the number of habitats they used over ontogeny (for the transition “new settler to juvenile stage”: 25% of species; for the transition “juvenile to adult stage”: 25% of species). Moreover, the majority of studied species (53%) showed a spatial variability in their ontogenetic pattern of habitat use according to alternate reef states (coral reef vs algal reef), suggesting that reef state can influence the dynamics of habitat associations in coral reef fish.     

The cuticle proteins of Drosophila melanogaster: stage specificity

Five stage-specific cuticles are produced during the development of Drosophila. Urea-soluble proteins were extracted from each developmental stage and compared by gel electrophoresis. Proteins from first and second instar cuticle are identical except for minor differences in two proteins. Each subsequent stage, third instar, pupa, and adult, has a unique set of cuticle proteins. Qualitative changes within stages are seen in proteins from third instar and adult cuticle. Third instar cuticle proteins can be divided into “early” [proteins 2a, 3, 4, 5, 7, and 8] and “late” [proteins 2 and 1] groups. Adult cuticle proteins change in relative amounts during pharate adult development and change mobility at eclosion. The lower abdominal pupal cuticle lacks a protein found in the pupal cuticle covering the head and thorax. Cuticle proteins from each stage are immunologically related. Nonetheless, electrophoretic variants of three larval proteins do not affect any major changes in the electrophoretic mobility of proteins from other stages. We propose that each stage (except first and second instar) has proteins encoded by discrete genes.     

Acclimation responses to short‐term temperature treatments during early life stages causes long lasting changes in spontaneous activity of adult Drosophila melanogaster

Ecotherms adjust their physiology to environmental temperatures. Long‐term exposures to heat or cold typically induce acclimation responses that generate directional, but reversible shifts in thermal tolerance and performance. However, less is known about how short exposure in different life stages will affect the adult phenotype. In the present study, we compared the effects of long‐term temperature exposure to 15, 19 and 31 °C with that of brief (16 h) exposure periods at the same temperatures in Drosophila melanogaster eggs, larvae, pupae, or adults, respectively. The acclimation responses are evaluated using activity measurements at 11, 15, 19, 27, 31 and 33 °C and by measuring upper and lower thermal limits (CT_max and CT_min) in 5‐day‐old adult males. As expected, long‐term cold exposure reduces relative CT_min, whereas long‐term heat exposure increases relative CT_max. By contrast, we find little effect on thermal limits when using short‐term exposures at different life stages. Long‐term exposures to 31 and 15 °C both suppressed activity relative to the 19 °C control, suggesting that development at high and low temperatures may lead to reduced activity later in life. Short‐term cold exposure early in development reduces activity in the adult stage, whereas the effects of short‐term heat exposure on behaviour are dependent on life stage and test temperature. Together, our results highlight how the thermal sensitivity of the trait measured determines the ability to detect acclimation responses.     

Contrasting patterns of density‐dependent selection at different life stages can create more than one fast–slow axis of life‐history variation

There has been much recent research interest in the existence of a major axis of life‐history variation along a fast–slow continuum within almost all major taxonomic groups. Eco‐evolutionary models of density‐dependent selection provide a general explanation for such observations of interspecific variation in the "pace of life." One issue, however, is that some large‐bodied long‐lived “slow” species (e.g., trees and large fish) often show an explosive “fast” type of reproduction with many small offspring, and species with “fast” adult life stages can have comparatively “slow” offspring life stages (e.g., mayflies). We attempt to explain such life‐history evolution using the same eco‐evolutionary modeling approach but with two life stages, separating adult reproductive strategies from offspring survival strategies. When the population dynamics in the two life stages are closely linked and affect each other, density‐dependent selection occurs in parallel on both reproduction and survival, producing the usual one‐dimensional fast–slow continuum (e.g., houseflies to blue whales). However, strong density dependence at either the adult reproduction or offspring survival life stage creates quasi‐independent population dynamics, allowing fast‐type reproduction alongside slow‐type survival (e.g., trees and large fish), or the perhaps rarer slow‐type reproduction alongside fast‐type survival (e.g., mayflies—short‐lived adults producing few long‐lived offspring). Therefore, most types of species life histories in nature can potentially be explained via the eco‐evolutionary consequences of density‐dependent selection given the possible separation of demographic effects at different life stages.     

Trypanosoma lewisi: Enhanced resistance in naive lactating rats and their suckling pups

Lactating rats and their suckling offspring were shown to have a naturally occurring resistance to Trypansoma lewisi which was greater than that seen in normal adult rats and was not dependent on previous exposure to the parasite. In the case of the pups maximum protection was dependent on suckling being the sole source of nutriment and also on the peak of the parasitemia falling within the nursing period. Supplementary solid food reduced survival rates in concurrent Hemobartonella muris—T. lewisi infections: pups infected at 10 days of age and totally dependent on nursing showed 82.5% survival but only 6.6% survival when solid food was allowed to supplement milk. However, supplementary food did not reduce survival in pups infected with only T. lewisi. When pups were totally dependent upon nursing until the normal time of weaning (21 days of age), infection with H. muris—T. lewisi at the following days of age allowed the indicated mean survival rates: 10 days (82.5%), 20 (32.5%), 30 (14.3%), and 40 (100%). Infection with T. lewisi alone at the following days of age allowed the indicated mean survival rates: 10 days (100%), 15 (76%), 17 (52%), 20 (100%), and 30 (100%). Lactating rat serum agglutinated “adult forms” of T. lewisi, which correlated well with the observed sudden resolution of parasitemia in lactating animals at the time of the antigenic transition from “juvenile” to “adult” type. The “adult” stage parasitemia in suckling pups was selectively reduced when compared to that of nonlactating adult rats. The lactating rat serum factor could be passively transferred with lactating rat serum to animals already weaned.     

Photoperiodic regulation of the diapause of the progeny in <Emphasis Type="Italic">Trichogramma embryophagum</Emphasis> Htg. (Hymenoptera,Trichogrammatidae): Dynamics of sensitivity to photoperiod at the immature stages of maternal females

Photoperiodic regulation of T. embryophagum progeny prepupal diapause was investigated under laboratory conditions. Maternal females developed at 20°C, the position of the photosensitive period was detected by transferring immature stages between the diapause-inducing “short” day (L:D = 12:12) and the diapause-averting “long” day (L:D = 20:4). Progeny generation developed at L:D = 12:12 under the near-threshold temperature of 13°, 14°, or 15°C. Experiments showed that, at least under the used photo-thermal regimens, only the pupal stage was sensitive to the photoperiod influence. The highest photosensitivity was recorded during the last two days before the adult emergence. In this critical period, even a single short day cycle induced diapause in most of individuals. However, the reaction to the long day was weaker: at least two long light days immediately preceding the maternal adult emergence were necessary to avert the diapause. If a short day was preceded by a long day by more than 2–3 days before the adult emergence, the stepwise photoperiodic response caused an increase in the tendency to diapause. The results of this study could be used in pest biocontrol practice for elaboration of optimal methods for Trichogramma mass rearing and storage and also for prediction of its seasonal cycles under natural conditions.     

The environmental control of cold acclimation in apple

The role of photoperiod and temperature in the cold acclimation of living Haralson apple (Pyrus malus L.) bark was studied in the autumn under field conditions in Minnesota. Whole trees, or different parts of the same tree, were exposed to either natural conditions, artifically lengthened days, or artificially warmed nights, or they were subjected to manual leaf removal. The results indicate that acclimation occurs in two stages which are induced by short days and frost (or low temperature), respectively. Leaves were stimulated by short days to produce translocatable substance(s) which promoted cold acclimation of the living bark. Leaves of plants grown under long days were the source of a translocatable substance(s) which inhibited acclimation. The second stage of hardiness, induced by frost (or low temperature), did not involve translocatable factors.     

Identification and pyramiding of QTLs for cold tolerance at the bud bursting and the seedling stages by use of single segment substitution lines in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Cold stress is one of the main constraints in rice production, and damage from cold can occur at different developmental stages in rice. Understanding the genetic basis of cold tolerance is the key for breeding cold-tolerant variety. In this study, we used single segment substitution lines (SSSLs) derived from a cross between cold-tolerant japonica variety “Nan-yang-zhan” and a popular indica variety “Hua-jing-xian 74” to detect and pyramid QTLs for cold tolerance at the bud bursting and the seedling stages. Evaluation of cold tolerance of these SSSLs and their recurrent parent helped identify two cold-tolerant QTLs (qCTBB-5 and qCTBB-6) at the bud bursting stage and two cold-tolerant QTLs (qCTS-6 and qCTS-12) at the seedling stage. The SSSLs carrying these QTLs showed stronger cold tolerance than their recurrent parent HJX74 did in three independent experiments. The qCTBB-6 and qCTS-6 were mapped to the same chromosomal region. QTL pyramiding was performed by intercrossing of SSSLs carrying the respective QTLs for cold tolerance at the bud bursting stage and the seedling stage and marker-assisted selection (MAS). The selected pyramiding line SC1-1 with different cold-tolerant QTLs showed cumulative effects on cold tolerance. Our results suggest that different genes (QTLs) control cold tolerance at bud bursting and seedling stages, and pyramiding of stable expression QTLs for cold tolerance at different developmental stages through MAS is a good strategy to prevent cold damage in rice.     

Cold storage of <Emphasis Type="Italic">Trichogramma nerudai</Emphasis> using an acclimation period

The availability of suitable storage methods for parasitoids is a valuable tool in biological control programs. Studies were conducted to investigate the effects of cold storage with acclimation period on the quality of Trichogramma nerudai Pintureau and Gerding. Prepupae were stored 50, 75 and 100 days at 5 °C with a previous acclimation period of 10 or 20 days at 12 °C. It was possible to arrest the development of T. nerudai. All the treatments with acclimation period of ten days had emergence values under 10% that were not useful to establish a cold storage protocol. Twenty days of acclimation had a positive impact on cold storage tolerance at 50 and 75 days. The adult emergence, the emergence time, the sex ratio, the parasitism and the progeny quality have not been affected by the storage of T. nerudai using an acclimation period of 20 days and until 50 days under cold temperature.     

Life stage-related differences in hardening and acclimation of thermal tolerance traits in the kelp fly, Paractora dreuxi (Diptera, Helcomyzidae)

It is widely appreciated that physiological tolerances differ between life stages. However, few studies have examined stage-related differences in acclimation and hardening. In addition, the behavioural responses involved in determining the form and extent of the short-term phenotypic response are rarely considered. Here, we investigate life stage differences in the acclimation and hardening responses of the survival of a standard heat shock (SHS) and standard low temperature (or cold) shock (SCS), and the crystallization temperature (or supercooling point, SCP) of adults and larvae of the sub-Antarctic kelp fly, Paractora dreuxi. These stages live in the same habitat, but differ substantially in their mobility and thus environmental temperatures experienced. Results showed that neither acclimation nor hardening affected the lower lethal limits in larvae or adults. Adults showed an increase in survival of upper lethal limits after low temperature acclimation, whilst larvae showed a consistent lack of response. The acclimation × hardening interaction significantly affected the SCP in adults, but no response to either acclimation or hardening was found in the larvae. This study further demonstrates the complexities of thermal tolerance responses in P. dreuxi.     

High cold tolerance through four seasons and all free-living stages in an ectoparasite

Off-host stages of temperate parasites must cope with low temperatures. Cold tolerance is often highest in winter, as a result of diapause and cold acclimation, and low during the active summer stages. In some blood-feeding ectoparasites, offspring provisioning determines cold tolerance through all the non-feeding, off-host stages. Large size increases survival in the cold, but so far seasonal variation in within-female offspring size has not been associated with offspring cold tolerance. The deer ked (Lipoptena cervi) reproduces on cervids from autumn to spring. Newborn pupae drop off the host, facing frosts without any acclimation. We examined cold tolerance through 4 seasons and from birth to adulthood by means of short- and long-term frost exposure. We expected females to produce more tolerant offspring in winter than in spring. Large spring pupae survived prolonged frosts better than did small winter pupae. Thus more tolerant offspring were not produced when the temperature outside the host is at its lowest. Unexpectedly, the freezing points were -20 °C or below all year round. We showed that high cold tolerance is possible without acclimation regardless of life stage, which presumably correlates with other survival characteristics, such as the starvation resistance of free-living ectoparasites.     

Cold hardiness in summer and winter diapause and post-diapause pupae of the cabbage armyworm, Mamestra brassicae L. under temperature acclimation

Cold hardiness and biochemical changes were investigated in winter and summer pupae of the cabbage armyworm Mamestra brassicae at the diapause and post-diapause stages under temperature acclimation. Diapause pupae were successively acclimated to 25, 20 and then 10 degrees C (warm-acclimated group). Pupae at the diapause and post-diapause stages were successively acclimated to 5, 0, -5 and then -10 degrees C (cold-acclimated groups). Supercooling point values in winter and summer pupae remained constant regardless of the diapause stages and acclimated temperatures. Warm-acclimated pupae at the diapause stage did not survive the subzero temperature exposure, whereas, cold-acclimated pupae achieved cold hardiness to various degrees. Winter pupae were more cold hardy than summer pupae, and pupae at the post-diapause stage were more cold hardy than those at the diapause stage. Trehalose contents in winter pupae rose under cold acclimation. Summer pupae accumulated far lower trehalose contents than winter pupae, with the maximal level occurring in winter pupae at the post-diapause stage. Glycogen content remained at a high level in diapause pupae after warm acclimation, whereas it decreased after cold acclimation. Alanine, the main free amino acid in haemolymph after cold acclimation, increased at lower temperatures in both diapause and post-diapause pupae, but the increase was greater in the diapause pupae. These results suggest that cold hardiness is more fully developed in winter pupae than in summer pupae, and cold acclimation provides higher cold hardiness in winter pupae at the post-diapause stage than at the diapause stage.     

Shivering and non-shivering thermogenesis in a marsupial,the eastern barred bandicoot (Perameles gunnii)

We investigated the metabolic rate of the Tasmanian marsupial, the eastern barred bandicoot, Perameles gunnii, before and after acclimation to cold temperature (5 °C) for a 2-week period. Although body temperature did not change significantly, we observed a significant increase in the metabolic rate (MR) when measured at 5 °C before and after cold acclimation. Nor-epinephrine had a significant effect on the metabolic rate when measured in the thermoneutral zone and when measured at 5 °C after cold acclimation; however, there was no significant increase when measured at 5 °C before cold acclimation. Nor-epinephrine also resulted in a small but significant decrease in body temperature. Electromyography (EMG) measurements were obtained before and after cold acclimation during shivering. Shivering decreased after two weeks of cold exposure indicating that the bandicoot had acclimated to that temperature. Nor-epinephrine (NE) significantly reduced shivering before but not after cold acclimation. The metabolic rate and shivering decreased in the adult eastern barred bandicoot after acclimation at 5 °C and nor-epinephrine had similar effects to cold acclimation. Our findings of minor changes in thermal conductance suggest that insulation differences were unlikely explanations for our results. These experiments indicate that this marsupial is able to increase its heat production by non-shivering thermogenesis.     

Cold-hardiness of the arctic beetle, Pytho americanus Kirby Coleoptera,Pythidae (Salpingidae)

The arctic beetle, Pytho americanus Kirby, is frost tolerant in both larval and adult stages. This is the first demonstration that an insect can tolerate freezing in more than one life stage, a situation which would be congruous with its northern distribution and allow it to spread its life cycle over a number of growing seasons. The main biochemical correlates during the cold hardening process of low temperature acclimation are increasing glycerol and decreasing glycogen concentrations. Glycerol is the only polyol to be synthesized during acclimation, and it accumulates to a maximum of 8.2 and 12.2% of the fresh body weight in larvae and adults respectively. This coincides with the peak of frost tolerance. In addition to its normally assumed roles in cryoprotection it is suggested that glycerol may further serve to minimize dehydration in the overwintering insect by increasing the level of ‘bound’ water. Evidence is presented that indicates that glycerol is synthesized mainly from carbohydrate reserves, especially glycogen, but it does not rule out the possibility that a proportion of free glycerol comes from glyceride sources.P. americanus larvae and adults have low supercooling potential and maintain their supercooling points in the region of ?4° to ?8°C. It is hypothesized that these elevated supercooling points are a result of the presence in the haemolymph of nucleating agents which ensure ice formation at high sub-zero temperatures. It is believed that this beetle overwinters in a frozen state within its microhabitat, which is under bark of fallen spruce which is, in turn, covered by an insulating blanket of snow. The advantages of this overwintering strategy are discussed.     

植物冷驯化相关基因研究进展

摘要 冷驯化是与提高植物抗冷性有关的生物化学及生理学过程, 主要包括寒驯化(cool acclimation)和冻驯化(freezing acdimation)。在冷驯化过程中, 植物体内许多基因在转录水平上的表达受到影响, 已经克隆了大量的相关基因,它们组成复杂的分子调控网络。目前研究表明不依赖ABA的低温信号转导途径是植物冷驯化机制的重要组成部分, 其中CBF/DREB1是该调控过程的关键转录因子, 与植物通过冷驯化而提高冰冻耐受能力密切相关。进一步利用转基因技术, 可有效地改善作物的耐冷性状。     

Effects of crowding on life history parameters of four green leafhoppers,Nephotettix spp. (Homoptera: Cicadellidae)

The effects of intraspecific crowding on the different life history parameters of the three tropical and subtropical green leafhoppers, Nephotettix virescensStål ,N. nigropictusDistant and N. malayanusIshihara etKawase , and one temperate species, N. cincticepsUhler , were studied under 25±1°C using the susceptible Japonica rice variety “Nipponbare”. Crowding during the immature stages of the four species prolonged the nymphal developmental period and resulted to smaller adults with reduced fecundity and shorter life span. Similarly, crowding during the adult stage also shortened the life span of both sexes of adults and decreased the fecundity of females of the four species. The effect of crowding during the adult stage was more adverse than the effect of crowding during the immature stages. N. cincticeps seemed more sensitive to crowding even at lower densities whereas the three tropical species were sensitive mostly at higher densities.