Cryoprotective role of polyols independent of the increase in supercooling capacity in diapausing adults of Pyrrhocoris apterus (Heteroptera: Insecta)

Effect of cooling rates on the cold hardiness and cryoprotectant profiles of locust eggs

To examine the relationship between cooling rate and cold hardiness in eggs of the migratory locust, Locusta migratoria, the survival rates and cryoprotectant levels of three embryonic developmental stages were measured at different cooling rates (from 0.05 to 0.8 degrees C min(-1)) in acclimated and non-acclimated eggs. Egg survival rate increased with decreasing cooling rate. The concentration of cryoprotectants (myo-inositol, trehalose, mannitol, glycerol, and sorbitol) increased in non-acclimated eggs, but varied significantly in response to different cooling rates in acclimated eggs. The acclimation process (5 degrees C for 3 days) did not increase eggs resistance to quick cooling ("plunge" cooling and 0.8 degrees C min(-1)). Earlier stage embryos were much more sensitive than later stage embryos to the same cooling rates. Time spent at subzero temperatures also had a strong influence on egg survival.

Metabolic responses to cold in subterranean crustaceans

Changes in polyol, sugar and free amino acid (FAA) body contents were investigated in the aquatic, subterranean (i.e. hypogean) crustaceans Niphargus rhenorhodanensis and Niphargus virei and in a morphologically close aquatic, surface-dwelling (i.e. epigean) crustacean Gammarus fossarum acclimated to 12°C, 3°C and –2°C. With decreasing temperature, G. fossarum significantly increased its alanine and glutamine levels, while trehalose body content was found to increase above control levels only at –2°C. N. virei showed moderate increases of alanine and glycine, and no change in trehalose level was observed in this species. By contrast, N. rhenorhodanensis was the only species showing a significant rise in its total FAA pool, mainly explained by alanine, glycine, arginine and glutamine accumulations. This species also gradually increased its trehalose body content with decreasing temperature. Several cold-hardy ectotherms show metabolic responses to cold that are identical to those observed in N. rhenorhodanensis. A previous comparative study showed that the hypogean N. rhenorhodanensis exhibited a survival time (Lt50) at–2°C that was 26.3 times and 2.6 times higher than the hypogean N. virei and the epigean G. fossarum, respectively. Thus,crustacean levels of FAA and trehalose were correlated with their respective cold tolerances. Such differences in metabolic responses to cold in both hypogean organisms were unexpected since they both live in thermally buffered biotopes. Considering the current distribution areas of the two subterranean crustaceans studied, we assume that the cold hardiness found in the hypogean N. rhenorhodanensis could be correlated with its biogeography history during the quaternary glaciations.

On the nature of pre-freeze mortality in insects: water balance, ion homeostasis and energy charge in the adults of Pyrrhocoris apterus

Three acclimation groups [i.e. non-diapause (LD), diapause (SD) and diapause, cold-acclimated (SDA)] of the adult bugs Pyrrhocoris apterus differed markedly in their levels of chill tolerance. Survival time at a sub-zero, but non-freezing, temperature of -5 degrees C (Lt50) extended from 7.6 days, through 35.6 days, to >60 days in the LD, SD and SDA insects, respectively. The time necessary for recovery after chill-coma increased linearly with the increasing time of exposure to -5 degrees C, and the steepness of the slope of linear regression decreased in the order LD>SD>SDA. The capacity to prevent/counteract leakage of Na(+) down the electrochemical gradient (from haemolymph to tissues) during the exposure to -5 degrees C increased in the order LD<SD<SDA. As a result, the rates of counteractive outward movement of K(+), and of the E(K) dissipation, decreased in the same order. The least chill-tolerant insects (LD) showed the highest rate of body-water loss. Most of the water was lost from the haemolymph compartment. The ability to regulate a certain fraction of ion pools into the hindgut fluid was the highest in the SDA group, medium in the SD group and missing in the LD group. The adenylate energy charge in the fat body cells was constant in all three groups. The total pools of ATP, ADP and AMP, however, decreased in the SD and SDA groups but remained constant in the LD group. The inability of insects to maintain ion gradients at sub-zero temperature is discussed as an important cause of pre-freeze mortality.

Comparing the freezing susceptibility of third-instar larvae of Gnorimus variabilis (Cetoniidae: Trichiinae) from three distant geographical regions

We compared the freezing susceptibility of three populations of Gnorimus variabilis (L., 1758) (Coleoptera: Cetoniidae) sampled from three distant locations in France. To separate the effects of habitat quality from those of genetics, we reared imagos from all field populations in a common garden experiment until the emergence of the third-instar larvae. The local climate appeared to determine the cold hardiness of the third-instar larvae, which live exclusively in cold seasons. The geographical location had an effect on the frost resistance (supercooling point) of the third-instar larvae of G. variabilis. We found no significant differences between the supercooling points of the populations originating from separate latitudes but from the same longitude, Sare (–10.9 ± 4.1 °C) and Husson (–11.5 ± 3.8 °C). Nonetheless, significant differences occurred between the larvae originating from the same northern latitude but from separate longitudes, Husson and La Robertsau (–17.8 ± 2.9 °C). It is the first time that a highly significant difference in the supercooling point of one stage within a single species has been observed along geographical gradients. Moreover, 19 of the 24 larvae originating from La Robertsau were alive after they were kept for 22 h at –10 °C compared with only 7 of the 24 larvae originating from Sare. When the three populations were reared in the laboratory under the same experimental conditions, the differential cryoresistance was preserved. It is likely that the greater freezing resistance found in the third-instar larvae of G. variabilis from La Robertsau could have a genetic component.

Adjustments of the enzymatic complement for polyol biosynthesis and accumulation in diapausing cold-acclimated adults of Pyrrhocoris apterus

The capacity to accumulate winter polyols (mainly ribitol and sorbitol) during cold-acclimation in Pyrrhocoris apterus is restricted only to the adults that have previously entered diapause. The enzymatic complement involved in polyol biosynthesis was found to differ in a complex manner between diapause and non-diapause adults. Nearly 100% of glycogen phosphorylase (GPase) was present in its active form in non-diapause adults irrespective of their acclimation status. In contrast, less than 40% of GPase was present in its active form in diapause adults prior to cold-acclimation and the inactive form was rapidly activated upon transition from 5 to 0 degrees C, concomitantly with the start of rapid polyol accumulation. The flow of carbon released by activation of glycogen degradation might be routed to the pentose cycle because the activity of glucose-6-P dehydrogenase (G(6)P-DH) was significantly higher and it increased with cold-acclimation in diapause adults while it was relatively low and it decreased with cold-acclimation in non-diapause adults. Reducing equivalents in the form of NADPH, which were generated in the pentose cycle, might require re-oxidation. Such re-oxidation might be achieved during reduction of sugars to polyols. The activity of NADP(H)-dependent aldose reductase (AR) was about 20-fold higher in diapause than in non-diapause adults. Similarly, the activity of NAD(H)-dependent polyol dehydrogenase (PDH) was higher in diapause adults. In addition, we found a very high activity of an unusual enzyme, NADP(H)-dependent ketose reductase (KR), exclusively in diapause adults. KR might be involved in reduction of fructose to sorbitol. Although its affinity for fructose as a substrate was low (K(M)=0.64M), its activity was about 10-fold higher than that of PDH with fructose. Moreover, the activity of KR significantly increased with cold-acclimation while that of PDH remained unchanged. Different electrophoretic mobilities in PAGE gel suggested that KR and PDH are two different enzymes with specific requirement for NADP(H) or NAD(H), respectively, as co-factors.

Entering diapause is a prerequisite for successful cold-acclimation in adult Graphosoma lineatum (Heteroptera: Pentatomidae)

In diapause adults of Graphosoma lineatum overwintering in a field-cage, high chill-tolerance (CT) developed gradually, within 5 months from August to December. In laboratory-acclimation experiments, the diapause state appeared to be an essential pre-condition for successful cold-acclimation and overwintering. First, diapause prevented elevation of the median supercooling point (SCP) by about 5.5 degrees C that accompanies the onset of reproductive activity in non-diapause specimens. Second, diapause allowed subsequent physiological changes resulting in cold-acclimation during a gradual (18-day) decrease of temperature from 25 to 0 degrees C. No, or very modest, cold-acclimation was observed in non-diapause specimens. Decrease of temperature led to a rapid loss of ca. 1/3 of the body water in both non-diapause and diapause specimens. Approximately 0.1 M of trehalose accumulated in tissues of diapause specimens only, and haemolymph osmolality rose from 347 mOsm (at 25 degrees C) to 444 mOsm after an 18-day cold-acclimation and to 764 mOsm during further storage at 0 degrees C for 100 days. Upon transfer of cold-acclimated diapause specimens back to 25 degrees C for one week (de-acclimation), the high CT was lost, the SCP elevated by about 2.5-3 degrees C, and the levels of trehalose, water content and haemolymph osmolality returned to pre-acclimation or non-diapause levels.

Photoperiodic regulation of the phospholipid molecular species composition in thoracic muscles and fat body of Pyrrhocoris apterus (Heteroptera) via an endocrine gland, corpus allatum

In the conventional view, the winter adaptation of membrane lipids is induced by temperature decrease. We propose that winter remodelling of membranes in Pyrrhocoris apterus is triggered by short-day photoperiod before the temperature decrease and changes caused by cold temperature represent the later phase of adaptation. The induction of diapause by short-day photoperiod results in an accumulation of phosphatidylethanolamine (PE) molecular species with C(16:0)/C(18:2) acyl chains esterified to sn-1/sn-2 positions of glycerol at the expense of C(18:0)/C(18:2). Proportions of C(16:0)/C(18:2)-PE are enhanced in short-day compared to long-day insects in both thoracic muscles (TM, 15.0 vs. 8.2%) and fat bodies (FB, 24.9 vs. 13.6 %). Proportions of C(16:0)/C(18:2)-PE are further enhanced during cold acclimation (to 26.5% in TM, 33.6 % in FB) at the expense of a more saturated species, C(18:0)/C(18:1)-PE. These changes are less prominent in phosphatidylcholines (PC). The effect of photoperiod seems to be mediated via the corpus allatum. Long-day non-diapause females deprived of their corpus allatum have the phospholipid molecular species profile similar to that found in short-day diapausing females. While the acyl chain remodelling is regulated by both photoperiod and temperature, the head group composition is regulated by temperature only. Similar to most other organisms, the level of PE is higher (50.3 vs. 43.5% in TM, 44.3 vs. 37.8% in FB) and that of PC is lower (35.9 vs. 40.2% in TM, 41.6 vs. 46.1 % in FB) at cold temperatures (</=1 degrees C) compared to warm temperatures (>/=16 degrees C). In contrast to a general rule, the PE is less unsaturated than PC. In both TM and FB, proportions of unsaturated/unsaturated molecular species are consistently high in PC (56.3-67.5% in TM, 59.2-66.6% in FB), while they are consistently low in PE (19.1-26.7% in TM, 12.1-15.1% in FB). An adaptive significance of changes in the phospholipid composition for the low temperature and/or dehydration stress is discussed in relation to known physical properties of phospholipids.