Yohei Izumi

Physiological response to low temperature in the freshwater apple snail, Pomacea canaliculata (Gastropoda: Ampullariidae)

SUMMARY Cold hardiness of the freshwater apple snail, Pomacea canaliculata, varies seasonally. We investigated lethal factors and physiological changes arising from exposure of P. canaliculata to low temperatures. Snails did not survive freezing. The supercooling point of cold-acclimated (cold tolerant) snails (–6.6±0.8°C) did not differ significantly from that of non-acclimated ones (–7.1±1.5°C) under laboratory conditions. Furthermore, snails died even under more moderately low temperatures approaching 0°C. These results indicate that indirect chilling injury is a factor in the death of P. canaliculata at low temperatures. Regardless of whether the snails were acclimated to low temperatures, all of the dead, and even some of the snails still alive at 0°C, had injured mantles, indicating that the mantle may be the organ most susceptible to the effects of low temperatures. The concentration of glucose in the posterior chamber of the kidney and concentration of glycerol in the digestive gland were significantly higher in cold-acclimated snails than in non-acclimated ones, suggesting carbohydrate metabolic pathways are altered in snails during cold acclimation.

Quality Control of Photosystem II: Lipid Peroxidation Accelerates Photoinhibition under Excessive Illumination

Environmental stresses lower the efficiency of photosynthesis and sometimes cause irreversible damage to plant functions. When spinach thylakoids and Photosystem II membranes were illuminated with excessive visible light (100–1,000 µmol photons m−1 s−1) for 10 min at either 20°C or 30°C, the optimum quantum yield of Photosystem II decreased as the light intensity and temperature increased. Reactive oxygen species and endogenous cationic radicals produced through a photochemical reaction at and/or near the reaction center have been implicated in the damage to the D1 protein. Here we present evidence that lipid peroxidation induced by the illumination is involved in the damage to the D1 protein and the subunits of the light-harvesting complex of Photosystem II. This is reasoned from the results that considerable lipid peroxidation occurred in the thylakoids in the light, and that lipoxygenase externally added in the dark induced inhibition of Photosystem II activity in the thylakoids, production of singlet oxygen, which was monitored by electron paramagnetic resonance spin trapping, and damage to the D1 protein, in parallel with lipid peroxidation. Modification of the subunits of the light-harvesting complex of Photosystem II by malondialdehyde as well as oxidation of the subunits was also observed. We suggest that mainly singlet oxygen formed through lipid peroxidation under light stress participates in damaging the Photosystem II subunits.

Comparison of cold hardiness and sugar content between diapausing and nondiapausing pupae of the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

Abstract.  To understand overwintering of the cotton boll worm Helicoverpa armigera, cold hardiness and sugar content are compared between diapausing and nondiapausing pupae. Diapausing and nondiapausing pupae reared at 20 °C under short and long photoperiods are acclimatized with a reduction of 5 °C per 5 days to 0 °C. When the acclimation temperature reaches 0 °C, the survival of diapausing pupae is assessed. The survival gradually decreases as the period of treatment progresses and approximately half survive for 112 days. However, nondiapausing pupae survive only 14 days after exposure to 0 °C. The surpercooling points of nondiapausing, diapausing and acclimatized pupae are approximately −17 °C. The major sugars contained in pupae are trehalose and glucose. Even though trehalose contents in diapausing pupae (initial level: 0.6 mg 100 mg−1 fresh weight) increase significantly during cold acclimation and continue increasing until 58 days after exposure to 0 °C (maximum level: 1.8 mg 100 mg−1), glucose is maintained at low levels (0.02 mg 100 mg−1) for 56 days at 0 °C. However, glucose contents increase (maximum level: 0.8 mg 100 mg−1) with decreasing contents of trehalose 84 days after exposure to 0 °C. Glycogen content gradually decreases during cold acclimation. When nondiapausing pupae are acclimatized with a reduction of 5 °C per 5 days to 5 °C from the beginning of pupation until the eyespots move, trehalose content increases (maximum level: 1.0 mg 100 mg−1). Glucose contents in nondiapausing pupae increase before eclosion (0.09 mg 100 mg−1). From these results, diapausing pupae of H. armigera can overwinter in regions where average winter temperatures are higher than 0 °C, but nondiapausing pupae cannot.

Changes in chemical components in the freshwater apple snail, Pomacea canaliculata (Gastropoda: Ampullariidae), in relation to the development of its cold hardiness.

The apple snail, Pomacea canaliculata, is an invasive freshwater snail. It increases its cold hardiness before winter. However, the physiological mechanism of cold hardiness in molluscs is poorly understood, especially in freshwater molluscs. In this study, we examined the changes in low molecular weight compounds, glycogen and lipids, in the body of P. canaliculata in association with the development of cold hardiness. When snails without cold hardiness were experimentally cold-acclimated, the amount of glycerol, glutamine, and carnosine increased, while glycogen and phenylalanine decreased. Overwintering cold-tolerant snails collected from a drained paddy field in November also showed increased glycerol in their bodies with decreasing glycogen concentration, compared to summer snails collected from a submerged field. Water content also decreased during the cold acclimation, although the water loss was minimal. These results indicate that the freshwater snail, P. canaliculata enhances cold hardiness by accumulation of some kinds of low molecular weight compounds in its body as some insects do. However, the actual function of each low molecular compound is still unknown.

Pupal diapause of Helicoverpa armigera (Lepidoptera: Noctuidae): sensitive stage for thermal induction in the Okayama (western Japan) population

Helicoverpa armigera (Hübner) exhibits a facultative pupal diapause, which depends on temperature and photoperiod. Pupal diapause is induced at 20 degrees C by short photoperiods and inhibited by long photoperiods during the larval stage. However, in some pupae (35% of males and 57% of females) of a non-selected field population from Okayama Prefecture (34.6 degrees N), diapause is not induced by short photoperiods. In the present experiment, the importance of temperature for diapause induction was studied in the non-diapausing strain, which was selected from such individuals reared at 20 degrees C under a short photoperiod of 10L:14D. Furthermore, the sensitive stage for thermal determination of pupal diapause was determined by transferring larvae of various instars and pupae between 20 degrees C and 15 degrees C. Diapause was induced by 15 degrees C without respect to photoperiod. When larvae or pupae reared from eggs at 20 degrees C under a short or a long photoperiod were transferred to 15 degrees C in the periods of the middle fifth instar to the first three days after pupation, the diapause induction rate was significantly reduced in both males and females, especially in females. In contrast, when larvae or pupae reared at 15 degrees C were transferred to 20 degrees C in the same periods, diapause was induced in males, but not in females. However, the diapause induction rate of pupae transferred to 20 degrees C on the fourth day after pupation was significantly increased in females. The results show that temperature is the major diapause cue in the photoperiod-insensitive strain and the periods of middle fifth larval instar to early pupal stage are the thermal sensitive stages for pupal diapause induction with some different responses to temperatures between males and females in H. armigera.

Identification of tissues showing the lowest tolerance to freezing in larvae of the rice stem borer, Chilo suppressalis

Abstract.  Even though overwintering larvae of the rice stem borer, Chilo suppressalis, are freeze‐tolerant, they cannot survive below −30 °C. Furthermore, nondiapausing larvae cannot survive freezing. However, the cause of death due to freezing is unclear. To identify the cause of death by freezing in larvae, those tissues most injured by low temperatures are identified using the vital stain trypan blue. In overwintering larvae, the midgut of dead larvae stains blue, and remarkable colour density differences between dead and surviving larvae are observed in the midgut. In nondiapausing larvae incubated at −10 °C for several hours, the fat body of dead larvae is strongly stained. Furthermore, increases in mortality with treatment time correspond with increases in the area of the fat body stained. Sterile nondiapausing larvae with lower supercooling points, below −20 °C, do not freeze at −10 °C and survive the treatment. However, all the larvae die when subjected to inoculative freezing at −10 °C, and the fat body stains blue. These results suggest that the midgut in overwintering larvae and the fat body in nondiapausing larvae have the lowest tolerance to freezing.

Seasonal changes of phospholipids in last instar larvae of rice stem borer Chilo suppressalis Walker (Lepidoptera: Pyralidae)

Phospholipids of the cell membrane have been studied from the viewpoint of how overwintering insects inhabiting temperate zones adapt to low temperature. The transition of cell membrane phospholipids from a liquid crystalline phase to a gel phase is a crucial cause of cold or freezing injuries. We determined the qualitative and quantitative changes of phospholipids in the last instar larvae of the rice stem borer in summer and winter. We found that the total amount of their phospholipids did not change significantly between summer and winter and that the sum of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) comprised about 85% of their total phospholipids. In summer, the ratio of their PE to PC was almost one, while from autumn to mid‐winter it increased and reached three in February. The fatty acid compositions of PC hardly changed, and the percentage of unsaturated fatty acids did not exceed 50%. In contrast, the percentage of unsaturated fatty acids of PE in overwintering larvae increased up to 80% as ambient temperatures fell and oleic acid mainly contributed to the high percentage of unsaturation. In the present study, the relationship between the quantitative and qualitative changes of phospholipids and adaptation of the rice stem borer to cold winter climate are discussed.

Effects of pesticide practices on insect biodiversity in peach orchards

To examine the effects of pesticides on insect biodiversity, a population survey was conducted in conventionally managed, low-input, and organic peach orchards. Pitfall traps were used to sample a total of 6489 insects representing 151 species at seven study sites. Results of a population survey suggest that pesticide application adversely affected insect biodiversity in peach orchards and that the magnitude of the adverse effect might be greater for herbicide application than for insecticide application. The usefulness of carabids and ants as biological indicators of the effects of pesticides was also evaluated. Results suggest that the ant species Tetramorium tsushimae is a good indicator in peach orchards. In contrast, carabids are not suitable indicators.

A small HSP gene is not responsible for diapause and cold tolerance acquisition in Chilo suppressalis

Abstract:  The cDNA sequence of a small heat shock protein (hsp19.7) was cloned and sequenced from the rice stem borer, Chilo suppressalis Walker. The cDNA encoded a protein of 177 amino acids with a calculated molecular weight of 19.7 kDa. The deduced amino acid sequence showed the highest identity of 90% to Bombyx morihsp19.9. Expression levels of hsp19.7 were similar between diapausing and non‐diapausing larvae. In non‐diapausing larvae, but not in diapausing ones, hsp19.7 expression was upregulated by cold acclimation. Involvement of hsp19.7 in larval diapause and cold tolerance in C. suppressalis is discussed.

Temperature and Water Availability Affect Decrease of Cold Hardiness in the apple Snail, Pomacea canaliculata

ABSTRACT The apple snail Pomacea canaliculata is a freshwater snail originating in tropical and temperate South America. We investigated the effect of temperature acclimatization, moisture levels, and food availability on the loss of cold hardiness in overwintering snails. Cold hardiness broke down by four days in snails maintained in aquatic condition at 25°C and by eight days at 20°C. However, snails held at 15°C retained their cold hardiness even after 64 days. In addition, the cold hardiness of snails kept moist (wrapped in wet towel) was maintained for at least 64 days even at 25°C. These results indicate that warm temperature together with increase of water availability decreases the cold hardiness of overwintering snails. Glycerol content reduced as cold hardiness decreased; supporting a previous hypothesis that glycerol has a major physiological role in the development of cold hardiness in this species.