Lower Developmental Temperature Research Articles

Physiological Time Model for Predicting Adult Emergence of Western Corn Rootworm (Coleoptera: Chrysomelidae) in the Texas High Plains

Field observations at three locations in the Texas High Plains were used to develop and validate a degree-day phenology model to predict the onset and proportional emergence of adult Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) adults. Climatic data from the Texas High Plains Potential Evapotranspiration network were used with records of cumulative proportional adult emergence to determine the functional lower developmental temperature, optimum starting date, and the sum of degree-days for phenological events from onset to 99% adult emergence. The model base temperature, 10 degrees C (50 degrees F), corresponds closely to known physiological lower limits for development. The model uses a modified Gompertz equation, y = 96.5 x exp (-(exp(6.0 - 0.00404 x (x - 4.0), where x is cumulative heat (degree-days), to predict y, cumulative proportional emergence expressed as a percentage. The model starts degree-day accumulation on the date of corn, Zea mays L., emergence, and predictions correspond closely to corn phenological stages from tasseling to black layer development. Validation shows the model predicts cumulative proportional adult emergence within a satisfactory interval of 4.5 d. The model is flexible enough to accommodate early planting, late emergence, and the effects of drought and heat stress. The model provides corn producers ample lead time to anticipate and implement adult control practices.

Why get big in the cold? Towards a solution to a life-history puzzle

The temperature-size rule (TSR), which states that body size increases at lower developmental temperatures, appears to be a near-universal law for ectotherms. Although recent studies seem to suggest that the TSR might be adaptive, the underlying developmental mechanisms are thus far largely unknown. Here, we investigate temperature effects on life-history traits, behaviour and physiology in the copper butterfly Lycaena tityrus in order to disentangle the mechanistic basis for the above rule. In L. tityrus the larger body size produced at a lower temperature was proximately due to a greater increase in mass, which was caused by both behavioural and physiological mechanisms: a much-increased food intake and a higher efficiency in converting ingested food into body matter. These mechanisms, combined with temperature-induced changes at the cellular level, may provide general explanations for the TSR. Body fat and protein content increased in butterflies reared at the higher temperature, indicating favourable growth conditions. As predicted from protandry theory, males showed reduced development times, caused by higher growth rates compared to females. The latter was itself related to a higher daily food consumption, while the total food consumption (due to the females' longer developmental period) and assimilation was higher in females and may underly the sexual body size dimorphism.

Environmental change and the phenology of European aphids

AbstractAphids, because of their short generation time and low developmental threshold temperatures, are an insect group expected to respond particularly strongly to environmental changes. Forty years of standardized, daily data on the abundance of flying aphids have been brought together from countries throughout Europe, through the EU Thematic Network ‘EXAMINE’. Relationships between phenology, represented by date of first appearance in a year in a suction trap, of 29 aphid species and environmental data have been quantified using the residual maximum likelihood (REML) methodology. These relationships have been used with climate change scenario data to suggest plausible changes in aphid phenology. In general, the date of first record of aphid species in suction traps is expected to advance, the rate of advance varying with location and species, but averaging 8 days over the next 50 years. Strong relationships between aphid phenology and environmental variables have been found for many species, but they are notably weaker in species living all year on trees. Canonical variate analysis and principal coordinate analysis were used to determine ordinations of the 29 species on the basis of the presence/absence of explanatory variables in the REML models. There was strong discrimination between species with different life cycle strategies and between species feeding on herbs and trees, suggesting the possible value of trait‐based groupings in predicting responses to environmental changes.

Physiological and morphological development of Anagyrus ananatis at constant temperatures

The lower developmental temperature threshold (T0) and the Degree Days (DD) required for the encyrtid endoparasitoid Anagyrus ananatis Gahan to develop from egg to adult on the pink pineapple mealybug (PPM), Dysmicoccus brevipes (Cockerell) (Hemiptera: Pseudococcidae), were determined. The T0 was estimated to be about 12.65 °C for both females and males. In contrast, females and males required about 275 and 265 DD, respectively, to complete development from egg to adult. Temperatures from 19 to 29 °C were optimal for mass rearing of A. ananatis, with the optimal temperature being around 24 °C. At this temperature, A. ananatis could complete almost two generations in the time it takes PPM to complete only one generation. Although A. ananatis is a koinobiont, the mealybug host was killed within a few (6–8) days after parasitization. The developmental stages of A. ananatis were described (e.g., appearance, size, color) and their time periods quantified when reared on PPM at 23.5 ± 0.5°C. Encyrtiform eggs were inserted through the dorsal surface of the PPM and were attached to the host via a slender stalk. This immature parasitoid remained attached to the host cuticle via the stalk until entering the prepupal stage. The host mealybug mummified during the parasitoid’s prepupal stage. First adult eclosion occurred at 24 days post-parasitization.

Effects of temperature on the post‐diapause embryonic development and the hatching time in three grasshopper species (Orth., Acrididae)

Abstract:A study was conducted to determine the effects of six constant temperatures (15, 20, 25, 30, 35 and 40°C) on the post‐diapause embryonic development and the hatching time in three grasshopper species –Omocestus haemorrhoidalis (Charp.), Calliptamus abbreviatus Ikonn. and Chorthippus fallax (Zub.) – from the Inner Mongolian steppe. The results indicate that the species differ in the developmental rates, survival curves and cumulative hatching probabilities. The eggs of O. haemorrhoidalis had the fastest developmental rate with a low developmental threshold temperature of 9.9°C and the sum of effective temperature (SET) 211.2 degree‐days (DD). The corresponding values were 10.9°C and 210.6 DD for C. abbreviatus, 10.5°C and 240.2 DD for Ch. fallax respectively. The SET at which 50% of post‐diapause eggs hatched were 252.0 DD for O. haemorrhoidalis, 262.8 DD for C. abbreviatus, and 273.3 DD for Ch. fallax. The predicted maximal hatch ability of O. haemorrhoidalis (91.17%), C. abbreviatus (75.67%) and Ch. fallax (94.07%) occurred at 23.7, 29.0 and 31.3°C, respectively. The thermal death points of each species were reached at 43.3, 45.0 and 48.6°C. The optimal temperature ranges were 12.2–35.2°C for O. haemorrhoidalis, 21.7–36.3°C for C. abbreviatus and 20.9–41.7°C for Ch. fallax respectively. These results suggest that O. haemorrhoidalis adapt to hatch at a lower temperature range, C. abbreviatus adapt to mid‐temperature range, while Ch. fallax adapt to hatch at a higher temperature range. Although the SET of Ch. fallax is more than that of the other two species, it is not sufficient to explain the hatching sequence of the species in springtime. The results also indicate that Ch. fallax and O. haemorrhoidalis have wider adaptive temperature range than C. abbreviatus.

Roles of bootstrap resampling, measure of central tendency, and method for calculating thermal units when developing phenology models for pest management

The successful development of phenology models from field studies depends on many factors, some of which are entirely under the control of pest managers. For example, one such factor is the choice of method for calculating thermal units. In this study, we have demonstrated that four methods for calculating thermal units provided for acceptable predictions of one phenological event of one insect species, while another method for calculating thermal units did not. The measure of central tendency (mean or median) that is used to estimate lower developmental temperatures and required thermal summations is another factor that pest managers can control when developing phenology models from field studies. Here, we show that predictions that were made when using phenology models based on median lower developmental temperatures and median required thermal summations were superior to predictions that were made when using phenology models based on mean lower developmental temperatures and mean required thermal summations. The use of bootstrap vs. non-bootstrap estimates of lower developmental temperatures and required thermal summations is yet another factor that pest managers can control when developing phenology models from field studies. In this study, we found that calculating and using bootstrap estimates of lower developmental temperatures and required thermal summations in phenology models did not improve the predictions of one phenological event for one insect species. The implications of these and other findings are discussed.

Divergent strategies in low temperature environment for the sibling species Drosophila melanogaster and D. simulans: overwintering in extension border areas of France and comparison with African populations

The two sibling species D. /melanogaster and D. simulans adopt different overwintering strategies in northern border areas situated in France. If the winter is mild, both species reappear in early spring to refound the population. If the winter is cold, with several weeks of temperatures below 0 °C, D. melanogaster leave their shelters in April whereas D. simulans, which do not use shelters, reappear in late June, probably after returning from further south. Here, we tried to identify life-history characteristics responsible for this difference. For this, we studied developmental duration, viability, fecundity, fertility and longevity, and compared the abilities of French and African populations to survive when food supplies were inadequate, at different temperatures (14, 11, 7 °C). These temperatures are lower than those commonly used in the laboratory but closer to real conditions encountered in the wild. When the temperature was mild (14 or 11 °C) and the food supply was adequate, D. simulans performed better than D. melanogaster: it had a higher fecundity, a longer life expectancy and the males remained fertile, allowing outdoor reproduction late in winter. However, D. simulans was less resistant in more extreme conditions. At 7 °C D. simulans survived shorter on normal medium and its ability to survive when food supplies were inadequate was insufficient to allow outdoor overwintering. In contrast, D. melanogaster could not reproduce during winter: its fecundity was low and males were sterile at 11 °C. Nevertheless, if only protein-deficient resources were available, temperate D. melanogaster could survive for longer than D. simulans at all the temperatures tested. This greater resistance to underfeeding allows the species to survive until spring, in shelters for several months. A comparison of French and African population performances showed differences in the evolution of the two species during the colonization of more northern areas. African D. simulans, which are efficient at mild temperatures, underwent few modifications. In contrast, the viability of D. melanogaster improved at low developmental temperatures. This species also displayed higher fecundity, longer survival and higher underfeeding resistance at low temperatures. The relationship between the long retention genotype and underfeeding resistance or survival ability observed in French D. melanogaster populations may not exist in African populations.

Computer-simulated evaluation of insect phenology models developed in the tropics: Chilo suppressalis (Walker) as an example

Computer-simulated studies were conducted to see if lower developmental temperatures and required thermal summations could be estimated for tropical insects through field studies. In addition, computer-simulated studies were conducted to see if the use of such estimates, in phenology models, would provide for more reliable (less variable) predictions of insect phenological events than the use of calendar-based methods. Finally, we wished to see if predictions of tropical insect phenology, when using phenology models that had 'low quality' estimates of lower developmental temperatures and required thermal summations, would give predictions that are just as reliable as those from phenology models that make use of 'high quality' estimates of lower developmental temperatures and required thermal summations. Our results indicate that it is possible to estimate lower developmental temperatures and required thermal summations of tropical insects from studies that are conducted in the field. They also indicate that predictions of insect phenology in the tropics, when made by using some phenology models, might be more reliable than the use of calendar-based methods. Finally, our results indicate that predictions of insect phenology in the tropics, when using phenology models that made use of low quality estimates of lower developmental temperatures and required thermal summations, are just as reliable as predictions that are made when using phenology models which made use of high quality estimates. From these results we conclude that the development and use of insect phenology models may prove beneficial for predicting insect phenological events in the tropics.

Effect of Temperature on Development, Survivorship, and Fecundity ofLysiphlebia mirzai(Hymenoptera: Aphidiidae), a Parasitoid ofToxoptera citricida(Homoptera: Aphididae)

Development, survivorship, longevity, and reproduction of Lysiphlebia mirzai Shuja-Uddin, a parasitoid of Toxoptera citricida (Kirkaldy), was studied at constant temperatures in the laboratory. Larval development ranged from 8.3 d at 32°C to 20.3 d at 10°C, and pupal development ranged from 3.7 d at 30°C to 15.4 d at 10°C. No adults emerged from mummies at 32°C. Duration of larval development was longer than pupal development. The respective low developmental temperatures and degree-days (DD) were estimated at 3.7°C and 142.7 DD for larval stage, 4.8°C and 94.2 DD for pupal stage, and 6.2°C and 181.2 DD for combined immature stage. The respective optimal temperatures of 30.3, 33.3, and 31.1°C for larval, pupal, and combined immature development were determined from a nonlinear biophysical model. The pupal survivorship was relatively constant within 10–20°C (90–84.5%). The average number of eggs laid per female in the initial 24-h period varied from 106.3 at 25°C to 41.8 at 32°C. The average longevity of L. mirzai adults decreased linearly with increasing temperature. At high (37 and 40°C) and low (5°C) temperature, the average pupal survivorship decreased as treatment time increased. Adult parasitoids failed to emerge from the mummies at 40°C after a 4-h treatment. In addition, the mean longevity of adult parasitoids was enhanced when they were fed with honey and water. The optimal constant temperature range for L. mirzai population growth was 15–25°C. This parasitoid may be more effective in control of brown citrus aphid in cooler months than in summer months.

Development, larval voracity, and greenhouse releases of Stethorus punctillum (Coleoptera: Coccinellidae)

Stethorus punctillum Weise (Coleoptera: Coccinellidae) is a Palearctic species first found in North America in the 1940s (Putman 1955). Commercial mass-rearing methods were developed in the late 1990s (Agriculture and Agri-Food Canada, London, Ontario, and Applied Bio-nomics Ltd, Saanich, British Columbia). The beetle is now released in North America to control two-spotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae). Knowledge of the life-history traits of S. punctillum is necessary for effective use of the predator in greenhouses. Putman (1955) provides useful information, but his results cannot be readily interpreted with respect to larval voracity, lower developmental temperature threshold, and developmental time in degreedays (°d), traits that affect efficacy. In this note I report new results relating to these life-history traits and to beetle releases in greenhouse vegetable crops.

Genetic and molecular characterization of a variegating hsp70-acZ fusion gene in the euchromatic 31 B region of Drosophila melanogaster.

A hsp70-lacZ fusion gene introduced into Drosophila melanogaster at the euchromatic 31B region by Pelement transformation displayed a variegated expression with respect to the lacZ fusion protein in the salivary gland cells under heat-shock conditions. The variegation is also reflected by the chromosome puffing pattern. Subsequent transposition of the 31B P element to other euchromatic positions restored wild-type activity, that is, a nonvariegated phenotype. A lower developmental temperature reduced the amount of expression under heat-shock conditions, similar to genes undergoing position-effect variegation (PEV). However, other modifiers of PEV did not affect the expression pattern of the gene. These results show a novel euchromatic tissue-specific variegation that is not associated with classical heterochromatic PEV.

Low Temperature Development under Intermittent Light Conditions Results in the Formation of Etiochloroplasts

Summary The effects of developmental temperature on etioplast to chloroplast conversion were examined in winter rye ( Secale cereale L. cv. Muskateer) upon exposure to intermittent light. One cycle of intermittent light is defined as exposure to 2 min of incandescent light (10 µE m- 2 ·s -1 ) followed by 118 min of darkness. Although 5 °C intermittent light seedlings developed almost 2-fold higher chlorophyll levels after 48 to 80 cycles than 20 °C seedlings, both exhibited similar maximum photosystem I activities. Development of maximum photosystem I and photosystem II activity was delayed at 5 °C relative to that observed at 20 °C. Etiolated seedlings exposed to 48 cycles of intermittent light at 20 °C exhibited the normal light-dependent breakdown of prolamellar bodies and protochlorophyllide oxidoreductase followed by the appearance of thylakoid membranes. In contrast, the plastids developed at 5 °C appeared to be etiochloroplasts which exhibited prolamellar bodies, high levels of protochlorophyllide reductase in addition to thylakoid membranes. We suggest that during chloroplast biogenesis intermittent light at low developmental temperature inhibits the plastid proteases involved in the light-dependent breakdown of protochlorophyllide reductase.

Effects of Temperature on the Development of the Water Strider, Gerris paludum insularis (Hemiptera: Gerridae)

The water strider, Gerris paludum insularis (Motschulsky), was reared in a growth cabinet at seven constant temperatures from 10 to 40°C under a 16:8 (L:D) photoperiod. There were highly significant differences among development times for all temperature regimes and the immature stages. Eggs failed to hatch at 10 and 40°C. The lower developmental temperature threshold was estimated to be 11.0°C and the upper, 35.6°C. Total development required 500 Celsius degree-days (DD) above threshold. The optimal temperatures for development and survival were 25 and 30°C, respectively. The presence or absence of this species in its normal range of distribution and habitats may be a useful indicator of thermal pollution.

Chloroplast biogenesis at cold-hardening temperatures. Kinetics of trans-?3-hexadecenoic acid accumulation and the assembly of LHCII

Etiolated seedlings developed at cold-hardening temperatures (5°C) exhibited etioplasts with considerable vesiculation of internal membranes compared to etioplasts developed at 20°C regardless of the osmotic concentration employed during sample preparation. This vesiculation disappeared during exposure to continuous light at 5°C. This transformation of 5°C and 20°C etioplasts to chloroplasts under continuous light at 5° and 20°C respectively proceeded normally with the initial development of non-appressed lamellae and the subsequent appearance of granal stacks. However, chloroplasts developed at 5°C exhibited fewer lamellae per granum than chloroplasts developed at 20°C.Although the polypeptide complements of etioplasts and chloroplasts developed at 5° or 20°C were not significantly different, monomeric light harvesting complex (LHCII3) was assembled into oligomeric light harvesting complex (LHCII1) during chloroplast biogenesis at 20°C (oligomer:monomer =1.8) whereas monomeric LHCII predominated at 5°C (oligomer:monomer =0.3). Low temperature fluorescence emission spectra of isolated thylakoids indicated that both the F685/F735 and F695/F735 were significantly higher after greening at 5°C than at 20°C. In addition, chloroplast biogenesis at 5°C was associated with a low ratio of trans-Δ3-hexadecenoic acid (0.5) in phosphatidylglycerol whereas at 20°C biogenesis was associated with a high ratio (1.6). Comparative kinetics indicated that the maximization of the trans-Δ3-hexadecenoic acid level precedes the assembly of monomeric LHCII into oligomeric LHCII during biogenesis at 20°C. It is suggested that low developmental temperatures modulate the assembly of LHCII by reducing the trans-Δ3-hexadecenoic acid content of phosphatidylglycerol such that monomeric or some intermediate form of LHCII predominates.

Low Temperature Development Induces a Specific Decrease in trans-Δ3-Hexadecenoic Acid Content which Influences LHCII Organization

Lipid and fatty acid analyses were performed on whole leaf extracts and isolated thylakoids from winter rye (Secale cereale L. cv Puma) grown at 5 degrees C cold-hardened rye (RH) and 20 degrees C nonhardened rye (RNH). Although no significant change in total lipid content was observed, growth at low, cold-hardening temperature resulted in a specific 67% (thylakoids) to 74% (whole leaves) decrease in the trans-Delta(3)-hexadecenoic acid (trans-16:1) level associated with phosphatidyldiacylglycerol (PG). Electron spin resonance and differential scanning calorimetry (DSC) indicated no significant difference in the fluidity of RH and RNH thylakoids. Separation of chlorophyll-protein complexes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the ratio of oligomeric light harvesting complex:monomeric light harvesting complex (LHCII(1):LHCII(3)) was 2-fold higher in RNH than RH thylakoids. The ratio of CP1a:CP1 was also 1.5-fold higher in RNH than RH thylakoids. Analyses of winter rye grown at 20, 15, 10, and 5 degrees C indicated that both, the trans-16:1 acid levels in PG and the LHCII(1):LHCII(3) decreased concomitantly with a decrease in growth temperature. Above 40 degrees C, differential scanning calorimetry of RNH thylakoids indicated the presence of five major endotherms (47, 60, 67, 73, and 86 degrees C). Although the general features of the temperature transitions observed above 40 degrees C in RH thylakoids were similar to those observed for RNH thylakoids, the transitions at 60 and 73 degrees C were resolved as inflections only and RH thylakoids exhibited transitions at 45 and 84 degrees C which were 2 degrees C lower than those observed in RNH thylakoids. Since polypeptide and lipid compositions of RH and RNH thylakoids were very similar, we suggest that these differences reflect alterations in thylakoid membrane organization. Specifically, it is suggested that low developmental temperature modulates LHCII organization such that oligomeric LHCII predominates in RNH thylakoids whereas a monomeric or an intermediate form of LHCII predominates in RH thylakoids. Furthermore, we conclude that low developmental temperature modulates LHCII organization by specifically altering the fatty composition of thylakoid PG.

Variations in Numbers of Median Dorsal Thorns and Rows of Teeth in Thorny Skate (Raja radiata) of the Northwest Atlantic

Data on the number of median dorsal thorns (anterior to the first dorsal fin) and the number of rows of teeth of the thorny skate from various areas of the Northwest Atlantic in 1949-72 were analyzed. The results showed a cline of mean number of median dorsal thorns from the Baffin Island and Ungava Bay areas southward along Labrador and eastern Newfoundland to the Grand Bank and thence westward to the Gulf of St. Lawrence, Scotian Shelf and Georges Bank, the higher numbers occurring at lower developmental temperatures and vice versa. The mean tooth-row counts showed a cline which was opposite to that for median dorsal thorns, increasing from the Baffin Island and Ungava Bay areas southward to the Scotian Shelf and Georges Bank. There was a slight tendency toward increased tooth-row numbers with fish length, but the within-area differences were not significant for lengths greater than 24 em which were used in the comparisons among areas. Males tended to have slightly more rows of teeth than females, but there was no clear evidence of sexual dimorphism. The number of tooth-rows in the lower jaw was, on the average, slightly higher than in the upper jaw, but the upper-jaw counts exceeded the lower-jaw counts in small samples from the geographical extremes of the populations studied.

Hybrid dysgenesis in Drosophila melanogaster: influence of temperature on cytotype determination in the P-M system.

In Drosophila melanogaster, the P-M system of hybrid dysgenesis is a syndrome of germ line abnormalities, including temperature dependent gonadal dysgenesis (GD sterility), high rates of mutation and male recombination, which occurs in some interstrain hybrids but only from one of the two crosses. In the P-M system, hybrid dysgenesis results from interaction between chromosomally transposable elements of the P element family and a particular extrachromosomal state referred to as the M cytotype. Cytotype (M or P) is known to be determined by the absence or presence of chromosomal factors, but principally with limited cytoplasmic transmission. In a series of experiments in which F1 hybrid females from various P and M strains were submitted to different preadult and ageing temperature treatments, it was found that the cytotype switch is strongly temperature-dependent in the F1 females from M female X P male but not in the reciprocal cross. In the F1 females from the former cross, a strong M cytotype occurs at a low developmental temperature (18 degrees C) and a weak M cytotype occurs at a high developmental temperature (26.5 degrees C). On the other hand, a high ageing temperature applied after a low developmental temperature switches the cytotype from M to P and reciprocally, a low ageing temperature applied after a high developmental temperature switches the cytotype from P to M.(ABSTRACT TRUNCATED AT 250 WORDS)

Environmental Control of the Seasonal Life Cycle of Adalia bipunctata (Coleoptera: Coccinellidae)

Adalia bipunctata L. has low developmental thresholds ( t ) for postdiapause and nondiapause development that adapt it to activity early in the season. Postdiapause reproductive development leading to oviposition requires an accumulation of 87 degree-days ( K ) above a relatively low developmental threshold temperature ( t ) of 6.8°C. Overwintering adults disperse and initiate oviposition during early to mid-May. The linearly derived ( t ) and ( K ) values of 263 ± 9.0 degree-days above 9.0°C for preimaginal development and daily maximum-minimum temperatures from outdoors accurately predicted outdoor developmental rates. Preimaginal development outdoors during June and July required 269 ± 12 degree-days above 9.0°C under conditions of excess prey. In the field, there are two to three generations per year in the Ithaca, N.Y., area. The critical photoperiod for diapause induction in the Ithaca, N.Y., (~42°N) population lies between LD 13:11 and LD 14:10 (23°C). Short daylengths maintain diapause, and during autumn the duration of diapause is quantitatively related to photoperiod. Diapause completion does not require chilling: during winter-early spring. photoperiod may exert some influence on the timing of diapause termination.

Maximum Reproductivity by the Hymenopterous Female, an Effect of Larval Nutrient Conservation Incidental to the Delayed Midgut-hindgut Connection

The derivation of highly reproductive hymenopterous females from larvae exposed to a very low developmental temperature during their final instars as reported in the literature is considered to be an effect of prolonged larval metabolism and the consequent utilization of ingested nutrients which would have been voided from the alimentary canal had higher (normal) temperatures prevailed. Instances that appear to illustrate this relation and its influence on caste differentiation in social Hymenoptera are discussed.

HYBRID DYSGENESIS IN DROSOPHILA MELANOGASTER: A SYNDROME OF ABERRANT TRAITS INCLUDING MUTATION, STERILITY AND MALE RECOMBINATION

A syndrome of associated aberrant traits is described in Drosophila melanogaster. Six of these traits, mutation, sterility, male recombination, transmission ratio distortion, chromosomal aberrations and local increases in female recombination, have previously been reported. A seventh trait, nondisjunction, is described for the first time. All of the traits we have examined are found nonreciprocally in F(1) hybrids. We present evidence that at least four of the traits are not found in nonhybrids. Therefore we have proposed the name hybrid dysgenesis to describe this syndrome.-A partition of tested strains into two types, designated P and M, was made according to the paternal or maternal contribution required to produce hybrid dysgenesis. This classification seems to hold for crosses of strains from within the United States and Australia, as well as for crosses between strains from the two countries. Strains collected recently from natural populations are typically of the P type and those having a long laboratory history are generally of the M type. However, a group of six strains collected from the wild in the 1960's are unambiguously divided equally between the P and M types. The dichotomy of this latter group raises interesting questions concerning possible implications for speciation.-Temperature often has a critical effect on the manifestation of hybrid dysgenesis. High F(1 ) developmental temperatures tend to increase the expression of sterility, sometimes to extreme levels. Conversely, low developmental temperatures tend to inhibit the expression of some dysgenic traits.-There are potentially important practical implications of hybrid dysgenesis for laboratory experimentation. The results suggest that care should be exercised in planning experiments involving strain crosses.