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.     

Geographic variation of reproductive diapause in the Drosophila auraria complex (Diptera: Drosophilidae)

ABSTRACT. In the Drosophila auraria complex (D. auraria Peng, D. biauraria Bock & Wheeler, D. triauraria Bock & Wheeler, D. quadraria Bock & Wheeler and D. subauraria Kimura), D. triauraria from the island of Yaku (30.3N in latitude) and D. quadraria from Taiwan (24.1N) had no or weak diapause, while strains of all the four species from the main islands of Japan had a firm reproductive diapause. These strains entered diapause at short daylengths and the critical daylength was longer and the incidence of diapause higher at a low rather than high temperature. In each species, the critical daylength was longer in strains from higher latitudes. Among sympatric populations of different species, D. subauraria always had the longest critical daylength, while D. auraria had the shortest one. The populations of D. biauraria and D. triauraria had longer critical daylengths than the sympatric populations of D. auraria , but this difference became smaller northwards. Since a longer critical daylength induces diapause earlier in autumn, D. subauraria would enter diapause earliest among these species, and D. auraria would do so latest. The late start of diapause in D. auraria may be related to its adaptation to domestic environments. On the other hand, the adaptive significance of the late start of diapause in northern populations of D. biauraria and D. triauraria is not clear.     

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.     

Geographic variation and genetic aspects of reproductive diapause in Drosophila triauraria and D. quadraria

ABSTRACT. Geographic variation and genetic aspect of reproductive diapause were studied in Drosophila triauraria Bock & Wheeler and D. quadraria Bock & Wheeler, in relation to their quantitative response to photoperiods. D. quadraria from the subtropical region had no photoperiodic diapause. In D. triauraria strains, diapause was induced under LD 10:14 or 12:12, but not under LD 14:10 or longer photoperiods. Their diapause was induced more effectively and maintained longer by LD 10:14 than by LD 12:12. The duration of diapause was longer in a northern strain, but the diapause inducing process varied little among different geographical strains. Diapause incidence was 50% or lower in F₁ hybrids between D. triauraria and D. quadraria and backcross progenies between F₁ and D. quadraria , and about 70% in backcross progenies between F₁ and D. triauraria under LD 10:14, but very low under LD 12:12. The lower incidence of diapause in these F; and backcross progenies is assumed to be due to the less efficient induction of diapause, since once diapause was induced in them, it was maintained for a long time, especially in the backcross progenies between F₁ and D. triauraria. These experiments suggest that diapause induction and maintenance are different physiological processes controlled by different genetic systems.     

Cold and heat tolerance of drosophilid flies with reference to their latitudinal distributions

The relation between thermal tolerance and latitudinal distribution was studied with 30 drosophilid species collected from the cool-temperate region (Sapporo), the warm-temperate region (Tokyo and Kyoto) and the subtropical region (Iriomote island) in Japan. In addition, intraspecific variation was examined for five species collected from two localities. The subtropical strains of Scaptodrosophila coracina, Drosophila bizonata and D. daruma were less tolerant to cold than their temperate strains. However, the difference of cold tolerance between these two geographic strains was much smaller than the difference between the species restricted to the subtropical region and those occurring in the temperate region. In D. auraria and D. suzukii, no difference was observed in thermal tolerance between their cool- and warm-temperate strains. Thus, geographic variation in thermal tolerance within species was low or negligible. Interspecific comparisons by phylogenetic independent contrasts revealed that species which had the northern boundaries of their distributions at higher latitudes were generally more tolerant to cold than those which had their boundaries at lower latitudes. However, the data for some species did not agree with this trend. The use of man-protected warm places for overwintering, competition or predation would also affect their distributions. It also appeared that species which had their southern boundaries at higher latitudes were generally more cold-tolerant. The acquisition of cold tolerance may lower a flys capacity to compete, survive or reproduce in warmer climates. On the other hand, no relation was observed between heat tolerance and latitudinal distribution. Heat tolerance was higher in species inhabiting openlands or the forest canopy than in those inhabiting the forest understorey.     

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.     

Geographic variability in diapause response of Japanese Orius species

Interspecific and latitudinal variation in diapause characteristics were examined in 12 strains of Orius species (Heteroptera: Anthocoridae) including O. sauteri (Poppius), O. nagaii Yasunaga, O. minutus (L.), O. strigicollis (Poppius), and O. tantillus (Motschulsky) from Japan. A latitudinal cline was found in the photoperiodic response controlling reproductive diapause: the lower the latitude, the lower the diapause incidence and the shorter the critical daylength. To examine the overwintering success, eight strains including four species derived from different latitudes (26–43° N) were reared outdoors in Tsukuba (36° N), central Japan from the autumn of 1998 to the summer of 1999, and their winter survival and spring fecundity were recorded. Most females of the northern strains entered diapause in the autumn when the temperature was still high, and died before winter without oviposition. In the southern strains including a nondiapause Okinawa (26° N) strain of O. strigicollis, females overwintered as well as native strains in a state of quiescence. In all strains, the winter survival was significantly lower in males than in females, and eggs and nymphs failed to overwinter.     

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.     

The influence of gene flow on latitudinal clines of photoperiodic adult diapause in the Drosophila auraria species-complex

Temperate species belonging to the Drosophila auraria species complex, D. auraria Peng, D. biauraria Bock & Wheeler, D. triauraria Bock & Wheeler and D. subauraria Kimura, enter reproductive diapause to pass the winter in response to short daylengths. These species from Japan showed latitudinal clines in critical daylength which is longer in populations from higher latitudes. The slopes of these clines coincided well with that of the cline which is approximately predicted from climatic data, suggesting that these clines result from adaptation of the species to the latitudinal gradient of climatic conditions. Between the mainlands and the surrounding islands of Japan, the slopes of clines did not differ significantly, but the deviation from the regression line was usually smaller in mainland populations. It is assumed that gene flow reduces the genetic variation among mainland populations and results in the development of smooth clines. In the plain of east China, D. triauraria did not show clinal variation in critical daylength, although the development of the cline is expected from climatic data. Extensive gene flow among Chinese populations is considered to prevent the development of a cline.     

Climatic adaptations in the Drosophila immigrans species group: seasonal migration and thermal tolerance

Abstract. 1. In central Japan, Drosophila curviceps Okada and Kurokawa was collected in spring and autumn but not in summer at lowlands (alt. 500–1200 m), while it was collected only in summer at highlands (1500–2000 m). Experiments on its thermal tolerance suggested that summer heat at the lowlands and winter low temperatures at the highlands were adverse to this species. It is considered that this species escapes from these extreme temperatures by undergoing seasonal migration between the lowlands and the highlands. This species had no photo-periodic diapause and bred at both lowlands and highlands.
2. D.immigrans Sturtevant was less cold-hardy but more heat-tolerant than D.curviceps. It is considered that this species is unable to overwinter outdoors at least in the study areas (i.e. alt. 500m or higher in central Japan) and its populations in these areas originate with migrants from warmer areas.
3. D.albomicans Duda, a subtropical species, was less cold-hardy but more heat-tolerant than the above two species.
4. Climatic adaptations and distributions of these species are discussed with reference to their thermal tolerance.     

A genetic analysis of photoperiodic reproductive diapause in Drosophila triauraria

Abstract. Recombinant inbred lines were produced from hybrids between diapausing and non-diapausing strains of Drosophila triauraria Bock & Wheeler by twenty generations of full-sib mating. The analysis of these inbred lines suggests that the difference in the photoperiodic response between the original diapausing and non-diapausing strains is due to genes at three or four loci and at least one of these loci is located on the X chromosome. The original diapausing strain and two inbred lines exhibited circadian peaks when subjected to the Nanda-Hamner protocol, although they had different critical daylengths. This suggests that critical daylength is modified independently of the circadian oscillation system in this species. On the other hand, diapause duration was correlated with critical daylength among the inbred lines and the original diapausing strain, suggesting that these two traits are under the control of the same genetic system.     

Feeding,reproductive and locomotor activities in diapausing and non-diapausing adults ofDrosophila

Feeding, reproductive and locomotor activities of fourDrosophila species were studied under short and long daylengths at 15°C. A short daylength induced firm reproductive diapause in experimental strains ofD. subauraria andD. triauraria from northern Japan, but very shallow diapause in those ofD. lutescens andD. rufa from southern Japan. A subtropical strain ofD. triauraria had no diapause. The influence of diapause on feeding activity was detected only in aged (> 12 day old) females; that is, the feeding activity was lower in diapausing females than in non-diapausing ones. Females that do not produce eggs would not require so much energy. On the other hand, young adults of the study species exhibited a high feeding activity and rapidly increased bodyweight irrespective of sex and the diapause state. They would need nutrition to build up their adult body. In males, the feeding activity decreased with age irrespective of the diapause state. Males would not require so much energy for reproductive activity. Diapausing males became heavier than non-diapausing males, perhaps because they accumulated triacylglycerols in fat bodies. However, female bodyweight did not differ by the diapause state, perhaps because diapausing females accumulated triacylglycerols and reproducing females had eggs in their ovaries. InD. triauraria, diapausing individuals exhibited somewhat lower locomotor activity than non-diapausing ones.     

The polytene chromosomes of Drosophila triauraria and D. quadraria, sibling species of D. auraria.

The polytene chromosomes of Drosophila triauraria and D. quadraria, two of the sibling species of D. auraria, were examined. The polytene chromosomes of all three species exhibit very clear homology. Unlike the stock of D. auraria that we studied, D. triauraria and D. quadraria carry heterozygous paracentric inversions. In both species, 2R and 3R are the arms where these inversions are concentrated. In addition, in D. quadraria, the 3L chromosome arm is very complicated because of heterozygous inversions. The mode of inheritance of these rearrangements was studied. A homozygous strain for all chromosome arms of D. triauraria was isolated, while a homozygous strain was obtained only for the arms X, 2L, 3L, and 4 of D. quadraria. Like D. auraria, both species show a large number of inverted tandem duplications in the paired condition, even in the chromosomes of their hybrids. Small deletions were also detected, one of which, in D. triauraria, is homozygous terminal. Hypotheses are discussed concerning the relationships of the species and the existence of inverted tandem duplications.     

Overwintering cost associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae)

Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) may have important effects on the evolution of resistance. We investigated overwintering costs in pink bollworm, Pectinophora gosypiella (Saunders), strains with different degrees of resistance to Bt cotton. Frequency of resistant individuals in a strain was not associated with induction of diapause or emergence from diapause in early winter. Emergence from diapause in the spring was 71% lower in three highly resistant strains than in two heterogeneous strains from which the resistant strains were derived. This underestimates the overwintering cost because the frequency of the resistance allele was relatively high in the heterogeneous strains. Emergence in the spring in hybrid progeny from crosses between the resistant and heterogeneous strains was greater than in resistant strains but did not differ from susceptible strains, showing that the overwintering cost was recessive to some extent.     

Evolutionary Relationship between Diapause and Cold Hardiness in the House Spider,Achaearanea tepidariorum (Araneae: Theridiidae)

The relationship between diapause and cold hardiness of the house spider, Achaearanea tepidariorum, differed geographically. In a cool-temperate population, enhanced chilling tolerance and supercooling ability were observed in diapause individuals, whereas a subtropical population showed only chilling tolerance. Because this spider is considered to be of tropical origin, it would follow that the ancestral diapause of this spider was equipped with chilling tolerance, but not with an increased supercooling ability. It seems that the ability to lower the supercooling point evolved through natural selection in the course of expansion of this species to the northern climates. Copyright 1997 Elsevier Science Ltd. All rights reserved     

Dispersion of flightless adults of the Asian lady beetle, Harmonia axyridis, in greenhouses containing cucumbers infested with the aphid Aphis gossypii: effect of the presence of conspecific larvae

Nezara viridula (L.) (Heteroptera: Pentatomidae) recently expanded its distribution range in Japan and reached Osaka. In the southern temperate zone, the species overwinters in the adult stage and reproductive diapause is associated with a body colour change from green to russet. In Osaka, the reproductive diapause is only induced in September–October and nymphs from late egg masses are destined to die during winter. However, the fate of adults emerging late in the season remained unknown. Survival, body colour change, and post‐diapause reproduction were studied under quasi‐natural conditions in Osaka in 1999–2000 in those adults that attained adulthood as late as in November. Two experimental cohorts were used: in the Outdoor cohort, insects were reared outdoors starting in their second instar (28 September); in the Laboratory cohort, nymphs and subsequently adults were reared from the same day under diapause‐inducing conditions (L10:D14 at 25 °C), then acclimatized (5 days at 20 °C and 5 days at 15 °C; L10:D14) and transferred outdoors on 1 December. Adults in both cohorts did not reproduce in autumn and survived the winter with a low mortality. More than 20% of adults in the Outdoor cohort failed to change body colour from green to russet during winter, apparently because of the low ambient temperature, suggesting that the environmental conditions required for colour change do not completely coincide with those required for diapause induction, and that the colour of the adults is not always a reliable indicator of diapause in this species. After overwintering, females from the Outdoor cohort produced significantly fewer egg masses and eggs and had a significantly shorter period of oviposition than females that entered diapause under short‐day conditions in the Laboratory cohort. Thus, if progeny from the late egg masses attain adulthood late in the autumn, these adults have high chances of successful overwintering, but their reproductive output after the winter diapause is significantly reduced.     

Mating prior to overwintering in the imported willow leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae)

Abstract. 1. We report on the mating status of overwintering adults of the imported leaf beetle, Plagiodera versicolora Laicharting. This species overwinters as adults, under the bark of willow trees.
2. Individuals were collected both in the autumn, just after initiation of diapause, and in the spring, just before natural termination of diapause. From the autumn sample, we discovered that diapause can be terminated by husbanding the adults at 4°C for 4 weeks.
3. Most females lay fertile eggs upon the termination of diapause; however, a significant number of females lay non-fertile eggs after overwintering in the field.
4. Many females mate with more than one male before entering diapause. Progeny analysis using allozyme genetic markers shows that, on average, females who are fertile upon termination of diapause utilize sperm from 1.2 males to fertilize their eggs.     

Association between circadian clock genes and diapause incidence in Drosophila triauraria

Diapause is an adaptive response triggered by seasonal photoperiodicity to overcome unfavorable seasons. The photoperiodic clock is a system that controls seasonal physiological processes, but our knowledge about its physiological mechanisms and genetic architecture remains incomplete. The circadian clock is another system that controls daily rhythmic physiological phenomena. It has been argued that there is a connection between the two clocks. To examine the genetic connection between them, we analyzed the associations of five circadian clock genes (period, timeless, Clock, cycle and cryptochrome) with the occurrence of diapause in Drosophila triauraria, which shows a robust reproductive diapause with clear photoperiodicity. Non-diapause strains found in low latitudes were compared in genetic crosses with the diapause strain, in which the diapause trait is clearly dominant. Single nucleotide polymorphism and deletion analyses of the five circadian clock genes in backcross progeny revealed that allelic differences in timeless and cryptochrome between the strains were additively associated with the differences in the incidence of diapause. This suggests that there is a molecular link between certain circadian clock genes and the occurrence of diapause.     

Nutrition,sex and season contribute to variation in fat and glycerol levels in the long‐lived moth Caloptilia fraxinella

The ash leaf cone roller Caloptilia fraxinella Ely (Lepidoptera: Gracillariidae) is an invasive leaf‐mining moth pest of horticultural ash Fraxinus spp. in the Canadian Prairie Provinces. Caloptilia fraxinella overwinter as adults in reproductive diapause and mating occurs after overwintering in the spring. The effect of a carbohydrate food source on fat and glycerol reserves throughout the long adult life stage of this moth is investigated. Insects collected as pupae are given access to either water or sugar water upon adult eclosion. Moths held under the different feeding regimes are sampled before (summer and autumn) and after overwintering in the spring. Analysis of either glycerol or lipid content is conducted for male and female moths from each collection period. Both moth weight and glycerol concentration are affected by moth sex, food regime and season of collection. Although female moths are heavier than males, a higher glycerol concentration occurs in males. Moths fed sugar are heavier and have a higher glycerol concentration than water‐fed moths late in reproductive diapause and after overwintering. Moths collected in the spring after overwintering are lighter and have a lower glycerol content than moths collected before winter. There is a significant influence of feeding regime and season on moth body lipid content, with sugar‐fed moths having more fat than water‐fed moths; however, this difference is smaller in the summer than the autumn or spring. An initial understanding of the overwintering biology and diapause of this pest is provided in the present study.     

Effects of nutrition and methoprene treatment upon reproductive diapause in Caloptilia fraxinella (Lepidoptera: Gracillariidae)

Abstract Female Caloptilia fraxinella exhibit a prolonged reproductive diapause immediately post adult emergence in mid‐summer until the next spring when mating, egg development and oviposition on fresh Fraxinus spp. leaflets occur. Factors that effect the termination of reproductive diapause are investigated in this species. Caloptilia fraxinella diapausing adults held in overwintering conditions (2 °C, LD 0 : 24 h) for 24 weeks terminate diapause after placement for 2 weeks in simulated summer conditions (24 °C, LD 16 : 8 h) only if they are provided with 10% sugar water. Exogenous application of the Juvenile Hormone (JH) analogue methoprene to moths in both early‐ (summer) and mid‐ (autumn) reproductive diapause demonstrates that JH affects diapause termination but a carbohydrate nutrition source also mediates mating and vitellogenesis. Mating between moth pairs early in diapause occurs only after treatment with methoprene and provision with sugar water. However, there is no impact of mating on the propensity of females to produce vitellogenic oöcytes. Moths collected in the autumn in mid‐diapause respond in a dose‐dependent fashion to methoprene treatment and the response is greater than that of moths early in diapause collected in the summer. Treatment with methoprene and access to sugar water results in vitellogenic oöcytes in 18.75% of females from mid‐diapause moth pairs treated with 0.01 μg methoprene per insect and in all females from pairs treated at the two highest doses of methoprene (0.1 and 1 μg per insect). Mating occurs only between moths in mid‐diapause treated with the two highest doses of methoprene and these doses induce 91% and 100% mating, respectively. Both control and methoprene‐treated males in mid‐diapause held under summer conditions mate successfully and pass a spermatophore to their methoprene‐treated female partner. These data demonstrate that female C. fraxinella undergo a prolonged reproductive diapause in which termination is dependent on JH and further mediated by a carbohydrate nutrition source. The production of vitellogenic oöcytes is independent of mating. These data also provide evidence that response of moths in diapause to exogenous applications of methoprene differs throughout the diapause period and between male and female C. fraxinella.