Interaction between genetic and inductive factors controlling the expression of dispersal and dormancy morphs in dimorphic astigmatic mites

Some astigmatic mites display dimorphic deutonymphs (hypopus) which are facultatively intercalated in their development cycle between protonymph and tritonymph. Such species, among them Glycyphagus privatus and Glycyphagus ornatus show three potential developmental pathways: (1) to bypass the hypopus stage and develop directly from the protonymph to the tritonymph and the subsequent reproductive stage when conditions are favorable; (2) to leave the original site and disperse by means of a phoretic hypopus morph; or (3) to survive inimical life conditions in the natal environment by means of a sedentary hypopus morph. By producing both dispersing (and afterwards at the arrival site reproducing) and sedentary (drought-hardy and dormancy-prone) progeny each single parent attains a selective advantage through a risk-reducing insurance against irregularly fluctuating and often fatal life conditions of their temporary patch habitats. Both genetic heterogeneity and ecological plasticity for hypopus production adapt the Glycyphagus species to cope with variation in the environment. Both traits (for dispersal and survival) are extremely polymorphic with genotypes ranging from low to high propensities for production of each hypopus type. There is a substantial environmental effect on genetic expression such that expression of both morphs depends on the quality of food. This ecological response allows a fast reaction of the mite to the current trophic environment. Phoretic morphs are predominantly expressed at favorable trophic conditions and sedentary morphs at poor trophic conditions. Ecological influences may override genetic propensities and vice versa. Although selection imposed by changing environmental patterns adjusts the frequencies of genotypes over generations and provides for long-term adaptation, the short-term process of environmental induction adapts the population within a generation to transient-habitat disturbances. The interaction of genetic and ecological determinants explains the varying proportions of directly developing mites, phoretic hypopodes, and sedentary hypopodes, in a population at any moment.     

Trophic determinants of hypopus induction in the stored-product mite Lepidoglyphus destructor (Acari: Astigmata)

A unique (synapomorphic) characteristic of astigmatic mites is the heteromorphic deuteronymph also called hypopus. It is a non-feeding and facultative instar between protonymph and tritonymph. The hypopus is adapted for dispersal and sometimes also for dormancy, as in Lepidoglyphus destructor. The experiments reveal a correlation between the composition of the foodstuff, the duration of development of homomorphic instars, the mortality of protonymphs and the production of hypopodes. As food quality decreases, development lasts longer, mortality increases and hypopodes are produced in greater numbers. Disadvantageous trophic conditions of varied chemical nature favour the induction of hypopodes. The experimental data show that hypopus incidences (as percentage individuals of a population) depend on the relative proportions of constituents of an ingested foodstuff. What matters is the ratio between nourishing foodstuff components and those that are of little or no nutritional value. When a certain ratio does not meet a presumed metabolically required level of nutrients a nutritional deficiency results and hypopus induction is triggered, provided that adequate genetic propensities for hypopus production are present (L. destructor is highly polymorphic for hypopus production). Specific key substances are apparently not involved, and composite properties of a foodstuff are crucial for hypopus induction. Decrease of food quality (not poor food per se) during the hypopus-inducible period (late larval to early protonymphal phase) promotes hypopus induction. The interpretation matches the ecological scene. When trophic deterioration of a patch habitat sets in, often as a result of overcrowding, conditions will eventually become untenable. As a response to incurring nutritional deficiencies the mites will induce hypopodes, which provide for escape from or survival at the decaying habitat patch. Experiments support the threshold model of quantitative genetics for hypopus expression as previously inferred from other experiments with L. destructor.     

Genetic variability and ecological adaptability of hypopus formation in a stored product mite

Occurrence inLepidoglyphus destructor of a facultative developmental stage (hypopus) adapted for dispersal and dormancy depends on genotype x environment interaction. Dietary factors affect hypopus formation, but the response of individuals to food quality greatly differs according to genotype. Large genetic variation in the control of hypopus formation exists within as well as between populations. Response to selection is rapid and reversible. The trait has substantial potential for selective adaptation to unpredictably varying environments and for evolutionary change.     

Genetic and environmental determinants of hypopus duration in the stored-product miteLepidoglyphus destructor

This paper reports a series of experiments over many years on hypopus duration and extends the preceding investigation (1987) on hypopus formation inLepidoglyphus destructor (Schrank, 1781). The length of time required for hypopus physiogenesis (diapause development) is genetically programmed but influenced by environmental factors. This span of time is highly variable, and may extend from one week to more than a year. Spreading out the potential for hypopus completion over time is adaptive, since a pool of hypopodes with prolonged and staggered dormancies serves to spread the risk of emergence of tritonymphs over extended periods of time; it buffers the population against sudden drought to which all other stages of the life-cycle succumb.The additive structure and large variance of the genetic system underlying the length of time required for hypopus physiogenesis allows for the reconstitution of a broad spectrum of genotypes in every generation through the process of meiotic segregation and recombination during sexual reproduction. It favours stored variability, provides a fail-safe device both for survival as well as development in irregularly fluctuating environments, and facilitates the adaptation of populations to local conditions. The trait for hypopus physiogenesis varies independently from that of hypopus formation, and is apparently free to adjust, without genetic constraints, towards an adaptive optimum. The response to selection is fast.Low environmental humidities and high temperatures accelerate physiogenesis of the hypopus. Completion of the hypopus stage and moulting to the tritonymph is triggered by high humidities at moderate temperatures. If environmental conditions preclude moulting, the hypopus following ending of physiogenesis enters a state of quiescence.In contrast the seasonal and largely predictably varying environments, in which essentially anticipatory and season-related token cues like photoperiod regulate the timing of so many arthropod lifecycles,L. destructor copes with sudden and fatal drought, as well as with unheralded and favourable humidities in its ephemeral habitats, mainly by excessive genetic polymorphism in hypopus duration and formation; some genotypes are always instantaneously fit to meet the respective environmental situation.The mite faces gradual food deterioration of its patchily distributed microhabitats by a short-term anticipatory and environmentally cued developmental switch mechanism, which lowers the threshold for hypopus induction.On top of genetic variability and phenotypic plastivity, any genotype×environment interaction provides for increasing flexibility above that from genetic polymorphism and environmental polyphenism alone. This extraordinary measure of adaptedness fitsL. destructor for life in irregularly fluctuating environments.     

Inter- and intra-population variations in diapause attribute of the two-spotted spider mite,Tetranychus urticae Koch,in Japan

Populations of the two-spotted spider mite, Tetranychus urticae Koch collected from various localities and from various host plants in Japan showed wide variations in diapause attribute. Diapause percentages at 18°C/9L15D varied from nearly 100% in the north to 0% in the south-west. At intermediate latitudes the mites showed wide inter-population variations. Populations on herbaceous hosts in vinyl- or glass-houses gave significantly lower incidence of diapause than those on roses and deciduous fruit trees. Presence of winter hosts and better host quality under protected environments seemed to favour non-diapausing mites. The temperature threshold for diapause expression also varied widely among local populations. Northern populations consistently had higher and less variable thresholds than populations at intermediate latitudes with thresholds between 15 and 18°C. Inbred lines derived from a population in Kyoto exhibited a wide variation in diapause percentage at 18°C. These results show that diapause in T. urticae is a quantitative threshold trait and that populations in central Japan consist of a variety of genotypes with different diapause traits. This might provide a genetic source for adaptation to local and temporal variations in environmental conditions.     

Adaptation in a spider mite population after long-term evolution on a single host plant

Evolution in a single environment is expected to erode genetic variability, thereby precluding adaptation to novel environments. To test this, a large population of spider mites kept on cucumber for approximately 300 generations was used to establish populations on novel host plants (tomato or pepper), and changes in traits associated to adaptation were measured after 15 generations. Using a half-sib design, we investigated whether trait changes were related to genetic variation in the base population. Juvenile survival and fecundity exhibited genetic variation and increased in experimental populations on novel hosts. Conversely, no variation was detected for host choice and developmental time and these traits did not evolve. Longevity remained unchanged on novel hosts despite the presence of genetic variation, suggesting weak selection for this trait. Hence, patterns of evolutionary changes generally matched those of genetic variation, and changes in some traits were not hindered by long-term evolution in a constant environment.     

MAINTENANCE OF POLYGENIC VARIATION IN SPATIALLY STRUCTURED POPULATIONS: ROLES FOR LOCAL MATING AND GENETIC REDUNDANCY

Although heritable genetic variation is critical to the evolutionary process, we know little about how it is maintained. Obviously, mutation-selection balance must play a role, but there is considerable doubt over whether it can account for heritabilities as high as 0.5, which are commonly found in natural populations. Most models of mutation-selection balance assume panmictic populations. In this paper we use Monte Carlo simulations to examine the effect of isolation by distance on the variation maintained by mutation in a polygenic trait subject to optimizing selection. We show that isolation by distance can substantially increase the total variation maintained in continuous populations over a wide range of dispersal patterns, but only if more than one genotype produces the optimal phenotype (genetic redundancy). Isolation by distance alone has only a slight effect on the variation maintained in the total population for neutral alleles. The combined effect of isolation by distance and genetic redundancy, however, allows the maintenance of substantial variation despite strong stabilizing selection. The mechanism is straightforward. Isolation by distance allows mutation and drift to operate independently in different parts of the population. Because of their independent evolutionary histories, different parts of the population independently draw from the available set of redundant genotypes. Because the genotypes are redundant, selection does not discriminate among them, and they will persist until eliminated by drift. The population as a whole maintains many distinct genotypes. We show that this process allows mutation to maintain high levels of variation, even under strong stabilizing selection, and that over a moderate range of dispersal patterns the amount of variation maintained in the entire population is independent of both the strength of selection and the variance of the dispersal distance. Furthermore, we show that individual heterozygosity is increased in locally mating populations when selection is strong. Finally, our simulations provide a rough picture of how selection and the dispersal pattern influence the spatial distribution of genetic and phenotypic variation.     

ON THE BIOLOGY OF THE MITE, GLYCYPHAGUS DOMESTICUS DE GEER. (TYROGLYPHIDAE, ACARINA)

1. A list is given of the mites most commonly found in houses. The most important of these is Glycyphagus domesticus , the furniture mite, and this has been mainly studied during this investigation.
2. An account is given of the occurrence of G. domesticus in houses and the conditions under which it is found.
3. The life history is described. It is characterised by the occurrence of the hypopus, a cyst-like stage which is found only amongst the Tyroglyphidae.
5. Humidity and temperature have a marked effect on the life of the mite. Tables are given showing the effect of ( a ) humidity on the adult, ( b ) humidity and temperature on the hypopus and egg.
6. Some experiments on the effect of carbon tetrachloride and methyl salicylate on the hypopus are recorded.     

Habitat-specific genetic effects on growth rate and morphology across pH and water-level gradients within a population of the moss Sphagnum angustifolium (Sphagnaceae)

To study genetic adaptations in bryophytes on small ecological and spatial scales and to assess the adaptive significance of morphological trait variation, genotypes of Sphagnum angustifolium originating from habitats characterized by different pH and height above water table were clonally propagated and grown along the same gradients that exist in the field. Clones from ombrotrophic habitats grew consistently better ombrotrophically than clones from minerotrophic habitats and vice versa, suggesting that the genotypes were adapted to different pH levels. Genetic variation was found in several morphological traits, but habitat-specific genetic effects were detected only in length of spreading branches. Covariation between morphology and growth was generally environmentally induced. Positive and negative cross-environment genetic correlations suggested the presence of constraints on adaptive reaction norm evolution. The indications of small-scale genetic adaptations suggest either selective establishment of genotypes adapted to specific habitats, strong selective forces operating at the later stages of the life cycle, restricted gene flow over short distances, or a combination of these. In contrast to prevailing views, these results indicate that bryophytes are likely to respond genetically to small-scale environmental gradients.     

Testing for microevolution in body size in three blue tit populations

Quantifying the genetic variation and selection acting on phenotypes is a prerequisite for understanding microevolutionary processes. Surprisingly, long-term comparisons across conspecific populations exposed to different environments are still lacking, hampering evolutionary studies of population differentiation in natural conditions. Here, we present analyses of additive genetic variation and selection using two body-size traits in three blue tit (Parus caeruleus) populations from distinct habitats. Chick tarsus length and body mass at fledging showed substantial levels of genetic variation in the three populations. Estimated heritabilities of body mass increased with habitat quality. The poorer habitats showed weak positive selection on tarsus length, and strong positive selection on body mass, but there was no significant selection on either trait in the good habitat. However, there was no evidence of any microevolutionary response to selection in any population during the study periods. Potential explanations for this absence of a response to selection are discussed, including the effects of spatial heterogeneity associated with gene flow between habitats.     

Comparison of genetic variation between the seed bank and above ground vegetation of a wetland species

The seed bank is thought to be important for the conservation and restoration of the genetic diversity of the above ground plant population. It has been proposed that it may be no use to compare the genetic diversity between seed bank and above ground plants if there was no habitat fragmentation. Whether this prediction holds for aquatic plants is still unknown. Here, ISSR analysis of a common wetland species Heleocharis valleculosa (Cyperaceae) was conducted to evaluate if the seed bank has higher, equal or lower genetic variation than the above ground vegetation. Our results showed high genetic diversity in the seed bank, so the seedbank can act as a storehouse of genetic variation for the above ground vegetation, especially in unfavorable conditions. Thus, unlike terrestrial habitats where genetic diversity is often low, wetlands that are connected with water flow can accumulate many genotypes in the seed bank and the question of how genetic diversity varies above and below ground is still open.     

The Conservation Value of Peripheral Populations and a Relationship Between Quantitative Trait and Molecular Variation

The adaptive potential of populations and therefore their ability to cope with rapid environmental changes is a question of paramount fundamental and applied importance. However, what is still not clear is the effect of population position within the species range (i.e. core vs. edge) on population adaptive potential, and whether the adaptive potential can be predicted from extent of neutral molecular variation. In this study, we compared the extent and structure of neutral (SSR) and presumably adaptive quantitative trait genetic variation in populations of Triticum dicoccoides sampled at the species range core and two opposite edges, and related this information to multigenerational performance of plants experimentally introduced beyond the range edge. The plants from the species arid edge performed worse than plants from the more mesic core in extreme desert conditions. The core and edge populations did not differ in extent of SSR variation. In contrast to the neutral genetic variation, there was lower quantitative trait variation in the two edge as compared with the core population for many traits, and no trait in any edge population had higher variation than the core population or either of its habitats. Reduced variation in selectively important traits indicates a lower adaptive potential of the two edge as compared with the core population. Our results imply (1) that extent of variation in quantitative traits can predict plant performance in novel environments while extent of variation in molecular markers can not; and (2) caution in usage of peripheral populations in such conservation actions as relocation and creation of new populations. We also warn against usage of neutral molecular variation as a surrogate for selectively important quantitative variation in conservation decisions.     

Trees to treehoppers: genetic variation in host plants contributes to variation in the mating signals of a plant‐feeding insect

Community genetics research has demonstrated ‘bottom‐up’ effects of genetic variation within a plant species in shaping the larger community with which it interacts, such as compositions of arthropod faunas. We demonstrate that such cross‐trophic interactions also influence sexually selected traits. We used a member of the Enchenopa binotata species complex of treehoppers (Hemiptera: Membracidae) to ask whether male mating signals are influenced by host plant genetic variation. We reared a random sample of the treehoppers on potted replicates of a sample of host plant clone lines. We found that treehopper male signals varied according to the clone line on which they developed, showing that genetic variation in host plants affects male treehoppers' behavioural phenotypes. This is the first demonstration of cross‐trophic indirect genetic effects on a sexually selected trait. We discuss how such effects may play an important role in the maintenance of variation and within‐population phenotypic differentiation, thereby promoting evolutionary divergence.     

Genetic variation underlying the expression of a polyphenism

Polyphenic traits are widespread and represent a conditional strategy sensitive to environmental cues. The environmentally cued threshold (ET) model considers the switchpoint between alternative phenotypes as a polygenic quantitative trait with normally distributed variation. However, the genetic variation for switchpoints has rarely been explored empirically. Here, we used inbred lines to investigate the genetic variation for the switchpoint in the mite Rhizoglyphus echinopus, in which males are either fighters or scramblers. The conditionality of male dimorphism varied among inbred lines, indicating that there was genetic variation for switchpoints in the base population, as predicted by the ET model. Our results also suggest a mixture between canalized and conditional strategists in R. echinopus. We propose that major genes that canalize morph expression and affect the extent to which a trait can be conditionally expressed could be a feature of the genetic architecture of threshold traits in other taxa.     

The maintenance of phenotypic and genetic variation in threshold traits by frequency-dependent selection

Many traits are phenotypically dimorphic but determined by the action of many loci, the phenotype being a result of a threshold of sensitivity. Quantitative genetic analysis has shown that generally there is considerable additive genetic variation for the trait, the average heritability being 0.52. In numerous cases threshold traits have been shown, or are assumed, to be under frequency-dependent selection; examples include satellite-territorial behaviour, sex-determination, wing dimorphism and trophic dimorphism. In this paper I investigate the potential for frequency-dependent selection to maintain both phenotypic and additive genetic variation in threshold traits. The qualitative results are robust to the particular form of the frequency-dependent selection function. The equilibrium proportion is more or less independent of population size but the heritability increases with population size, typically approaching its maximal value at a population size of 5000, when the mutation rate is 10^?4. A tenfold decrease in the mutation rate requires an approximate doubling of the population size before an asymptotic value is approached. Thus frequency-dependent selection can account for both the existence of two morphs in a population and the observed levels of heritability. It is also shown, both via simulation and theory, that the quantitative genetic model and a simple phenotypic analysis predict the same equilibrium morph proportion.     

Variability in life history traits of the aphid,Acyrthosiphon pisum (Harris), from sexual and asexual populations

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7–13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.     

Intra-specific variation in relative growth rate: impact on competitive ability and performance of Lychnis flos-cuculi in habitats differing in soil fertility

Plant species from unproductive or adverse habitats are often characterized by a low potential relative growth rate (RGR). Although it is generally assumed that this is the result of selection for specific trait combinations that are associated with a low rate of net biomass accumulation, few studies have directly investigated the selective (dis-)advantage of specific growth parameters under a set of different environmental conditions. Aim of the present study was to quantify the impact of inherent differences in growth parameters among phenotypes of a single plant species, Lychnis flos-cuculi, on their performance under different soil nutrient conditions. Growth analysis revealed significant variation in RGR among progeny families from a diallel cross between eight genotypes originating from a single population. Differences in RGR were due to variation in both leaf area ratio (LAR) and in net assimilation rate (NAR). A genetic trade-off was observed between these two components of growth, i.e. progeny families with high investment in leaf area had a lower rate of net biomass accumulation per unit leaf area. The degree of plasticity in RGR to nutrient conditions did not differ among progeny families. Inherent differences in growth parameters among progeny families had a significant impact on their yield in competition with Anthoxanthum odoratum and Taraxacum hollandicum. In nutrient-rich conditions, progeny families with an inherently high leaf weight ratio (LWR) achieved higher yield in competition, but variation in this trait could not explain differences in competitive yield under nutrient-poor conditions. Inherent differences in growth parameters among progeny families were poorly correlated with differences in survival and average rosette biomass (a good predictor of fecundity) among these progeny families sown in four field sites along a natural gradient of soil fertility. In the more productive sites none of the growth parameters was significantly correlated with rosette biomass, but in the least productive site progeny families with an inherently high specific leaf area (SLA) tended to produce smaller rosettes than low-SLA families. These results are consistent with the view that a selective advantage may accrue from either high or low values of individual RGR components, depending on habitat conditions, and that the selective advantage of low trait values in nutrient-poor environments may results in indirect selection for low RGR in these habitats.     

Morphological divergence and flow‐induced phenotypic plasticity in a native fish from anthropogenically altered stream habitats

Understanding population‐level responses to human‐induced changes to habitats can elucidate the evolutionary consequences of rapid habitat alteration. Reservoirs constructed on streams expose stream fishes to novel selective pressures in these habitats. Assessing the drivers of trait divergence facilitated by these habitats will help identify evolutionary and ecological consequences of reservoir habitats. We tested for morphological divergence in a stream fish that occupies both stream and reservoir habitats. To assess contributions of genetic‐level differences and phenotypic plasticity induced by flow variation, we spawned and reared individuals from both habitats types in flow and no flow conditions. Body shape significantly and consistently diverged in reservoir habitats compared with streams; individuals from reservoirs were shallower bodied with smaller heads compared with individuals from streams. Significant population‐level differences in morphology persisted in offspring but morphological variation compared with field‐collected individuals was limited to the head region. Populations demonstrated dissimilar flow‐induced phenotypic plasticity when reared under flow, but phenotypic plasticity in response to flow variation was an unlikely explanation for observed phenotypic divergence in the field. Our results, together with previous investigations, suggest the environmental conditions currently thought to drive morphological change in reservoirs (i.e., predation and flow regimes) may not be the sole drivers of phenotypic change.     

Shared and unique morphological responses of stream fishes to anthropogenic habitat alteration

Understanding population-level responses to novel selective pressures can elucidate evolutionary consequences of human-altered habitats. Stream impoundments (reservoirs) alter riverine ecosystems worldwide, exposing stream fishes to uncommon selective pressures. Assessing phenotypic trait divergence in reservoir habitats will be a first step in identifying the potential evolutionary and ecological consequences of stream impoundments. We tested for body shape divergence in four stream-adapted fishes found in both habitats within three separate basins. Shape variation among fishes was partitioned into shared (exhibited by all species) and unique (species-specific) responses to reservoir habitats. All fishes demonstrated consistent significant shared and unique morphological responses to reservoir habitats. Shared responses were linked to fin positioning, decreased body depths and larger caudal areas; traits likely related to locomotion. Unique responses were linked to head shape, suggesting species-specific responses to abiotic conditions or changes to their trophic ecology in reservoirs. Our results highlight how human-altered habitats can simultaneously drive similar and unique trait divergence in native populations.     

Maintenance of diapause variability in the two-spotted spider mite,tetranychus urticae, in a heterogeneous and stochastic environment

A simple nongenetic mathematical model analyzed the processes responsible for the variations in the diapause percentage among populations of the two-spotted spider mite,Tetranychus urticae Koch. This model incorporates the following assumptions. 1) Mites have diapause (DD), non-diapause (NN), and “plastic” (DN) populations (plasticity exists in the phenotypic expression of diapause in response to habitat conditions at the time of diapause induction). 2) A heterogeneous mite habitat consists of microhabitat L, in which all the non-diapausing mites die during the winter due to the lack of winter host plants, and microhabitat O with winter hosts capable of supporting some of the non-diapausing mites overwinter. 3) Temporal fluctuation of winter conditions which affect the survival and reproduction of non-diapausing mites. Using these assumptions, we compared the fitness functions of the three populations and analyzed the conditions under which each population is favoured over the other two, thereby elucidating the processes involved in the maintenance of variability in diapause. Our analysis revealed: 1) frequent mild winters are of primary importance for the non-diapause trait to be maintained, 2) the existence of winter hosts is also important for the nondiapause trait to be favoured, and this importance depends greatly on the degree of the adaptive diapause expression in the DN mites in response to habitat conditions, i.e., the better the phenotypeenvironment matching in DN, the higher the probability that DN will be favoured, 3) The combined effect of the temporal and spatial variation enhances the maintenance of variablity in the diapause trait of the mites.