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.     

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.     

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.     

How differing modes of non‐genetic inheritance affect population viability in fluctuating environments

Different modes of non‐genetic inheritance are expected to affect population persistence in fluctuating environments. We here analyse Caenorhabditis elegans density‐independent per capita growth rate time series on 36 populations experiencing six controlled sequences of challenging oxygen level fluctuations across 60 generations, and parameterise competing models of non‐genetic inheritance in order to explain observed dynamics. Our analysis shows that phenotypic plasticity and anticipatory maternal effects are sufficient to explain growth rate dynamics, but that a carryover model where ‘epigenetic’ memory is imperfectly transmitted and might be reset at each generation is a better fit to the data. We further find that this epigenetic memory is asymmetric since it is kept for longer when populations are exposed to the more challenging environment. Our analysis suggests that population persistence in fluctuating environments depends on the non‐genetic inheritance of phenotypes whose expression is regulated across multiple generations.     

DOES ENVIRONMENTAL ROBUSTNESS PLAY A ROLE IN FLUCTUATING ENVIRONMENTS?

Fluctuating environments are expected to select for individuals that have highest geometric fitness over the experienced environments. This leads to the prediction that genetically determined environmental robustness in fitness, and average fitness across environments should be positively genetically correlated to fitness in fluctuating environments. Because quantitative genetic experiments resolving these predictions are missing, we used a full‐sib, half‐sib breeding design to estimate genetic variance for egg‐to‐adult viability in Drosophila melanogaster exposed to two constant or fluctuating temperatures that were above the species’ optimum temperature, during development. Viability in two constant environments (25°C or 30°C) was used to estimate breeding values for environmental robustness of viability (i.e., reaction norm slope) and overall viability (reaction norm elevation). These breeding values were regressed against breeding values of viability at two different fluctuating temperatures (with a mean of 25°C or 30°C). Our results based on genetic correlations show that average egg‐to‐adult viability across different constant thermal environments, and not the environmental robustness, was the most important factor for explaining the fitness in fluctuating thermal environments. Our results suggest that the role of environmental robustness in adapting to fluctuating environments might be smaller than anticipated.     

Does fluctuating salinity induce branching of Fucus vesiculosus?

During the field surveys in the Bothnian Sea, i.e. towards low salinity areas (approx. 4), present authors observed that the frequency of irregularly branched Fucus vesiculosus plants increased. Salinity is known to decrease gradually towards the northern parts of the Baltic Sea. However, salinity is not steady but may fluctuate greatly on an annual and even a daily scale, and salinity can drop to zero for short periods. In order to demonstrate whether the fluctuating salinity induces irregular branching of F. vesiculosusus, a experiment was carried out in the Tvärminne archipelago, on the south coast of Finland in May–September 1997. First, plants were collected and then put into two containers both of which had a fresh water and a sea water flow throught. After 48 h of treatment, the plants were removed to the same place where they had been orginally collected. After the growing season, the plants were collected again, and the number of irregularly and normally branched tips were measured. The results shows that plants with the fresh water treatment have branched irregularly. In constrast, the control plants had only a few irregularly branched tips. This experiment brings us to conclude that low salinity during the critical growing season induces irregular branching.     

Experimental evolution of evolutionary potential in fluctuating environments

Variation is the raw material for evolution. Evolutionary potential is determined by the amount of genetic variation, but evolution can also alter the visibility of genetic variation to natural selection. Fluctuating environments are suggested to maintain genetic variation but they can also affect environmental variance, and thus, the visibility of genetic variation to natural selection. However, experimental studies testing these ideas are relatively scarce. In order to determine differences in evolutionary potential we quantified variance attributable to population, genotype and environment for populations of the bacterium Serratia marcescens. These populations had been experimentally evolved in constant and two fluctuating environments. We found that strains that evolved in fluctuating environments exhibited larger environmental variation suggesting that adaptation to fluctuations has decreased the visibility of genetic variation to selection.     

Parametric and jackknife confidence interval estimators for two-factor mating design genetic variance ratios

Summary Confidence interval estimators have not been defined for dominance to additive genetic variance () and average degree of dominance () for the nested, factorial, and backcross mating designs. The objective of this paper was to describe interval estimators for these parameters. Approximate F random variables were defined for expected mean square (EMS) ratios for linear models with one environmental effect. Approximate 1– parametric interval estimators were defined for and using these random variables. Random variables defined for linear models with no environmental effects are not approximately distributed as F random variables because common EMS are involved in the numerators and denominators of the EMS ratios. Delete-one jackknife (jackknife) interval estimators were defined for and for linear models with zero or one environmental effect(s); In transformed analysis of variance point estimates were used in pseudovalue estimators.Oregon Agricultural Experiment Station Technical Paper No. 8067     

The potential role of phoresy in the evolution of parasitism: radiolabelling (tritium) evidence from an astigmatid mite

Using tritium as a radiolabel marker of interspecific fluid transfer, we present experimental evidence that the heteromorphic deutonymph of an astigmatid mite (Hemisarcoptes cooremani) acquires materials (at least water) directly from the haemolymph of its beetle host (Chilocorus cacti). This acquisition is above that obtained from atmospheric vapour. The material acquired from the host is necessary for the completion of the ontogeny of H. cooremani and is likely procured through the action of the caudal ventral suckers of the heteromorphic deutonymph (hypopus). On gross morphological criteria, this mite-beetle relationship was previously defined as phoretic (for dispersal). Scanning electron photomicrographs of the physical relationship between the hypopodes and the heetles shed light on the parasitic nature of the hypopus of H. cooremani. Our findings are discussed in terms of the evolution of parasitism from a free-living astigmatid form. This transition into parasitism is facilitated by the heteromorphic hypopus and represents a classic wolf-insheep's-clothing strategy. The heteromorph retains the characteristic phoretic morphology while exploiting the host in transit.     

Genotype-by-environment interaction and the genetic covariance between environments: multilocus genetics

Explicit multi-locus expressions for the genetic variance of a phenotypically plastic trait over a range of environments are derived. The link becomes apparent between the genetic covariance, the genotype-by-environment interaction variance and the variance for the main effect genotype in a two-factor analysis of variance over two environments and many known genotypes. The genetic correlation can be +1 even if the genotype-by-environment interaction variance is greater than zero.     

Negative frequency‐dependent selection maintains coexisting genotypes during fluctuating selection

Natural environments are rarely static; rather selection can fluctuate on timescales ranging from hours to centuries. However, it is unclear how adaptation to fluctuating environments differs from adaptation to constant environments at the genetic level. For bacteria, one key axis of environmental variation is selection for planktonic or biofilm modes of growth. We conducted an evolution experiment with Burkholderia cenocepacia, comparing the evolutionary dynamics of populations evolving under constant selection for either biofilm formation or planktonic growth with populations in which selection fluctuated between the two environments on a weekly basis. Populations evolved in the fluctuating environment shared many of the same genetic targets of selection as those evolved in constant biofilm selection, but were genetically distinct from the constant planktonic populations. In the fluctuating environment, mutations in the biofilm‐regulating genes wspA and rpfR rose to high frequency in all replicate populations. A mutation in wspA first rose rapidly and nearly fixed during the initial biofilm phase but was subsequently displaced by a collection of rpfR mutants upon the shift to the planktonic phase. The wspA and rpfR genotypes coexisted via negative frequency‐dependent selection around an equilibrium frequency that shifted between the environments. The maintenance of coexisting genotypes in the fluctuating environment was unexpected. Under temporally fluctuating environments, coexistence of two genotypes is only predicted under a narrow range of conditions, but the frequency‐dependent interactions we observed provide a mechanism that can increase the likelihood of coexistence in fluctuating environments.     

Variable carbon isotope ratios of Dudleya species growing in natural environments

Summary Measurements are reported of carbon isotope ratios of Dudleya species growing in natural plant communities. Considerable variation in the ¹³C values are interpreted as indicating substantial flexibility of the photosynthetic pathways between C₃ and CAM. The variability in photosynthetic pathway was in response to genetic factors, stage of plant development, life-form, and environmental conditions. Species active during drought periods have less carbon isotope fractionation than species that are summer-dormant. Summer-active species from drier habitats have less negative ¹³C values than those from more mesic sites. On the same plant, leaf tissue had more negative ¹³C values than tissue from the inflorescence. The less negative carbon isotope ratios are indicative of an increased proportion of exogenous CO₂ fixed in dark vs light. The ecological significance of these results is discussed.CIW-DPB Publication No. 522     

Relationships of leaf diffusion resistance of Populus clones to leaf water potential and environment

Summary Leaf diffusion resistance (r ₁) of the upper and lower leaf surfaces of several Populus clones was related to leaf water potential (₁), light intensity, vapor pressure deficit (VPD), and temperature by intrinsicallylinear, logarithmic multiple regression analyses. Regression equations accounted for up to 80% of variation in r ₁ data. Light intensity and VPD varied among clones in importance in influencing r ₁. Pronounced sensitivity of r ₁ of certain clones to VPD was related to drought resistance in their parentage. Increasing r ₁ was significantly positively correlated with ₁, in apparent contradiction to prevailing concepts of stomatal response to water status, and this relationship was probably attributable to effects of other environmental variables on ₁ and r ₁. Leaf resistance decreased after a storm characterized by winds in excess of 160 km·h^-1. Cuticular disruption and altered stomatal response may have been responsible for the storminduced r ₁ decrease.     

Drought and drought tolerance

Drought tolerance is a nebulous term that becomes more nebulous the more closely we look at it, much as a newspaper photograph does when viewed through a magnifying glass. From the vantage point of an ecologist the features that distinguish xerophytic from mesophytic vegetation are clear. We can all tell that a cactus is more drought tolerant than a carnation. But when we look at crop plants, the features that confer drought tolerance are far from clear. The main reason for the contrast is that the traits we associate with xerophytes typically concern survival during drought, whereas with crops we are concerned with production—and insofar as the term drought tolerance has any useful meaning in an agricultural context, it must be defined in terms of yield in relation to a limiting water supply.Further, with the well-developed major crop plants, those of us trying to increase water-limited yield would be pleased to achieve improvements of just a few percent in environments that are highly variable in their water supply. This variability often means that several seasons are required to demonstrate the advantages of an allegedly improved cultivar. Traits that confer drought tolerance in such circumstances are subtle, and may manifest themselves in some types of drought but not in others. Indeed the most influential characters often have no direct connection to plant water relations at all, as I elaborate on below.I will concentrate on the agricultural rather than the natural environment (although there are no doubt lessons for us still to learn from analysing the behaviour of natural vegetation—see Monneveux, this volume), and will argue that drought tolerance is best viewed at an ontogenetic time scale—i.e. at the time scale of the development of the crop—weeks to months for an annual crop. The timing of the main developmental changes, like floral initiation and flowering, and the rate of development of leaf area in relation to the seasonal water supply, are the most important variables at this time scale. Occasionally though, rapid changes in the environment, such as a sudden large rise in air temperature and humidity deficit, perhaps associated with hot dry winds, make appropriate short-term physiological and biochemical responses essential for the survival of the crop. These short term responses may be amenable to cellular and sub-cellular manipulation, especially if the sudden environmental deterioration occurs at especially sensitive stages in development such as pollen meiosis or anthesis.Purists insist that drought is a meteorological term that refers only substantial to periods in which rainfall fails to keep up with potential evaporation. Within the spirit of this meeting it is appropriate to interpret the term more loosely than this definition, and to define it as circumstances in which plants suffer reduced growth or yield because of insufficient water supply, or because of too large a humidity deficit despite there being seemingly adequate water in the soil.     

Expression of a dispersal trait in a guild of mites colonizing transient habitats

Summary Dispersal as a means of escape from deteriorating habitats is of particular ecological relevance for organisms such as certain astigmatic mites that colonize habitats which vary unpredictably in space and time. The mites meet these ecological challenges by a facultative dispersal morph, the heteromorphic deutonymph, also called hypopus. The appearance or absence of hypopodes in natural populations is attributable to two fundamentally different, albeit interacting, causes. Genetic polymorphism for the propensity to induce a hypopus provides for heritable variation within the population and allows selection to favor or eliminate certain genotypes. The genotypic composition of a population reflects selection forces previously acting on the population. But it holds no predictive power. Rather, it adapts the population to cope with unpredictably varying living conditions because it ensures instantaneous fit of certain genotypes of the population (those displaying hypopus-free development) to favorable (moist) environmental conditions, and others (those expressing a hypopus) to detrimental (dry) conditions. In contrast, environmentally cued inducibility allows mites to anticipate food quality inasmuch as it allows each genotype of the population to adjust its development rapidly to impending adversity or benefit. Inducibility occurs by means of a developmental switching mechanism and leads either to a developmental pathway with a hypopus or else one without. The expression of a hypopus depends on interacting genetic and environmental (trophic) factors. High levels of additive genetic variation combine with considerable genetic-trophical interaction (comprising a threshold for phenotypic expression of the trait) to control hypopus induction. The results are consistent with a variable threshold whose level depends on diet quality. Different trophic conditions set the threshold at different points along the genetic scale resulting in different proportions of hypopus-forming and directly developing individuals within the population. The threshold, therefore, converts the concealed continuous genetic variation underlying the trait into a discontinuous response of the mite.     

Comparative mapping in F2∶3 and F6∶7 generations of quantitative trait loci for grain yield and yield components in maize

This study was conducted to compare maize quantitative trait loci (QTL) detection for grain yield and yield components in F₂₃ and F₆₇ recombinant inbred (RI) lines from the same population. One hundred and eighty-six F₆₇ RIs from a Mo17×H99 population were grown in a replicated field experiment and analyzed at 101 loci detected by restriction fragment length polymorphisms (RFLPs). Single-factor analysis of variance was conducted for each locus-trait combination to identify QTL. For grain yield, 6 QTL were detected accounting for 22% of the phenotypic variation. A total of 63 QTL were identified for the seven grain yield components with alleles from both parents contributing to increased trait values. Several genetic regions were associated with more than one trait, indicating possible linked and/or pleiotropic effects. In a comparison with 150 F₂₃ lines from the same population, the same genetic regions and parental effects were detected across generations despite being evaluated under diverse environmental conditions. Some of the QTL detected in the F₂₃ seem to be dissected into multiple, linked QTL in the F₆₇ generation, indicating better genetic resolution for QTL detection with RIs. Also, genetic effects at QTL are smaller in the F₆₇ generation for all traits.Abbreviations RFLPs Restriction fragment length polymorphisms - QTL quantitative trait loci - RIs recombinant inbreds Journal Paper no. J-16261 of the Iowa Agric and Home Economics Exp Stn Project no. 3134     

Relationship between ‘early’ and ‘late’ protein induced by 20-hydroxyecdysone in imaginal wing discs ofDrosophila melanogaster

The insect moulting hormone, 20-hydroxyecdysone, induces the synthesis of two groups of proteins in the imaginal wing discs ofDrosophila melanogaster. The early induced group appears about 4 h after hormone treatment, the late induced group appears about 12 h after treatment. Studies using -amanitin to inhibit mRNA synthesis during the period of hormonal treatment showed that the induction of the late proteins is dependent on mRNA synthesis and possibly the synthesis of the early proteins.     

Increased photosynthesis and water potentials in Silphium integrifolium galled by cynipid wasps

Interactions between drought, insect herbivory, photosynthesis, and water potential play a key role in determining how plants tolerate and defend against herbivory, yet the effects of insect herbivores on photosynthesis and water potential are seldom assessed. We present evidence that cynipid wasp galls formed by Antistrophus silphii on Silphium integrifolium increase photosynthesis (A), stomatal conductance (g), and xylem water potential (). Preliminary data showed that in drought-stressed plants galled shoots had 36% greater A, and 10% greater stem than ungalled shoots, while in well-watered plants leaf gas exchange was not affected by galls. We hypothesized that 1) galled shoots have higher , g, and A than ungalled shoots, but this differences diminishes if plant drought stress is reduced, and 2) galls can reduce decreases in A and g if water availability decreases. A field experiment testing the first hypothesis found that galls increased g and , but that differences between galled and ungalled shoots did not diminish after plants were heavily watered. A laboratory test of the second hypothesis using potted Silphium found that galled plants had smaller drops in A and g over a 4-day dry-down period. A vs g and A vs intercellular CO₂ concentration relationships were consistent with the explanation that increased allows galls to increase A by reducing stomatal limitation of A, rather than by altering sink-source relationships or by removing low- limitations on non-stomatal components of A. Our working hypothesis is that galls increase and A by reducing the shoot: root ratio so that the plant is exploiting a greater soil volume per unit leaf area. We argue that increased A is an ineffective way for Silphium to compensate for negative effects of gall insect attack. Instead, increased and A may protect gall insects from variation in resource availability caused by periodic drought stress, potentially reducing negative effects of drought on plant quality and on gall insect populations.     

Photosynthetic gas exchange response of poplars to steady-state and dynamic light environments

The steady-state and dynamic photosynthetic response of two poplar species (Populus tremuloides and P. fremontii) to variations in photon flux density (PFD) were observed with a field portable gas exchange system. These poplars were shown to be very shade intolerant with high light saturation (800 to 1300 mol photons m^–2 s^–1) and light compensation (70 to 100 mol m^–2 s^–1) points. Understory poplar leaves showed no physiological acclimation to understory light environments. These plants become photosynthetically induced quickly (10 min). Activation of Rubisco was the primary limitation for induction, with stomatal opening playing only a minor role. Leaves maintained high stomatal conductances and stomata were unresponsive to variations in PFD. Leaves were very efficient at utilizing rapidly fluctuating light environments similar to those naturally occurring in canopies. Post-illumination CO₂ fixation contributed proportionally more to the carbon gain of leaves during short frequent lightflecks than longer less frequent ones. The benefits of a more dynamic understory light environment for the carbon economy of these species are discussed.