W. Knülle

Water vapour uptake from subsaturated atmospheres by engorged immature ixodid ticks

Contrary to current opinion, fully engorged and detached larvae and nymphs of some ixodid ticks consistently take up substantial amounts of atmospheric water vapour and thereby display their regulative capacity for maintaining water balance in subsaturated air. Net uptake of vapour generally begins some days after detachment and the capability persists until shortly after initiation of apolysis, a period which in diapausing specimens may extend up to several months. This was shown forIxodes ricinus, Haemaphysalis punctata, and the North AmericanI. dammini. Apparently, some other engorged ixodid immatures fail to exhibit net vapour uptake, as was shown for both larvae and nymphs ofDermacentor marginatus and nymphs ofHyalomma anatolicum excavatum. But there is some evidence for engorged nymphs ofD. marginatus that active uptake of vapour does occur, masked by spiracular transpiration. Net uptake of vapour is apparently not possible during the pharate phases. InI. ricinus both teneral nymphs and adults are capable of achieving net water gains by active vapour uptake on the first day following ecdysis. There is new evidence from fully engorgedI. ricinus immatures for the decisive role of agranular alveoli in the production of the salivary secretion involved in vapour uptake.

Prevalence of borrelia burgdorferi in Ixodes ricinus ticks in Berlin (West)

In 1986, 1711 nymphal and adult Ixodes ricinus orginating from Berlin (West) forests were examined individually or in pools of up to 10 ticks for the presence of Borrelia burgdorferi, the etiologic agent of Lyme borreliosis. Detection of borreliae was carried out by means of a culture method using modified Barbour-Stoenner-Kelly-Medium (BSK II). Tick populations from 14 out of 15 locations contained positive specimens. The calculated minimal infection rate of pooled ticks was 2.5% in nymphs (n = 1365), 10.2% in females (n = 59), and 5.3% in males (n = 114). Among those ticks examined individually, none of the nymphs (n = 49) proved to be positive but B. burgdorferi was isolated from 8.2% of the females (n = 73) and 7.8% of the males (n = 51). Fifty-five out of 56 isolates were identified as B. burgdorferi by means of an indirect immunofluorescence test (IFT) using monoclonal antibody H 5332. From these results B. burgdorferi must be considered as being present in the Berlin area.

Genetic and environmental determinants of hypopus duration in the stored-product mite Lepidoglyphus 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.

Gross morphological changes in the salivary glands of Ixodes ricinus (Acari, Ixodidae) between bloodmeals in relation to active uptake of atmospheric water vapour.

The gross morphological changes in the salivary glands ofIxodes ricinus (L.) were investigated at the light microscopic level in various phases off the host with emphasis on the engorged nymph, in order to relate the capability of active vapour uptake in the course of postembryonal development to degeneration and regeneration of salivary-gland alveoli.Agranular alveoli in engorged immatures ofI. ricinus, from detachment to the following early pharate phase, do not appear different from those of the unfed instars. This is also true for the female up to approximately the end of oviposition. During moulting, the agranular alveoli of the immatures degenerate and new ones are formed which are apparently already functional in teneral nymphs and adults. In contrast, granular alveoli, much enlarged in freshly detached immatureI. ricinus, shrivel in the early post-repletion period and soon reach a highly reduced state which is maintained until apolysis. Subsequently, they disintegrate completely.The finding that engorged and detached immatures ofI. ricinus with markedly atrophied granular alveoli are capable of active vapour uptake until some days after initiation of apolysis suggests that only agranular alveoli are responsible for producing the primary secretion involved in vapour uptake.

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.

INTERACTION BETWEEN GENETIC AND INDUCTIVE FACTORS CONTROLLING THE EXPRESSION OF DISPERSAL AND DORMANCY MORPHS IN DIMORPHIC ASTIGMATIC MITES

Abstract 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.

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

SummaryDispersal 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.

Water vapour uptake from the atmosphere and critical equilibrium humidity of a feather mite

The feather miteProctophyllodes troncatus Robin takes up water vapour from subsaturated atmospheres down to relative humidities between 55% and 60%. Vapour uptake increases with rising humidity of the surrounding air and the time to attain maximum water gain decreases.

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.

Embryonic diapause and cold hardiness of Ixodes ricinus eggs (Acari: Ixodidae)

Ixodes ricinus is medically the most important tick species in temperate Europe. The present study was undertaken to assess the effect of photoperiod and low-temperature exposure on termination of embryonic diapause and on cold hardiness in I. ricinus eggs. Engorged I. ricinus females (n = 9) were kept at 90% r.h., 15 °C longday (L17:D7), or 15 °C shortday (L14:D10), or at an outdoor site providing natural temperatures and daylength. Eggs of defined ages were continuously kept under these conditions or subjected to changes in photoperiod and temperature, and the effect on diapause incidence was observed. At 15 °C, non-diapause and diapause eggs hatched after 70-120 and 140-240 days, respectively. Interestingly, most egg masses did not show an all or nothing reaction but a certain percentage of diapause eggs. Cold exposure of eggs to 4 °C for 6 weeks completely terminated diapause, whereas the effect of photoperiod was negligible. Also exposure to temperatures between -10 and -20 °C for 24 h terminated diapause. At the outdoor site, eggs were laid in winter, from mid October to early April. All eggs hatched between mid June and early July. The supercooling point of these eggs was around -28 °C between November and January and rose to -27 °C in April. At constant 15 °C, the supercooling point rose from -30 °C in young eggs to -24 °C in physiologically older eggs. The lower lethal temperature in diapause and non-diapause eggs was -21.6 and -18.0 °C, respectively, but the difference was not significant.