Night length measurements by the circadian clock controlling diapause induction in the mosquito Aedes atropalpus

Night interruption experiments were used to investigate the behavior of the clock controlling diapause induction in the mosquito, Aedes atropalpus. The data from these experiments indicated that the clock included a circadian oscillator which was phase set at dusk. Following this event the oscillator proceeded to drive a nightly rhythm of sensitivity to light. This rhythm included a photoinducible phase where light interruption inhibited diapause. The photoinducible phase was fixed, occurring 7 to 9 hr after dusk in all photoperiod regimens tested. The photoinducible phase was followed by a refractory phase, which continued until dawn. During the refractory period light did not inhibit diapause. These observations indicated that the circadian clock behaved like an interval timer which was set at dusk. The rhythm of sensitivity to light, an inherited time scale, limited the induction of diapause to seasonal periods when nights were longer than 9 hr. As a result, diapause was induced only when the daylength dropped below the critical photoperiod of L15:D9 (hours of light:hours of dark).A ‘T’ experimental design was used to confirm the importance of the length of the night in clock controlled induction of diapause in this mosquito.     

Feedback regulation of vitellogenin synthesis in Aedes aegypti and Aedes atropalpus

Blood-fed hemiovariectomized Aedes aegypti synthesized vitellogenin at the same rate as intact females. Removal of both developing ovaries from blood-fed females diminished vitellogenin synthesis gradually. To determine the concentration of circulating vitellogenin, vitellin was purified using hydrophobic affinity chromatography. A titration curve of antibody-antigen complex was developed and the concentration of vitellogenin circulating in ovariectomized females was found to be five-fold higher than that in intact females.Ovaries removed from vitellogenic donors did not induce vitellogenin synthesis in non-vitellogenic (blood-fed, decapitated) recipients. Implanting a pinocytotic ovary into blood-fed females in which vitellogenesis was arrested by ovariectomy, restored vitellogenin synthesis.These results indicate that vitellogenin synthesis in Aedes aegypti is controlled by a feedback-inhibition mechanism, rather than an ovarian factor, and the ovary acts as a sink.     

The present distribution and predicted geographic expansion of the floodwater mosquito Aedes sticticus in Sweden

The mass emergence of floodwater mosquitoes, in particular Aedes sticticus and Aedes vexans, causes substantial nuisance and reduces life quality for inhabitants of infested areas and can have a negative impact on the socio‐economic conditions of a region. We compared the previous, present, and predicted geographic distribution of Ae. sticticus in Sweden. Previous records from the literature until 1990 list the species in three out of 21 Swedish counties. Beginning in 1998, studies show that the present distribution of the species covers 11 counties, with highest abundances in an east‐west belt in Central Sweden. Using climate data from the present and predicted climate scenarios, the expected distribution of Ae. sticticus in 2020, 2050, and 2080 could be modelled using GIS. As variables, mean temperatures and cumulative precipitation between May and August and degree slope were chosen. The predicted geographic distribution of Ae. sticticus will continue to increase and include 20 out of 21 Swedish counties. The expected temperature rise will increase the suitable area towards the northern part of Sweden by 2050. Some non‐suitable areas can be found along the south‐east coast due to insufficient amount of precipitation in 2050 and 2080. Modelling the expected distribution of a species using predicted climate change scenarios provides a valuable tool for risk assessments and early‐warning systems that is easily applied to different species and scenarios.     

Costs of reproduction and geographic variation in the reproductive tactics of the mosquito Aedes triseriatus

Intraspecific host discrimination is widespread in solitary parasitoids whose adult females forage for and evaluate host suitability, whereas interspecific discrimination is less common. In some parasitoid species, mostly Diptera and Coleoptera, the larva performs the last step of host searching. It has been suggested that host discrimination will rarely occur in such host-seeking larvae because their low mobility results in a low host encounter rate. We determined the extent to which the larvae of Aleochara bilineata Gyllenhal (Coleoptera: Staphylinidae), a solitary parasitoid of aggregated Diptera pupae: (1) discriminated between unparasitized hosts and hosts parasitized by conspecifics; (2) used semiochemical cues to discriminate; (3) were influenced by life expectancy, presence of conspecifics and host availability in their host acceptance decision; and the extent to which (4) A. bilineata and A. bipustulata L., a species exploiting the same hosts and occurring sympatrically, showed interspecific host discrimination. A. bilineata larvae were able to discriminate between unparasitized hosts and hosts parasitized by conspecifics in a choice experiment. Such behavior has never previously been described for a coleopteran parasitoid or for a parasitoid species whose larvae perform host searching. Host discrimination in this species was not based on the presence of visual or tactile cues (e.g., entrance holes) but rather on chemical cues. The life expectancy of A. bilineata larvae was significantly shorter in the presence than in absence of hosts, and older larvae had lower parasitism success than young larvae in a 24-h experiment. However, the host acceptance decision of A. bilineata larvae was not influenced by larval age or the presence of conspecifics when the ratio of hosts per larva was greater than or equal to 1. When hosts were scarce, the degree of superparasitism increased significantly with the number of foraging conspecifics and the age of the larvae. Both species of Aleochara showed intra- and interspecific host discrimination in a choice experiment. In contrast to A. bipustulata, A. bilineata larvae more frequently parasitized hosts parasitized by A. bipustulata than those parasitized by conspecifics. We suggest that host discrimination will be frequent in solitary parasitoids with host-seeking larvae when hosts are aggregated. Received: 4 June 1998 / Accepted: 1 September 1998     

Polygenic regulation of fecundity in autogenous Aedes atropalpus

Genetic regulation of autogenous fecundity was examined in the mosquito Aedes atropalpus. Only one of four subspecies was found to contain all the genetic factors needed to produce, without blood-feeding, an average in excess of 150 eggs/female. This was the type form, A. a. atropalpus, which possessed the gene for autogeny (A) and a genetic background which enhanced fecundity. In contrast, strains of A. a. epactius, A. a. perichares and A. a. nielseni lacked both genetic systems. When the genome of A. a. atropalpus was progressively replaced by repeated crossing to the other subspecies, a stepwise decrease in autogenous fecundity was noted over several generations. However, this trend was stopped and in certain cases reversed by back-crossing to the type form. Thus it appears that in the type-form genome there are several modifier genes which enhance the expression of the gene A.

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Résumé Aedes atropalpus dont l'aire d'extension s'étend du Labrador au Panama présente quatre sous-espèces, dont une seule, la forme-type A. a. atropalpus, est autogène, les trois autres: A. a. epactius, A. a. perichares, A. a. nielseni étant anautogènes. Les femelles anautogènes sont homozygotes pour l'allèle récessif, a.Les hybrides F₁ issus de croisements entre les lignées autogènes et anautogènes sont autogènes, mais leur fécondité est réduite. Des croisements de retour répétés entre ces hybrides et les formes anautogènes aboutissent à une réduction progressive de la fécondité. Cependant cette réduction peut être stoppée et, dans certains cas, inversée par croisement de retour avec la forme-type autogène. Ces faits conduisent à envisager que le génome des femelles de type autogène contient, outre le gène dominant A, un certain nombre de gènes modificateurs qui en renforcent l'expression. Les trois sous-espèces anautogènes manqueraient de certains, sinon de tous ces gènes modificateurs. La réduction progressive de la fécondité chez les hybrides résulterait de la perte successive au cours des croisements de retour, de ces gènes modificateurs.

     

Transcript profiling reveals mechanisms for lipid conservation during diapause in the mosquito, Aedes albopictus

The Asian tiger mosquito, Aedes albopictus, is a medically important invasive species whose geographic distribution has expanded dramatically during the past 20 years, and one of the key elements of its success is its capacity to survive long distance transport as a diapausing pharate first instar larva, encased within the chorion of the egg. We report that pharate larvae entering diapause are larger and contain 30% more lipid than their nondiapausing counterparts. To improve our understanding of the molecular regulation of lipid metabolism during diapause, we assessed the relative mRNA abundance of 21 genes using qRT-PCR. Elevated expression of lipid storage droplet protein 2 during embryonic development likely contributes to the higher amounts of lipid we noted in diapausing individuals. The conservation of lipids during diapause is reflected in downregulation of genes involved in lipid catabolism, including lipase 2, lipase 3, lipase 4, acyl-CoA dehydrogenase 4, and isovaleryl-CoA dehydrogenase. Two genes involved in fatty acid synthesis and modification, Δ(9)-desaturase, and fatty acyl-CoA elongase, were both upregulated in diapausing pharate larvae, suggesting roles for their gene products in generating unsaturated fatty acids to enhance membrane fluidity at low temperatures and generating precursors to the surface hydrocarbons needed to resist desiccation, respectively. Together, the results point to substantial distinctions in lipid metabolism within the embryo as a consequence of the diapause program, and these differences occur both before the actual onset of diapause as well as during the diapause state.     

Genetic sexing strains for the population suppression of the mosquito vector Aedes aegypti

Aedes aegypti is the primary vector of arthropod-borne viruses including dengue, chikungunya and Zika. Vector population control methods are reviving to impede disease transmission. An efficient sex separation for male-only releases is crucial for area-wide mosquito population suppression strategies. Here, we report on the construction of two genetic sexing strains using red- and white-eye colour mutations as selectable markers. Quality control analysis showed that the Red-eye genetic sexing strains (GSS) is better and more genetically stable than the White-eye GSS. The introduction of an irradiation-induced inversion (Inv35) increases genetic stability and reduces the probability of female contamination of the male release batches. Bi-weekly releases of irradiated males of both the Red-eye GSS and the Red-eye GSS/Inv35 fully suppressed target laboratory cage populations within six and nine weeks, respectively. An image analysis algorithm allowing sex determination based on eye colour identification at the pupal stage was developed. The next step is to automate the Red-eye-based genetic sexing and validate it in pilot trials prior to its integration in large-scale population suppression programmes.This article is part of the theme issue ‘Novel control strategies for mosquito-borne diseases’.     

Identification of two mariner-like elements in the genome of the mosquito Ochlerotatus atropalpus

Two distinct mariner-like elements, Atmar-1 and Atmar-2, were isolated from the genome of the mosquito Ochlerotatus atropalpus. Full-sized Atmar-1 elements, obtained by screening a genomic library, have a 1293-bp consensus sequence with 27-bp inverted terminal repeats and a 1047-bp open reading frame (ORF) encoding the transposase. The Atmar-2 elements were amplified by polymerase chain reaction from genomic DNA and contain the central part of the transposase ORF. Individual clones of both mariner elements contain deletions, frameshifts, and stop codons. The Atmar-1 elements are present in 370-1200 copies, while the Atmar-2 elements are present in approximately 100-300 copies per haploid genome. One of the Atmar-1 elements, Atmar-1.33, could be mobilized, suggesting the presence of functional Atmar-1 elements elsewhere in the genome. Phylogenetic analysis demonstrated that Atmar-1 elements belong to the irritans subfamily and Atmar-2 elements to the cecropia subfamily of mariner elements.     

Larval diapause in the mosquito aedes triseriatus: effects of diet and temperature on photoperiodic induction

Larval diapause was induced in both a northern (Ohio) and a southern (Alabama) strain of Aedes triseriatus by photoperiodic treatment of the larval stage itself. The effect of a short-day (10 hr) photoperiod during larval development was cumulative and resulted in failure of the fourth instar to pupate. The larvae, which had been obtained from long-day (16 hr) eggs, underwent diapause when their development was slowed by a low-quantity diet or low temperature (16°C) for a sufficient number of days to allow the short-day photoperiod to exert a visible effect.     

The invasive mosquito Aedes japonicus in Central Europe

Complaints about a biting pest led to the recognition of invasive Aedes (Finlaya) japonicus japonicus (Theobald) (Diptera: Culicidae) in Central Europe. Larval collections from cemetery vases revealed a colonized area of approximately 1400 km² in northern Switzerland spreading into bordering Germany, suggesting that the mosquito has been established in this region for several years. Within this range, larvae of Ae. japonicus were recovered from more containers than the most common resident culicid species Culex pipiens. Possible introduction sites (used tyre yards and international airports) revealed few or no larvae, and the mode of introduction remains unclear. Given the vector potential of this species for arboviruses, implementation of surveillance and control measures should be considered.     

The effect of DDT and rearing temperature on pecten spine number in larvae of the mosquito Aedes aegypti L.

Pecten spine number has been studied in larvae of Aedes aegypti exposed during development to low temperatures or to DDT at the fourth instar, both of which caused substantial mortality. But neither contingency caused stabilising selection for spine number, in contrast to what was observed in a previous study when larvae were exposed to scum on the surface of the rearing medium. — Two larval colour morphs, yellow and brown, did not differ in spine number.     

Climate and geographic trends in hatch delay of the treehole mosquito,Aedes triseriatus Say (Diptera: Culicidae)

Eggs of Aedes triseriatus mosquitoes are stimulated to hatch when inundated with water, but only a small fraction of eggs from the same batch will hatch for any given stimulus. Similar hatching or germination patterns are observed in desert plants, copepods, rotifers, insects, and many other species. Bet hedging theory suggests that parents stagger offspring emergence into vulnerable life history stages in order to avoid catastrophic reproductive failures. For Ae. triseriatus, a treehole breeding mosquito, immediate hatching of an entire clutch leaves all of the parent's progeny vulnerable to extinction in the event of a severe drought. Natural selection has likely favored parents that pursued a bet hedging strategy where the risk of reproductive failure is distributed over time. Considering treehole mosquitoes, bet hedging theory could be used to predict that hatch delay would be positively correlated with the likelihood of drought. To test this prediction, we collected Ae. triseriatus from habitats that varied widely in mean annual precipitation and exposed them to several hatch stimuli in the laboratory. Here we report that, as predicted, Ae. triseriatus eggs from high precipitation regions showed less hatch delay than areas of low precipitation. This strategy probably allows Ae. triseriatus to cope with the wide variety of climatic conditions that it faces in its extensive geographical range.     

昆虫滞育诱导的分子调控机制

滞育是昆虫长期适应不利环境的重要对策之一,对昆虫的生存、繁衍和进化都具有重要意义.滞育一般分为滞育前期、滞育期和滞育后期3个阶段,其中,滞育前期包括滞育诱导期和滞育准备期,是确保昆虫顺利进入滞育的重要时期.本文首先简要概括了光周期、温度、湿度、食料等外界环境因素在不同昆虫滞育诱导中的作用;然后系统综述了滞育激素、保幼激...