A second diapause in Wyeomyia smithii: seasonal incidence and maintenance by photoperiod

Diapause and quiescence: dormancy mechanisms that contribute to the geographical expansion of mosquitoes and their evolutionary success

Mosquitoes are insects belonging to the order Diptera and family Culicidae. They are distributed worldwide and include approximately 3500 species, of which about 300 have medical and veterinary importance. The evolutionary success of mosquitoes, in both tropical and temperate regions, is due to the various survival strategies these insects have developed throughout their life histories. Of the many adaptive mechanisms, diapause and quiescence, two different types of dormancy, likely contribute to the establishment, maintenance and spread of natural mosquito populations. This review seeks to objectively and coherently describe the terms diapause and quiescence, which can be confused in the literature because the phenotypic effects of these mechanisms are often similar.

EFFECTS OF POSTGLACIAL RANGE EXPANSION ON ALLOZYME AND QUANTITATIVE GENETIC VARIATION OF THE PITCHER-PLANT MOSQUITO, WYEOMYIA SMITHII

We determined allozyme variability of 34 populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30°N) to northern Manitoba (54°N) and compared allozyme variability with the additive genetic variance for preadult development time and photoperiodic response determined previously for six populations over a similar range (30-50°N). Phylogenetic analysis of allozymes shows a well-defined split between Gulf Coast and lowland North Carolina populations, similar to previously observed phylogeographic patterns in a wide variety of taxa. A deeper split in the phylogeny of W. smithii coincides with the location of the maximum extent of the Laurentide Ice Sheet. Furthermore, both average heterozygosity and patterns of isolation-by-distance decline in populations north of the former glacial border. It is likely that northern populations are the result of a range expansion that occurred subsequent to the late-Wisconsin retreat of the Laurentide Ice Sheet and that these populations have not yet reached a drift-migration equilibrium. The northern decline in allozyme heterozygosity contrasts sharply with the northern increase in additive genetic variance of development time and photoperiodic response found in previous studies. These previous studies also showed that the genetic divergence of populations has involved stochastic variation in the contribution of dominance and epistasis to the genetic architecture underlying demographic traits, including preadult development time, and photoperiodic response. When taken together, the present and prior studies identify the genetic processes underlying the lack of concordance between geographic patterns of allozyme and quantitative genetic variation in natural populations of W. smithii. In the presence of nonadditive genetic variation, isolation and drift can result in opposite patterns of genetic variation for structural genes and quantitative traits.

Comparative Transcriptomics Reveals Key Gene Expression Differences between Diapausing and Non-Diapausing Adults of Culex pipiens

Diapause is a critical eco-physiological adaptation for winter survival in the West Nile Virus vector, Culex pipiens, but little is known about the molecular mechanisms that distinguish diapause from non-diapause in this important mosquito species. We used Illumina RNA-seq to simultaneously identify and quantify relative transcript levels in diapausing and non-diapausing adult females. Among 65,623,095 read pairs, we identified 41 genes with significantly different transcript abundances between these two groups. Transcriptome divergences between these two phenotypes include genes related to juvenile hormone synthesis, anaerobic metabolism, innate immunity and cold tolerance.

Mosquito diapause

Diapause, a dominant feature in the life history of many mosquito species, offers a mechanism for bridging unfavorable seasons in both temperate and tropical environments and serves to synchronize development within populations, thus directly affecting disease transmission cycles. The trait appears to have evolved independently numerous times within the Culicidae, as exemplified by the diverse developmental stages of diapause in closely related species. Its impact is pervasive, not only influencing the arrested stage, but also frequently altering physiological processes both before and after diapause. How the diapause response can be molded evolutionarily is critical for understanding potential range expansions of native and newly introduced species. The study of hormonal regulation of mosquito diapause has focused primarily on adult diapause, with little current information available on larval diapause or the intriguing maternal effects that regulate egg diapause. Recent quantitative trait locus, transcriptome, and RNA interference studies hold promise for interpreting the complex suite of genes that subserve the diapause phenotype.

Light, time, and the physiology of biotic response to rapid climate change in animals

Examination of temperate and polar regions of Earth shows that the nonbiological world is exquisitely sensitive to the direct effects of temperature, whereas the biological world is largely organized by light. Herein, we discuss the use of day length by animals at physiological and genetic levels, beginning with a comparative experimental study that shows the preeminent role of light in determining fitness in seasonal environments. Typically, at seasonally appropriate times, light initiates a cascade of physiological events mediating the input and interpretation of day length to the output of specific hormones that ultimately determine whether animals prepare to develop, reproduce, hibernate, enter dormancy, or migrate. The mechanisms that form the basis of seasonal time keeping and their adjustment during climate change are reviewed at the physiological and genetic levels. Future avenues for research are proposed that span basic questions from how animals transition from dependency on tropical cues to temperate cues during range expansions, to more applied questions of species survival and conservation biology during periods of climatic stress.

Quantitative trait loci associated with photoperiodic response and stage of diapause in the pitcher-plant mosquito, Wyeomyia smithii

A wide variety of temperate animals rely on length of day (photoperiodism) to anticipate and prepare for changing seasons by regulating the timing of development, reproduction, dormancy, and migration. Although the molecular basis of circadian rhythms regulating daily activities is well defined, the molecular basis for the photoperiodic regulation of seasonal activities is largely unknown. We use geographic variation in the photoperiodic control of diapause in the pitcher-plant mosquito Wyeomyia smithii to create the first QTL map of photoperiodism in any animal. For critical photoperiod (CPP), we detect QTL that are unique, a QTL that is sex linked, QTL that overlap with QTL for stage of diapause (SOD), and a QTL that interacts epistatically with the circadian rhythm gene, timeless. Results presented here confirm earlier studies concluding that CPP is under directional selection over the climatic gradient of North America and that the evolution of CPP is genetically correlated with SOD. Despite epistasis between timeless and a QTL for CPP, timeless is not located within any detectable QTL, indicating that it plays an ancillary role in the evolution of photoperiodism in W. smithii. Finally, we highlight one region of the genome that includes loci contributing to CPP, SOD, and hormonal regulation of development.

Evolutionary divergence of the genetic architecture underlying photoperiodism in the pitcher-plant mosquito, Wyeomyia smithii

We determine the contribution of composite additive, dominance, and epistatic effects to the genetic divergence of photoperiodic response along latitudinal, altitudinal, and longitudinal gradients in the pitcher-plant mosquito, Wyeomyia smithii. Joint scaling tests of crosses between populations showed widespread epistasis as well as additive and dominance differences among populations. There were differences due to epistasis between an alpine population in North Carolina and populations in Florida, lowland North Carolina, and Maine. Longitudinal displacement resulted in differences due to epistasis between Florida and Alabama populations separated by 300 km but not between Maine and Wisconsin populations separated by 2000 km. Genetic differences between New Jersey and Ontario did not involve either dominance or epistasis and we estimated the minimum number of effective factors contributing to a difference in mean critical photoperiod of 5 SD between them as nE = 5. We propose that the genetic similarity of populations within a broad northern region is due to their more recent origin since recession of the Laurentide Ice Sheet and that the unique genetic architecture of each population is the result of both mutation and repeated migration-founder-flush episodes during the dispersal of W. smithii in North America. Our results suggest that differences in composite additive and dominance effects arise early in the genetic divergence of populations while differences due to epistasis accumulate after more prolonged isolation.

Repeated larval diapause and diapause-free development in geographic strains of the burnet moth Zygaena trifolii Esp. (Insecta, Lepidoptera) : I. Discontinuous clinal variation in photoperiodically controlled diapause induction

Zygaena trifolii is a "long-day insect" with temperature-dependent photoperiodic responses. All larval instars are sensitive to photoperiod; however, diapause may occur at the third larval stage or any subsequent larval instars. There were quantitative differences within populations in the threshold photoperiod for diapause induction. The diapause response was polymorphic, so that larvae might enter diapause at different instars under the same culture conditions. Furthermore, decreasing photoperiods below a critical daylength shifted the diapausing instar towards earlier stages. Geographic strains of Z. trifolii showed discontinuous clinal variation. Near the northern edge of the distribution [Cologne (Köln), FRG], there is first an obligatory diapause, mainly during early instars, and additional facultative ("repeat") diapauses during later larval instars in subsequent years. In the southern part of its distribution, this burnet moth is partially bivoltine in the field with a facultative first developmental arrest and a decreased capacity for repeated diapause (Valencia, Spain; Marseille, France). Further experiments indicated that the photoperiodically controlled diapause reaction is also influenced by the number of photoperiodic cycles experienced during the period spent in each larval instar, which depends on temperature. The adaptive significance of obligatory and facultative repeated diapause, varying even among the offspring of a single female, may be to buffer the populations against the more extreme and, from year to year, unpredictable fluctuations in climatic conditions at the northern edge of the distribution.

Photoperiodism and the photic environment of the pitcher-plant mosquito, Wyeomyia smithii

Wyeomyia smithii Coq. (Diptera: Culicidae) overwinters as a larva in a state of diapause which is initiated, maintained, and terminated by photoperiod. Both in the dawn and in the dusk, diapausing larvae are photoperiodically most sensitive to blue light (390-450 nm) with a shoulder in response in the bluegreen and green (480-540 nm) region of the spectrum. The saturation curves for response to blue light in the dusk has a steeper slope than for response to blue-green and green light in the dusk, suggesting two distinct pigments or pigment complexes underly photoperiodic response in W. smithii.The photic environment of W. smithii during twilight is rich in yellow-green light but sufficient light is available at 390-540 nm to trigger photoperiodic response early during morning civil twilight and to sustain response until late in evening civil twilight. Comparison of action spectra with spectra of available light indicates that the zenith angles of the sun which would result in 50% response are 95°48' and 94°52' in the dawn and dusk, respectively. Using these zenith angles to approximate daylength in nature provides a resonable prediction of development in the field.The flux density of photons necessary to elicit 50% development a 454 nm is about 9×10⁷ photons/cm² s in the dawn and 3×10⁸ photons/cm² s in the dusk. This high degree of sensitivity enables W. smithii to cue to the rapidly changing light intensity which occurs around the nautical-civil twilight transition. At the same time, the chromophore is not likely to be stimulated by the light of the full moon.