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

Summary 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.     

Thermoperiodism and the thermal environment of the pitcher-plant mosquito,Wyeomyia smithii

Summary Wyeomyia smithii Coq. (Diptera: Culicidae) completes its pre-adult development only within leaves of the purple pitcher-plant, Sarracenia purpurea. Between early June and mid-October in northern New York State, the daily temperature cycle in leaves lagged the photic cycle by 0–6 h and exhibited a mean daily amplitude of 14.5°C.Thermoperiod acts as a potent zeitgeber. At constant temperatures, W. smithii respond to the shorter dark period of a symmetric skeleton photoperiod as day. However, a superimposed thermoperiod having the thermophase coincident with the longer dark period overrides this tendancy. Thermoperiods may also perturb the photoperiodic clock but W. smithii compensate for the range of phase relationships between the photic and thermal cycles observed in nature.Compared with constant temperatures, W. smithii develop more slowly but exhibit a 7-fold increase in fecundity when reared under fluctuating temperatures. The net result is a 50% greater capacity for increase in the latter regimen. These results suggest that maximum fitness in W. smithii is achieved through the action of, and not despite, thermal heterogeneity.     

Chromosomal polymorphisms in the pitcher-plant mosquito,Wyeomyia smithii

A local population of the pitcher-plant mosquito, Wyeomyia smithii (Diptera: Culicidae, Culicini) in western New York State contains naturally polymorphic salivary gland chromosomes. Maps depicting the proposed standard sequence of bands along each of the three chromosomes are presented. Structural conformations of heterokaryotypes reveal ten paracentric inversions. Four inversions occur in chromosome 1, and three in each of chromosomes 2 and 3. All the inversions are small, occupying 3–17% of the length of their respective chromosomes, and all are located in the terminal 1/5 to 1/2 of a chromosome arm. Three inversions are almost adjacent on chromosome 1. On chromosome 2 one inversion lies completely within another. Three inversions on chromosome 3 partially overlap. The remaining two inversions occur alone, one on chromosome 1 and the other on chromosome 2. Another polymorphism, with no visible alteration of band sequence, produces asynapsis of all but the tip of the left arm of chromosome 1, and shows a significant change in frequency between two successive years. No inversion exhibits a significant seasonal change in frequency, though some fluctuation in frequency was noted in two cases. The rarest inversion in the population exists at a frequency below 0.01, and the two most common inversions at frequencies of 0.44 and 0.52. The average inversion heterozygosity of an individual is between 1.7 and 2.4 based on two different estimation procedures. Heterokaryotypes for several of the inversions were significantly more frequent than they should have been, given binomial expectations. Cytogenetic analysis of W. smithii was deemed particularly desirable because this species has been and continues to be the subject of extensive biometrical-genetic, ecological, and evolutionary investigation.This paper is the seventh in a series on the ecology and evolution of the pitcher-plant mosquito     

Reproductive maturation in the pitcher-plant mosquito, Wyeomyia smithii

ABSTRACT. Females of Wyeomyia smithii (Coquillett) mate immediately following adult eclosion. Those from Massachusetts emerge with their ovaries at a more advanced stage of development than those from Florida. Decapitation of teneral females prevented the completion of ovarian maturation among individuals from Florida but not from Massachusetts. Application of a JH analogue to Florida pupae induced precocious ovarian development and compensated for decapitation of adults. Juvenile hormone analogue (Altosid) treatment after decapitation at emergence did not stimulate egg development in Florida females beyond the previtellogenic stage (Stage II). Precocious ovarian maturation apparently results from the action in pupae of JH which mediates an early release of a gonotropic factor from the head.     

Genetic coordination of demography and phenology in the pitcher-plant mosquito,Wyeomyia smithii

Demographic and phenological traits compose the basic elements of an insect's life-history strategy in a seasonal environment. An insect's long-term fitness depends on its ability to exploit favorable conditions, to avoid unfavorable conditions, and to convert from one life style to the other. For the pitcher-plant mosquito, Wyeomyia smithii, we show that genetic variation exists in both development time (a demographic trait) and critical photoperiod (a phenological trait) in six populations spanning much of the species' geographical range. During the northward range expansion of W. smithii in North America, these traits have evolved independently under strong directional and stabilizing selection. The correlated response in critical photoperiod to divergent selection on development time reveals significantly positive genetic correlations in five populations and a negative correlation in one population. The positive correlations form a genetically coordinated phenotype: faster developing individuals use a shorter photoperiodic switch point and are able to exploit the late favorable season; slower developing individuals use a longer photoperiodic switch point and are able to avoid extending development into the unfavorable season. This genetic coordination of demography and phenology has not, however, prevented their independent evolution. We propose that evolutionary flexibility in W. smithii may arise in part from the reorganization of their genetic architecture following repeated founder events during their northward invasion of North America.     

Factors influencing the seasonal life history of the pitcher-plant mosquito, Wyeomyia smithii

Abstract.  1. The effects of resource levels, thermal microclimate, and seasonal oviposition patterns on fecundity and survivorship in the pitcher-plant mosquito, Wyeomyia smithii (Coq.), were examined at a northern Wisconsin bog over the course of 2 years. Wyeomyia smithii are bivoltine at this locality, thereby enabling the study of summer and overwintering generations separately.
2. Nutrient resources of W. smithii were not limiting and there was no indication of density-dependent survivorship or fecundity.
3. Oviposition rates were highest in young, large pitchers and individual mosquitoes appeared to allocate only a few eggs to any one leaf.
4. Winter was the harsh season, and the principal manifestation of seasonal harshness was reduced survivorship.
5. Overwintering W. smithii that had been oviposited later in the summer had a higher odds of survival than those oviposited earlier in the summer.
6. It was concluded that dispersal of eggs among many pitchers serves to spread the risk of encountering lethal winter temperatures among spatially unpredictable patches.     

Rhythmic components of photoperiodic time measurement in the pitcher-plant mosquito, Wyeomyia smithii

Photoperiodic time measurement regulating larval diapause in the pitcher-plant mosquito, Wyeomyia smithii, varies in a close relationship with latitude. The critical photoperiod mediating the maintenance and termination of diapause is positively correlated with latitude (r ² = 0.977) among six populations from southern (30–31° N), intermediate (40° N), and northern (46–49° N) latitudes in North America. The developmental response to unnaturally short and to unnaturally long photoperiods declines with increasing latitude, so that longer critical photoperiods are associated with a downward rather than a lateral shift in the photoperiodic response curve. Exotic light and dark cycles of varying period (T) with a short (10 h) photophase and a scotophase ranging from 14 (T = 24) to 62 (T = 72) h, reveal two geographic patterns: a decline in perturbability of the photoperiodic clock with increasing latitude, and no change with latitude in the 21-h period of rising and falling development with increasing T. These results show (1) that there is a rhythmic component to photoperiodic time measurement in W. smithii, (2) that the period of this rhythm is about 21 h in all populations, and (3) that more northern populations show decreasing responsiveness to photoperiod and increasing stability against perturbation by exotic period lengths (T > 24). Previous studies on W.␣smithii indicate that this single temperate species of a tropical and subtropical genus has evolved from south to north. We therefore conclude that the evolution of increasing critical photoperiod in W. smithii during its adaptive radiation into North America has more likely involved the amplitude and not the period of the underlying circadian pacemaker. Received: 22 July 1996 / Accepted: 30 September 1996     

Evolutionary divergence of circadian and photoperiodic phenotypes in the pitcher-plant mosquito, Wyeomyia smithii

For decades, chronobiologists have investigated the relationship between the circadian clock that mediates daily activities and the photoperiodic timer that mediates seasonal activities. The main experiment used to infer a circadian basis for photoperiodic time measurement is the Nanda-Hamner protocol (NH). Herein, the authors compare additive and nonadditive (dominance and epistasis) genetic effects that lead to the divergence of populations of the pitcher-plant mosquito, Wyeomyia smithii, for critical photoperiod (CPP) and amplitude of the rhythmic response to NH for 3 temporal-geographic scales: 1) Over geological time between populations in northern and southern clades, 2) over millennial time between populations within the northern clade, and 3) over generational time between lines selected for long and short CPP from within a single population. The authors show that the pattern of additive, dominance, and epistatic effects depends on the time scale over which populations or lines have diverged. Patterns for genetic differences between populations for CPP and response to NH reveal similarities over geological and millennial time scales but differences over shorter periods of evolution. These results, and the observation that neither the period nor amplitude of the NH rhythm are significantly correlated with CPP among populations, lead the authors to conclude that the rhythmic response to NH has evolved independently of photoperiodic response in populations of W. smithii. The implication is that in this species, genetic modification of the circadian clock has not been the basis for the adaptive modification of photoperiodic time measurement over the climatic gradient of North America.     

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.     

Epistasis underlying a fitness trait within a natural population of the pitcher-plant mosquito, Wyeomyia smithii

We selected on divergent photoperiodic response in three separate lines from a natural population of the pitcher-plant mosquito, Wyeomyia smithii. Line crosses reveal that there exists within a population, diverse epistatic variation for a fitness trait that could contribute to adaptive potential following founder events or rapid climate change.     

Footprints in time: comparative quantitative trait loci mapping of the pitcher-plant mosquito,Wyeomyia smithii

Identifying regions of the genome contributing to phenotypic evolution often involves genetic mapping of quantitative traits. The focus then turns to identifying regions of ‘major’ effect, overlooking the observation that traits of ecological or evolutionary relevance usually involve many genes whose individual effects are small but whose cumulative effect is large. Herein, we use the power of fully interfertile natural populations of a single species of mosquito to develop three quantitative trait loci (QTL) maps: one between two post-glacially diverged populations and two between a more ancient and a post-glacial population. All demonstrate that photoperiodic response is genetically a highly complex trait. Furthermore, we show that marker regressions identify apparently ‘non-significant’ regions of the genome not identified by composite interval mapping, that the perception of the genetic basis of adaptive evolution is crucially dependent upon genetic background and that the genetic basis for adaptive evolution of photoperiodic response is highly variable within contemporary populations as well as between anciently diverged populations.     

Reproductive value in a complex life cycle: heat tolerance of the pitcher-plant mosquito, Wyeomyia smithii

Because mortality accumulates with age, Fisher proposed that the strength of selection acting on survival should increase from birth up to the age of first reproduction. Hamilton later theorized that the strength of selection acting on survival should not change from birth to age at first reproduction. As organisms in nature do not live in uniform environments but, rather, experience periodic stress, we hypothesized that resistance to environmental stress should increase (Fisher) or remain constant (Hamilton) from birth to age at first reproduction. Using the pitcher-plant mosquito, Wyeomyia smithii, we imposed heat stress by simulating the passage of a warm-weather front at different pre-adult and adult stages. Contrary to either Fisher or Hamilton, stress tolerance declined from embryos to larvae to pupae to adults. Consequently, reproductive value appears to have been of little consequence in the evolution of stage-specific tolerance of heat stress in W. smithii.     

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.     

Oviposition decreased in response to enriched water: a field study of the pitcher-plant mosquito, Wyeomyia smithii

Abstract.  1. Environmental cues are known to influence oviposition behaviour in mosquitoes, with important consequences for larval survival and insect population dynamics. Enriched microhabitats have been shown to be preferred oviposition sites.
2. In a field experiment designed to determine whether ovipositing mosquitoes are sensitive to different levels of nutrient enrichment, new pitcher-plant ( Sarracenia purpurea ) leaves were opened and enriched with 0, 2, or 20 dead ants, and the number of pitcher-plant mosquito ( Wyeomyia smithii ) larvae resulting from subsequent oviposition were measured.
3. Oviposition rates were higher in leaves with low levels of enrichment (0 and 2 ants per leaf), although larval development was enhanced at the highest enrichment level.
4. Results suggest that, although these mosquito larvae are nutrient limited, ovipositing females preferentially avoid highly enriched leaves. This counterintuitive result may be due to low oxygen concentrations or a masked cue in enriched leaves, and contrasts with other oviposition studies.     

The contribution of an hourglass timer to the evolution of photoperiodic response in the pitcher-plant mosquito, Wyeomyia smithii

Abstract Photoperiodism, the ability to assess the length of day or night, enables a diverse array of plants, birds, mammals, and arthropods to organize their development and reproduction in concert with the changing seasons in temperate climatic zones. For more than 60 years, the mechanism controlling photoperiodic response has been debated. Photoperiodism may be a simple interval timer, that is, an hourglasslike mechanism that literally measures the length of day or night or, alternatively, may be an overt expression of an underlying circadian oscillator. Herein, we test experimentally whether the rhythmic response in Wyeomyia smithii indicates a causal, necessary relationship between circadian rhythmicity and the evolutionary modification of photoperiodic response over the climatic gradient of North America, or may be explained by a simple interval timer. We show that a day-interval timer is sufficient to predict the photoperiodic response of W. smithii over this broad geographic range and conclude that rhythmic responses observed in classical circadian-based experiments alone cannot be used to infer a causal role for circadian rhythmicity in the evolution of photoperiodic time measurement. More importantly, we argue that the pursuit of circadian rhyth-micity as the central mechanism that measures the duration of night or day has distracted researchers from consideration of the interval-timing processes that may actually be the target of natural selection linking internal photoperiodic time measurement to the external seasonal environment.     

Genetic correlations and the evolution of photoperiodic time measurement within a local population of the pitcher-plant mosquito, Wyeomyia smithii

The genetic relationship between the daily circadian clock and the seasonal photoperiodic timer remains a subject of intense controversy. In Wyeomyia smithii, the critical photoperiod (an overt expression of the photoperiodic timer) evolves independently of the rhythmic response to the Nanda-Hamner protocol (an overt expression of the daily circadian clock) over a wide geographical range in North America. Herein, we focus on these two processes within a single local population in which there is a negative genetic correlation between them. We show that antagonistic selection against this genetic correlation rapidly breaks it down and, in fact, reverses its sign, showing that the genetic correlation is due primarily to linkage and not to pleiotropy. This rapid reversal of the genetic correlation within a small, single population means that it is difficult to argue that circadian rhythmicity forms the necessary, causal basis for the adaptive divergence of photoperiodic time measurement within populations or for the evolution of photoperiodic time measurement among populations over a broad geographical gradient of seasonal selection.     

Geographic and developmental variation in expression of the circadian rhythm gene, timeless, in the pitcher-plant mosquito, Wyeomyia smithii

Expression of the circadian rhythm gene timeless was investigated in the pitcher-plant mosquito, Wyeomyia smithii (Coq.), and was found to vary with time of day, instar of diapause, and latitude of origin. The temporal pattern of timeless expression differed between the two diapausing instars and was significantly higher in southern (38-40 degrees N) than in northern (50 degrees N) populations, when diapausing instar was held constant. Expression of timeless is therefore both developmentally and evolutionarily variable. This result provides the first example of a latitudinal difference in the expression of timeless, suggesting that, along with evidence from other insects, timeless has the potential to affect photoperiodic response and its adaptive evolution in temperate seasonal environments.     

A baculovirus from the mosquito Wyeomyia smithii

A Baculovirus was found in larvae of the pitcher plant mosquito Wyeomyia smithii. This virus is similar in size and appearance to Baculoviruses from other mosquitoes. A unique feature of the virus is the formation of polymorphic inclusions. The prepatent period for this virus in W. smithii is 3–5 days.     

Imperfect oviposition decisions by the pitcher plant mosquito (Wyeomyia smithii)

Summary Wyeomyia smithii mosquitoes distribute their eggs across available oviposition sites (water-holding pitcher plant leaves) of varying quality. I experimentally examined responses to three components of site quality: conspecific larval density, larval density of the pitcher plant midge,Metriocnemus knabi, and pitcher size. Responses to larval treatments were complex and apparently suboptimal. Although mosquito larval performance is better in leaves with fewer conspecific and more midge larvae, females did not lay more eggs in such pitchers. Instead, more eggs were laid in experimental pitchers containing either midge or mosquito larvae, but fewer eggs in pitchers with neither or both. More eggs were laid in larger pitchers, which tend to accumulate more resources and dry out less often. Therefore, although the oviposition decisions made were suboptimal, they were better than random.     

Hourglass and rhythmic components of photoperiodic time measurement in the pitcher plant mosquito, Wyeomyia smithii

The mosquito, Wyeomyia smithii, enters a larval dormancy or diapause that is initiated, maintained, and terminated by photoperiod. The median or critical photoperiod regulating diapause increases from 12 h of light per day along the Gulf of Mexico, USA (30° N), to over 15 h in southern Canada (49° N). Photoperiodic time measurement in W. smithii comprises both rhythmic and hourglass (interval timer) components. Using interrupted-night and resonance experiments, we show that both the rhythmic and hourglass components are prominent in the southern (ancestral) populations and that the influence of the rhythmic component declines with increasing latitude, while the hourglass component remains strong in northern (derived) populations. Previously, it has been shown that the genetic differences in critical photoperiod between northern populations and their southern ancestors involve not only the additive (independent) effects of genes, but also gene-gene interaction (epistasis). We therefore conclude that adaptive evolution of W. smithii has probably involved the progressive epistatic masking of the ancestral rhythmic component resulting in photoperiodic time measurement in northern populations accomplished principally through a day-interval timer. A comparison of W. smithii with previous studies indicates that the decline in critical photoperiod with increasing latitude represents an overall decrease in response to light rather than a shift in the timing of photosensitivity among arthropods in general. We propose that the underlying functional components of photoperiodic time measurement, as well as the overt photoperiodic response, are either homologous or are themselves responding directly to selection over latitudinal gradients in seasonality. Received: 18 May 1998 / Accepted: 14 September 1998