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.     

Circadian rhythmicity and photoperiodism in the pitcher-plant mosquito: can the seasonal timer evolve independently of the circadian clock?

The two major rhythms of the biosphere are daily and seasonal; the two major adaptations to these rhythms are the circadian clock, mediating daily activities, and the photoperiodic timer, mediating seasonal activities. The mechanistic connection between the circadian clock and the photoperiodic timer remains unresolved. Herein, we show that the rhythmic developmental response to exotic light:dark cycles, usually used to infer a causal connection between the circadian clock and the photoperiodic timer, has evolved independently of the photoperiodic timer in the pitcher-plant mosquito Wyeomyia smithii across the climatic gradient of eastern North America from Florida to Canada and from the coastal plain to the mountains. We conclude that the photoperiodic timing of seasonal events can evolve independently of the daily circadian clock.     

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.     

Extrinsic light:dark cycles,rather than endogenous circadian cycles,affect the photoperiodic counter in the pitcher-plant mosquito, <Emphasis Type="Italic">Wyeomyia smithii</Emphasis>

A wide diversity of organisms use photoperiod (daylength) as an environmental cue to anticipate the changing seasons and to time various life-history events such as dormancy and migration. Photoperiodic time measurement consists of two main components, (1) the photoperiodic timer that discriminates between long and short days, and (2) the photoperiodic counter that accumulates and stores information from the timer and then induces the phenotypic output. Herein, we use extended night treatments to show that light is necessary to accumulate photoperiodic information across the geographic range of the mosquito, Wyeomyia smithii and that the photoperiodic counter counts extrinsic (external) light:dark cycles and not endogenous (internal) circadian cycles.     

Mammalian photoperiodic system: formal properties and neuroendocrine mechanisms of photoperiodic time measurement

Photoperiodism is a process whereby organisms are able to use both absolute measures of day length and the direction of day length change as a basis for regulating seasonal changes in physiology and behavior. The use of day length cues allows organisms to essentially track time-of-year and to "anticipate" relatively predictable annual variations in important environmental parameters. Thus, adaptive types of seasonal biological changes can be molded through evolution to fit annual environmental cycles. Studies of the formal properties of photoperiodic mechanisms have revealed that most organisms use circadian oscillators to measure day length. Two types of paradigms, designated as the external and internal coincidence models, have been proposed to account for photoperiodic time measurement by a circadian mechanism. Both models postulate that the timing of light exposure, rather than the total amount of light, is critical to the organism's perception of day length. In mammals, a circadian oscillator(s) in the suprachiasmatic nucleus of the hypothalamus receives photic stimuli via the retinohypothalamic tract. The circadian system regulates the rhythmic secretion of the pineal hormone, melatonin. Melatonin is secreted at night, and the duration of secretion varies in inverse relation to day length; thus, photoperiod information is "encoded" in the melatonin signal. The melatonin signal is presumably "decoded" in melatonin target tissues that are involved in the regulation of a variety of seasonal responses. Variations in photoperiodic response are seen not only between species but also between breeding populations within a species and between individuals within single breeding populations. Sometimes these variations appear to be the result of differences in responsiveness to melatonin; in other cases, variations in photoperiod responsiveness may depend on differences in patterns of melatonin secretion related to circadian variation. Sites of action for melatonin in mammals are not yet well characterized, but potential targets of particular interest include the pars tuberalis of the pituitary gland and the suprachiasmatic nuclei. Both these sites exhibit uptake of radiolabeled melatonin in various species, and there is some evidence for direct action of melatonin at these sites. However, it appears that there are species differences with respect to the importance and specific functions of various melatonin target sites.     

Circadian rhythmicity and photoperiodism in the pitcher-plant mosquito: adaptive response to the photic environment or correlated response to the seasonal environment?

Many plants and animals use the length of day or photoperiod to cue their seasonal patterns of development, reproduction, dormancy, and migration. Among temperate arthropods, the median or critical photoperiod increases with latitude or altitude. Concomitantly, in beetles, moths, mites, flies, and mosquitoes, there is a declining expression of a rhythmic, presumably circadian-based, component of photoperiodic response. It has been proposed that the long summer days in the north select for a reduced response to light by the circadian clock, which results in this declining rhythmic expression and, consequently, longer northern critical photoperiods. However, these patterns might also be due to direct, seasonal selection on the critical photoperiod itself, which results in a correlated reduction in the rhythmic component as a result of internal physiological constraints within the organism. Using standard light duration and selection experiments, we show that evolution of photoperiodic time measurement in the mosquito, Wyeomyia smithii, results from the direct response of critical photoperiod to seasonal selection and a correlated response of the rhythmic component of photoperiodic time measurement. We conclude that expression of the circadian clock is necessary neither for the central mechanism of photoperiodic time measurement nor for the adaptive modification of critical photoperiod.     

Photoperiodic control of reproduction in the male lizardAnolis carolinensis

Summary In contrast to the higher vertebrates the photoperiodic time measuring system in the male lizardAnolis carolinensis seems to rely on an hourglass timer which lacks endogenous rhythmicity. This timer appears to measure the absolute length of the light portion of light-dark (LD) cycles. The present study further characterized the nature of theAnolis photoperiodic timer and demonstrated: (1) The gonadal response is quite sensitive to photostimulation. Exposure to as few as three 16 h photoperiods (over a 3 week period) can maintain testicular function in summer anoles whereas exposure to as few as six 16 h photoperiods (over a 3 week period) can elicit maximal testicular development in the fall. (2) The photoperiodic timer does not have to be reset daily by a dark interruption. (3) The dark portion of LD cycles may be involved in a complex fashion in reversing a light-initiated reaction and (4) Comparisons of entrained circadian activity rhythms with testicular responses to various light cycles argue against the participation of a circadian clock in photoperiodic time measurement.Abbreviation CRPP circadian rhythm of photoperiodic photo-sensitivity     

Photoperiodism of diapause induction in Thyrassia penangae (Lepidoptera: Zygaenidae)

Thyrassia penangae enters winter diapause as a prepupa in a cocoon. Photoperiodism of diapause induction was systematically investigated in this moth. The photoperiodic response curves under 24-h light-dark cycles showed that this insect is a typical long-day species. The critical daylength was 13 h 30 min at 25 °C, 13 h at 30 °C and 12 h 20 min at 28 °C. Transferring experiments from a short day (LD 12:12) to a long day (LD 15:9) or vice versa indicated that photoperiodic sensitivity mainly occurs during the larval period. In experiments using non-24-h light-dark cycles, when the length of photophase exceeded the critical daylength (13.5 h), was diapause inhibited effectively, even when the length of scotophase exceeded the critical nightlength (10.5 h). Only when a long scotophase was combined with a short photophase, diapause was induced effectively. This result suggests that daylength measurement is more important than nightlength measurement in T. penangae. Night interruption experiments under 24-h light-dark cycles exhibited two points of apparent light sensitivity, but the photosensitive position was highly influenced by temperature and the length of scotophase. Nanda-Hamner experiments failed to reveal the involvement of a circadian system in this photoperiodic time measurement. All light-dark cycles from LD 12:12 to LD 12:72 resulted in a short day response, and all cycles from LD 14:4 to LD 14:72 resulted in a long day response, suggesting that photoperiodic time measurement in this moth is performed by a day-interval timer or an hourglass-like clock.     

Photoperiodic clock of diapause induction in Pseudopidorus fasciata (Lepidoptera: Zygaenidae)

Pseudopidorus fasciata enters diapause as fourth instar larvae at short day lengths. Using 24-h light-dark cycles, the photoperiodic response curves in this species appeared to be similar with a critical night length of 10.5h at temperatures below 30 degrees C. At an average temperature of 30.5 degrees C, the critical night length had shifted to between 15 and 17h. In experiments using non-24-h light-dark cycles, it was clearly demonstrated that the dark period (scotophase) was the decisive phase for a diapause determination. In night interruption experiments using 24-h light-dark cycles, a 1-h light pulse at LD12:12 completely reversed the long night effect and averted diapause in all treatments. At LD 9:15 light pulses of 1-h, 30- or 15-min also averted diapause effectively when both the pre-interruption (D(1)) or the post-interruption scotophases (D(2)) did not exceed the critical night length. If D(1) or D(2) exceeded the critical night length diapause was induced. The most crucial event for the photoperiodic time measurement in this species is the length of the scotophase. A 10-min light pulse placed in the most photosensitive phase reversed diapause in over 50% of the individuals. Night interruption experiments under non-24-h light-dark cycles indicated that the photoperiodic clock measured only D(1) regardless of the length of D(2), suggesting that the most inductive cycles are often those in which L+D are close to 24h. In resonance experiments, this species showed a circadian periodicity at temperatures of 24.5 or 26 degrees C, but not at 30.5 and 23.3 degrees C. On the other hand, Bünsow and skeleton photoperiod experiments failed to reveal the involvement of a circadian system in this photoperiodic clock. These results suggest the photoperiodic clock in this species is a long-night measuring hourglass and the circadian effect found in the final expression of the photoperiodic response in the resonance experiments may be caused by a disturbing effect of the circadian system in unnatural regimes.     

Evolution of photoperiodic time measurement is independent of the circadian clock in the pitcher-plant mosquito, <Emphasis Type="Italic">Wyeomyia smithii</Emphasis>

For over 70 years, researchers have debated whether the ability to use day length as a cue for the timing of seasonal events (photoperiodism) is related to the endogenous circadian clock that regulates the timing of daily events. Models of photoperiodism include two components: (1) a photoperiodic timer that measures the length of the day, and (2) a photoperiodic counter that elicits the downstream photoperiodic response after a threshold number of days has been counted. Herein, we show that there is no geographical pattern of genetic association between the expression of the circadian clock and the photoperiodic timer or counter. We conclude that the photoperiodic timer and counter have evolved independently of the circadian clock in the pitcher-plant mosquito Wyeomyia smithii and hence, the evolutionary modification of photoperiodism throughout the range of W. smithii has not been causally mediated by a corresponding evolution of the circadian clock.

Photoperiodic time measurement in the male lizardAnolis carolinensis

Summary Photoperiod plays an important role in controlling the annual reproductive cycle of the male lizardAnolis carolinensis. Groups of anoles were exposed to various experimental lighting regimens to determine how the lizards were measuring the length of the day. The experimental regimens were designed to discriminate between the following two general classes of hypotheses: (1) Photoperiodic time measurement is based on an hourglass or interval timer which measures the length of the light or dark or (2) Photoperiodic time measurement is based on an endogenous circadian rhythm of photoperiodic photosensitivity. The experiments demonstrated thatAnolis uses an hourglass mechanism which measures the absolute length of the light period. This is in contrast to the higher vertebrates (birds and mammals) which measure photoperiodic time by means of a circadian oscillation of responsiveness to light.     

A circadian clock measures photoperiodic time in the male lizard Anolis carolinensis

Summary Photoperiod plays an important role in controlling the annual reproductive cycle of the male lizard Anolis carolinensis. The nature of photoperiodic time measurement in Anolis was investigated by exposing anoles to 3 different kinds of lighting paradigms (resonance, T cycles, and night breaks) to determine if photoperiodic time measurement involves the circadian system. Both the reproductive response and the patterns of entrainment of the activity rhythm were assessed. The results show that the circadian system is involved in photoperiodic time measurement in this species and that a discrete photoinducible phase resides in the latter half of the animals' subjective night. Significantly, the ability of the circadian system to execute photoperiodic time measurement is crucially dependent on the length of the photoperiod. Resonance, T cycle and night break cycles utilizing a photoperiod 10–11 h in duration reveal circadian involvement whereas these same cycles utilizing 6 or 8 h photoperiods do not.Abbreviation CRPP circadian rhythm of photoperiodic sensitivity     

Erwin Bünning and Tony Lees, two giants of chronobiology, and the problem of time measurement in insect photoperiodism

This paper examines the views of Erwin Bünning and Tony Lees on the mechanism of photoperiodic time measurement, the former advocating a circadian basis for the phenomenon and the latter a non-circadian hourglass-like timer. This difference in opinion led to a protracted split among workers on photoperiodism, some supporting an oscillatory clock and others an "hourglass", and gave rise to the often stated opinion that the two forms of time measurement were mutually exclusive. This paper, however, suggests that both oscillatory and hourglass-like properties are to be seen in insect photoperiodism. Furthermore, the differences between the two apparently conflicting models may be resolved if, following Bünning, "hourglasses" are regarded as damping circadian oscillators, with the more self-sustained (clearly oscillatory) and more highly damped (hourglass-like) responses being parts of a continuous series. Since circadian rhythmicity is an all-pervading and fundamental aspect of insect biology, currently opening up to genetic and molecular analysis, recognition of the basic similarity of a wide range of insect photoperiodic timers may help to unravel the biochemical nature of the mechanism(s) involved.     

Hourglass and oscillator expressions of photoperiodic diapause response in the cabbage moth Mamestra brassicae

Abstract. Both oscillator and hourglass features are found in the photoperiodic response that controls the pupal winter diapause of Mamestra brassicae. The expression of oscillatory response to extended long-night cycles is temperature dependent, i.e. circadian resonance appears at 23 and 25^oC but not at 20 and 28^oC. At 20^oC, scanning of extended scotophases by a short light pulse does not reveal any clear circadian rhythmicity. However, a circadian feature of the photoperiodic response is indicated even at 20^oC by a bistability phenomenon, i.e. either one of the two dark periods in symmetrical skeleton photoperiods determines the diapause response depending on the phase angle with the preceding (entraining) light-dark cycles. At 20 and 25^oC, the incidence of diapause increases as a function of the number of light–dark cycles regardless of the cycle length (T) , if T is 24 h or 2 X 24h (with a 12 h light period). A non-diel cycle (r=36h) is less effective, suggesting that disturbance of the circadian organization partly impairs the diapause-inducing function. The inductive effect of a long night is largely affected by temperature, and becomes saturated with eight cycles at 20^oC and 14 cycles at 25^oC. Presumably, an hourglass mechanism measures the dark time, and a circadian component involved in some later sequence of the photoperiodic response may or may not be expressed depending on the mode of interaction between them.     

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.     

Diet-dependent switch from circadian to hourglass-like operation of an insect photoperiodic clock

The results of resonance experiments gave evidence that circadian organization was involved in photoperiodic diapause induction of Pieris brassicae. However, giving the larvae an artificial diet instead of the host plant deeply altered the resonance effect and led to a response that could be accounted for by the operation of a noncircadian timer (hourglass). This important change, however, had no effect on the insect's ability to discriminate between short- and long-day 24-hr cycles. Thus, it is possible via the metabolism to switch the photoperiodic mechanism to a circadian or an hourglass-like mode of operation without impairing the photoperiodic time measurement.     

Deciphering time measurement: the role of circadian 'clock' genes and formal experimentation in insect photoperiodism

This review examines possible role(s) of circadian ‘clock’ genes in insect photoperiodism against a background of many decades of formal experimentation and model building. Since ovarian diapause in the genetic model organism Drosophila melanogaster has proved to be weak and variable, recent attention has been directed to species with more robust photoperiodic responses. However, no obvious consensus on the problem of time measurement in insect photoperiodism has yet to emerge and a variety of mechanisms are indicated. In some species, expression patterns of clock genes and formal experiments based on the canonical properties of the circadian system have suggested that a damped oscillator version of Pittendrigh's external coincidence model is appropriate to explain the measurement of seasonal changes in night length. In other species extreme dampening of constituent oscillators may give rise to apparently hourglass-like photoperiodic responses, and in still others there is evidence for dual oscillator (dawn and dusk) photoperiodic mechanisms of the internal coincidence type. Although the exact role of circadian rhythmicity and of clock genes in photoperiodism is yet to be settled, Bünning's general hypothesis (Bünning, 1936) remains the most persuasive unifying principle. Observed differences between photoperiodic clocks may be reflections of underlying differences in the clock genes in their circadian feedback loops.     

Photoperiodism in Neurospora crassa

Plants and animals use day or night length for seasonal control of reproduction and other biological functions. Overwhelming evidence suggests that this photoperiodic mechanism relies on a functional circadian system. Recent progress has defined how flowering time in plants is regulated by photoperiodic control of output pathways, but the underlying mechanisms of photoperiodism remain to be described. The authors investigate photoperiodism in a genetic model system for circadian rhythms research, Neurospora crassa. They find that both propagation and reproduction respond systematically to photoperiod. Furthermore, a nonreproductive light-regulated function is also enhanced under certain photoperiodic conditions. All of these photoperiodic responses require a functional circadian clock, in that they are absent in a clock mutant. Night break experiments show that measuring night length is one of the mechanisms used for photoperiod assessment. This represents the first formal report of photoperiodism in the fungi.     

Phylogeny and oscillating expression of period and cryptochrome in short and long photoperiods suggest a conserved function in Nasonia vitripennis

Photoperiodism, the ability to respond to seasonal varying day length with suitable life history changes, is a common trait in organisms that live in temperate regions. In most studied organisms, the circadian system appears to be the basis for photoperiodic time measurement. In insects this is still controversial: while some data indicate that the circadian system is causally involved in photoperiodism, others suggest that it may have a marginal or indirect role. Resonance experiments in the parasitic wasp Nasonia vitripennis have revealed a circadian component in photoperiodic time measurement compatible with a mechanism of internal coincidence where a two components oscillator system obtains information from dawn and dusk, respectively. The identity of this oscillator (or oscillators) is still unclear but possible candidates are the oscillating molecules of the auto-regulatory feedback loops in the heart of the circadian system. Here, we show for the first time the circadian oscillation of period and cryptochrome mRNAs in the heads of Nasonia females kept under short and long photoperiods. Period and cryptochrome mRNA levels display a synchronous oscillation in all conditions tested and persist, albeit with reduced amplitude, during the first day in constant light as well as constant darkness. More importantly, the signal for the period and cryptochrome oscillations is set by the light-on signal. These results, together with phylogenetic analyses, indicate that Nasonia’s period and cryptochrome display characteristics of homologous genes in other hymenopteran species.