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

Abstract

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.

Drosophila ezoana uses an hour‐glass or highly damped circadian clock for measuring night length and inducing diapause

“…In response to short‐day conditions, the third‐ or fourth‐instar W. smithii larvae enter diapause (Bradshaw & Lounibos, ). Formal, classical experiments show that a weak dampening oscillator participates in photoperiodic induction of diapause in W. smithii (Wegis et al ., ; Bradshaw et al ., ) and also that the influence of this oscillator decreases in populations living at high latitudes (Bradshaw & Holzapfel, ). Bradshaw & Holzapfel () assume that the circadian clock of W. smithii is robust and argue that it would appear to be maladaptive to couple the robust circadian clock to the highly variable and rapidly evolving photoperiodic calendar.…”

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

“…Saunders and Bertossa, 2011 proposed that this concept 'could be tested by using classical night interruption experiments in 24 h L:D cycles (Saunders, 2010a, b) to determine the time of night at which this phase occurs in different latitudinal strains.' Indeed, we have already run just such night-interruption experiments and have shown that the phases of maximum response do not change with respect to either dawn or dusk among six populations from 30-491N in either 24-or 48-h light-dark cycles and over a 3.25-h difference in CPP among these populations (Bradshaw et al, 1998). Hence, the evolution of critical photoperiod in W. smithii has not involved a shift in the light-sensitive phase of photoperiodic induction.…”

Variability in diapause propensity within populations of a temperate insect species: interactions between insecticide resistance genes and photoperiodism

“…In numerous insect species, critical photoperiods regularly increase with decreasing latitude (Danilevski, 1965; Tauber et al ., 1986; Danks, 1987). Such geographical clines of the critical photoperiods are now assumed to be adaptive products of natural selection acting locally on the timing of diapause (Taylor & Spalding, 1986; Bradshaw, Holzapfel & Davidson, 1998). Within populations of the freshwater copepod Diaptomus sanguineus (Forbes), adaptive response to fluctuating selection for fish predation avoidance was favoured by genetic variation associated with the timing of the production of diapausing eggs (Hairston & Munns, 1984; Hairston & Dillon, 1990).…”