Nanda-Hamner Curves Show Huge Latitudinal Variation but No Circadian Components in Drosophila Montana Photoperiodism

Abstract

Insect species with a wide distribution offer a great opportunity to trace latitudinal variation in the photoperiodic regulation of traits important in reproduction and stress tolerances. We measured this variation in the photoperiodic time-measuring system underlying reproductive diapause in Drosophila montana, using a Nanda-Hamner (NH) protocol. None of the study strains showed diel rhythmicity in female diapause proportions under a constant day length (12 h) and varying night lengths in photoperiods ranging from 16 to 84 h at 16°C. In the northernmost strains (above 55°N), nearly all females entered diapause under all photoperiods and about half of them even in continuous darkness, while the females of the southern strains showed high diapause proportions only in the circadian 24 h photoperiod. Significant correlation between the strains’ mean diapause proportions in ≥ 24 h photoperiods and critical day length (CDL; half of the females enter diapause) suggests at least partial causal connection between the traits. Interestingly, females of the northern strains entered diapause even in ≤ 24 h photoperiods, where the night length was shorter than their critical night length (24 h - CDL), but where the females experienced a higher number of Light:Dark cycles than in 24 h photoperiods. NH experiments, performed on the control and selection lines in our previous selection experiment, and completed here, gave similar results and confirmed that selection for shorter, southern-type CDL decreases female diapausing rate in non-circadian photoperiods. Overall, our study shows that D. montana females measure night length quantitatively, that the photoperiodic counter may play a prominent but slightly different role in extra short and extra long photoperiods and that northern strains show high stability against perturbations in the photoperiod length and in the presence of LD cycles. These features are best explained by the quantitative versions of the damped external coincidence model.

Adaptation and ecological speciation in seasonally varying environments at high latitudes: Drosophila virilis group

“…D. montana females show no circadian rhythmicity (high diapause peaks in photoperiods with a length of 24 h or its multiples; Figure 2(b )) in their diapause proportions in NH experiments, which suggests that the PPTM of this species is either based on heavily damping circadian oscillator(s) or that it lacks strong oscillators [ 43 , 44 ]. NH experiments have also shown that D. montana females measure the night length quantitatively, that their photoperiodic counter may play a slightly different role in extra short and long photoperiods and that the northern strains of this species show high stability against perturbations in photoperiod length and in the presence of light: dark (LD) cycles [ 45 ], see Figure 2(b ). Accordingly, the function of PPTM in D. montana is best explained by the quantitative versions of the damped external coincidence model [ 45 ].…”

“…NH experiments have also shown that D. montana females measure the night length quantitatively, that their photoperiodic counter may play a slightly different role in extra short and long photoperiods and that the northern strains of this species show high stability against perturbations in photoperiod length and in the presence of light: dark (LD) cycles [ 45 ], see Figure 2(b ). Accordingly, the function of PPTM in D. montana is best explained by the quantitative versions of the damped external coincidence model [ 45 ].…”

“…D. montana also shows latitudinal variation in the properties of the photoperiodic time-measuring system (PPTM) underlying reproductive diapause. In Nanda–Hamner (NH) experiment, the females of the northernmost strains enter diapause under both circadian (24 h and its multiples) and non-circadian photoperiods, and about half of them even in continuous darkness, while the females of the southern strains show high diapause proportions only in the 24 h photoperiod [ 45 ], see Figure 2(b ). This shows that the northern strains possess a high stability against perturbations in the photoperiod length and in the presence of LD cycles than the southern ones.…”

A Comparative Study of Circadian Rhythmicity and Photoperiodism in Closely Related Species of Blow Flies: External Coincidence, Maternal Induction, and Diapause at Northern Latitudes

“…The more complex response in N. vitripennis , initially interpreted as evidence for internal coincidence, however, has been re-interpreted as evidence for oscillator dampening in a clock of the external coincidence type ( Saunders, 2021 )–but this requires further experimental investigation. In some species such as the aphid Megoura viciae ( Lees, 1986 ) and drosophilids from high latitudes such as Drosophila ezoana ( Vaze and Helfrich-Förster, 2016 ) and D. montana ( Lankinen et al, 2021 ) ‘negative’ NH responses, lacking peaks and troughs in diapause incidence, have been recorded. In these cases, PPTM resembles a non-circadian hourglass-like timer.…”

“…In these cases, PPTM resembles a non-circadian hourglass-like timer. This hourglass-like clock mechanism, however, may be due to a rapidly dampening circadian oscillation, based upon the same molecular components as clocks showing ‘positive’ responses but with extreme reduction of their output ( Lewis and Saunders, 1987 ; Lankinen et al, 2021 ).…”