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

Abstract

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 = 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.

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

“…In Fig. 1C, the``valleys'' or short-day responses occur when T 24, 45, and 66 h, indicating a 21-h period (s 21 h) of the underlying pacemaker (Wegis et al 1997). Note also that the height (long-day response) of the response curves declines among populations from progressively more northern latitudes.…”

“…Previous results with W. smithii (Wegis et al 1997) have shown that in populations from more northern latitudes, Fig. 4 Developmental (longday) response of W. smithii to ASPPs with a 10-h main photophase and a 38-h scotophase (T=48).…”

“…Fig. 2 Origin of W. smithii populations in North America, with the latitudes of origin (Lat) and critical photoperiods (CPP) for each location (Loc) from Wegis et al (1997) To quantify the area under a peak, we summed the percentages of development at the peak, 1 h earlier, and 1 h later.…”

“…A photophase of 13 h represents a long photophase for southern populations and a short photophase for intermediate and northern populations. A photophase of 14 3/4 h represents a long photophase for southern and intermediate populations and a short photophase for northern populations (Wegis et al 1997). The experiment was run in a single block in one 12-chamber photoperiod cabinet.…”

“…In this case, the 1-h pulse serves to set the photosensitivity rhythm and the end of the main pho- (Bradshaw 1980). C Developmental (long-day) response of W. smithii to T-experiments with a 10-h photophase and T varied from 24 to 72 h (Wegis et al 1997). Note that s 21 h (valley-to-valley interval) for all latitudes and that the magnitude of the long-day response (peak height) declines with increasing latitude.…”