Pankaj Yadav

Correlated changes in life history traits in response to selection for faster pre-adult development in the fruit fly Drosophila melanogaster.

Insects including the fruit fly Drosophila melanogaster are under intense pressure to develop rapidly because they inhabit ephemeral habitats. We have previously shown that when selection for faster development was artificially imposed on D. melanogaster in the laboratory, reduction of pre-adult development time and shortening of the clock period occurs, suggesting a role for circadian clocks in the regulation of life history traits. Circadian clocks in D. melanogaster have also been implicated in the control of metabolic pathways, ageing processes, oxidative stress and defense responses to exogenous stressors. In order to rigorously examine correlations between pre-adult development time and other life history traits, we assayed pre-adult survivorship, starvation and desiccation resistance, body size and body weight, fecundity and adult lifespan in faster developing populations of D. melanogaster. The results revealed that selection for faster pre-adult development significantly reduced several adult fitness traits in the faster developing flies without affecting pre-adult survivorship. Although overall fecundity of faster developing flies was reduced, their egg output per unit body weight was significantly higher than that of controls, indicating that reduction in adult lifespan might be due to disproportionate investment in reproduction. Thus our results suggest that selection for faster pre-adult development in D. melanogaster yields flies with higher reproductive fitness. Because these flies also have shorter clock periods, our results can be taken to suggest that pre-adult development time and circadian clock period are correlated with various adult life history traits in D. melanogaster, implying that circadian clocks may have adaptive significance.

A model based on oscillatory threshold and build-up of a developmental substance explains gating of adult emergence in Drosophila melanogaster

SUMMARY Adult emergence (eclosion) of fruit flies Drosophila melanogaster under constant laboratory conditions follows a circadian pattern with bouts of eclosion recurring at approximately 24 h intervals. Under periodic light:dark (LD) cycles, adults emerge only during a specific time of the day followed by little or no emergence for the rest of the day. This phenomenon is therefore equated to a gate of emergence that, when open, allows adults to emerge and when closed, no emergence takes place. In this study, we attempt to understand the mechanism underlying adult emergence rhythm in D. melanogaster using a model based on interplay between developmental and circadian clock systems. The model is composed of an oscillatory threshold of a substance that builds up during pre-adult development. Computer simulations based on this model enabled us to make specific predictions about the ‘gate width’ of the adult emergence rhythm under conditions of fast/slow pre-adult development and short/long circadian periods, which we subsequently tested empirically. The main predictions from the simulations are: (1) flies with faster development have greater gate width and vice versa, and (2) flies with faster circadian clocks have shorter gate width and vice versa. To empirically validate these predictions, we carried out experiments on D. melanogaster populations known to have fast/slow pre-adult development, short/long circadian periods and narrow/wide gate width. Additionally, we manipulated the rate of pre-adult development of the above flies by increasing/decreasing ambient temperature to further examine the influence of developmental rates on gate width of adult emergence rhythm by a complementary approach. The results show that gate width is greatly influenced by the duration of pre-adult development and the length of circadian cycles. This suggests that the adult emergence rhythm of D. melanogaster may be based on mechanisms involving oscillatory threshold and build-up of a developmental substance.

Extent of mismatch between the period of circadian clocks and light/dark cycles determines time-to-emergence in fruit flies

Circadian clocks time developmental stages of fruit flies Drosophila melanogaster, while light/dark (LD) cycles delimit emergence of adults, conceding only during the “allowed gate.” Previous studies have revealed that time‐to‐emergence can be altered by mutations in the core clock gene period (per), or by altering the length of LD cycles. Since this evidence came from studies on genetically manipulated flies, or on flies maintained under LD cycles with limited range of periods, inferences that can be drawn are limited. Moreover, the extent of shortening or lengthening of time‐to‐emergence remains yet unknown. In order to pursue this further, we assayed time‐to‐emergence of D. melanogaster under 12 different LD cycles as well as in constant light (LL) and constant dark conditions (DD). Time‐to‐emergence in flies occurred earlier under LL than in LD cycles and DD. Among the LD cycles, time‐to‐emergence occurred earlier under T4–T8, followed by T36–T48, and then T12–T32, suggesting that egg‐to‐emergence duration in flies becomes shorter when the length of LD cycles deviates from 24 h, bearing a strong positive and a marginally negative correlation with day length, for values shorter and longer than 24 h, respectively. These results suggest that the extent of mismatch between the period of circadian clocks and environmental cycles determines the time‐to‐emergence in Drosophila.

Environmentally-induced modulations of developmental rates do not affect the selection-mediated changes in pre-adult development time of fruit flies Drosophila melanogaster

In a previous study we had shown that 55 generations of selection for faster egg-to-adult development in fruit flies Drosophila melanogaster results in shortening of pre-adult duration by ~29-h (~12.5%) and speeding-up of circadian clock period (τ) by ~0.5-h, implying a positive correlation between development time and τ. In Drosophila, change in ambient temperature is known to alter the rate of pre-adult development but not the speed of circadian clocks whereas 12:12-h warm/cold (WC) cycles are likely to alter both pre-adult development rate and τ (via entrainment). To study the effect of overall speeding-up/slowing-down of pre-adult development and circadian clocks on the selection-mediated difference in pre-adult development time, we subjected developing flies to the following conditions: (i) different ambient temperatures (18, 25 and 29°C) under constant darkness (DD) to alter the rate of pre-adult development, or (ii) cyclic WC conditions (WC1-25:18 or WC2-29:25°C) to alter rate of development and τ. The results revealed that the selected (FD) stocks develop faster than controls (BD) by ~52, 28 and 21-h, at 18, 25 and 29°C, respectively, and by 28 and 26-h under WC1 and WC2, respectively. The τ of activity/rest rhythm decreased considerably at 18°C but it did not differ between the FD and BD flies, which suggests a break-down of correlation between development time and τ, seen under their normal rearing conditions (constant darkness--DD at 25°C). While the absolute difference in development time between FD and BD stocks increased or decreased under cooler or warmer conditions, the relative difference in their pre-adult development time remained largely unaltered. These results suggest that manipulations in ambient conditions independently changes development time and τ, resulting in a break-down of the genetic correlation between them.

Analysis of circadian locomotor rhythms in vg and cry b mutants of Drosophila melanogaster under different light:dark regimens

The fruit fly (Drosophila melanogaster) offers an extremely useful system to study circadian clock function at the behavioral, molecular, cellular, and organismal levels. We reared wild type (Oregon R+) and mutant flies – vestigial (vg) and cryptochrome depleted (cryb ) flies in 12:12 light:dark cycles (LD 12:12) from the embryo to adult stage. Later, the locomotor activity rhythms of these newly eclosed flies were monitored in different light:dark (LD) regimens such as T20 (LD 10:10), T22 (LD 11:11), T24 (LD 12:12), T26 (LD 13:13), T28 (LD 14:14), continuous light (LL), and continuous darkness (DD) for seven to eight cycles. The circadian locomotor characteristics, such as period length, percentage activity, day and nighttime activity, anticipation index (AI), and phase relationships, were analyzed. Under these LD regimens, the above-mentioned characteristics were altered in these mutants when compared to wild-type (WT) flies. Altered circadian locomotor rhythms in vg mutants under these LD regimens might be due to its reduced wing structure. cryb flies have defective photic input to clock and it showed altered circadian locomotor activity rhythms when compared to WT flies under all these LD regimens. From this study, we suggest that under different LD regimens blue-light component cryptochrome as well as wing structure play an important role for the expression of circadian locomotor rhythms in D. melanogaster.

Response to 'Remarks on the article on life-history traits in Drosophila populations selected for rapid development by Yadav and Sharma'

While we agree that citations of the review article of Prasad and Joshi ([Prasad and Joshi, 2003][1]) could have been done more perceptively in our recent paper ([Yadav and Sharma, 2014][2]), most other comments made by Prasad and Joshi ([Prasad and Joshi, 2015][3]) are certainly out of place. We

Breakdown of selection-mediated correlation between development time and clock period.

Previously we have reported that selection for faster pre-adult development in fruit flies speeded-up development by ~29-h and shortened the clock period (τ) by ~0.5h, which suggests that development time and τ are correlated. Since it is known that τ is altered following exposure to light/dark (LD) cycles, we asked whether this correlation persists in the faster developing (FD) and control (BD) flies by examining the τ of the activity/rest rhythm and its difference between the two stocks following exposure to a variety of cyclic conditions. We assayed the activity/rest behavior of FD and BD flies under DD, following a week-long exposure to (a) LD cycles of 10:10h, 12:12h and 14:14h, or (b) LD12:12h with different light intensities (10, 100 and 1000lx), or (c) 12:12h warm/cold (WC) cycles of 25:18°C (WC1) and 29:25°C (WC2), or (d) WC1 or WC2, in-phase or out-of-phase with LD. The results revealed that both LD and WC altered the τ of FD and BD flies, and considerably reduced the selection-mediated difference between the two stocks. LD10:10 caused more severe after-effects on τ compared to LD12:12 and LD14:14. Among the WC cycles, WC1 which had a higher contrast caused period shortening. Irrespective of the phase relationship, imposition of LD cycles on WC cycles made no difference to the extent of after-effects; however, interestingly there was a reversal in the trend, in that, now WC2 with LD caused most drastic reduction in τ. These results suggest that cyclic environments modulate the circadian organization of Drosophila melanogaster altering the selection-mediated correlation between pre-adult development time and clock period.