The circadian clock interacts with metabolic physiology to influence reproductive fitness

Abstract

Circadian rhythms are regulated by a synchronized system of central and peripheral clocks. Here, we show that a clock in the Drosophila fat body drives rhythmic expression of genes involved in metabolism, detoxification, the immune response, and steroid hormone regulation. Some of these genes cycle even when the fat body clock is disrupted, indicating that they are regulated by exogenous factors. Food is an important stimulus, as limiting food availability to a 6 hr interval each day drives rhythmic expression of genes in the fat body. Restricting food to a time of day when consumption is typically low desynchronizes internal rhythms because it alters the phase of rhythmic gene expression in the fat body without affecting the brain clock. Flies maintained on this paradigm produce fewer eggs than those restricted to food at the normal time. These data suggest that desynchrony of endogenous rhythms, caused by aberrant feeding patterns, affects reproductive fitness.

Figure 1. Rhythmic gene expression in the fat body is affected by expression of DN-CLK

Microarray analysis was used to detect cycling transcripts in the fat bodies of wild type flies (left) and flies expressing DN-Clk flies (right). Individual transcripts were median normalized, sorted by phase, and plotted as a heatmap (yellow = high expression, blue = low expression).

Figure 2. Restricted feeding between CT9 and CT15 drives rhythmic expression of clock genes and clock-regulated genes in the Drosophila fat body

Quantitative PCR was performed to assay expression of seven cyclically expressed genes at different times of day in the fat body (A–G) of flies maintained on ad lib food (red) or restricted to food between CT9 and CT15 (black). Flies were transferred to a 1% agar vial from a normal food vial on day 1 in DD. Restricted feeding was started on day 2 and continued until day 6 at which point the flies were collected. Of the seven genes assayed - tim, per, cyp6a21, cyp6a20, CG31189, Jhe, and pli –all except per and CG31189 were also assayed in the brain. Expression values are normalized to α-tubulin. Each experiment was repeated at least three times, and the error bars represent S.E.M. Asterisks denote the time of peak expression of genes (p<0.05 compared to the trough point by two-tailed Student’s t-test with unequal variance) in flies that underwent restricted feeding. The cycling was also independently verified by ANOVA.

Figure 3. Restricted feeding does not drive rhythmic gene expression in the brain

As in Figure 2, flies were maintained on ad lib food (red) or restricted to food between CT9 and CT15 (black) from days 2–6 of DD. Quantitative RT-PCR was performed to assay expression of five genes expressed cyclically in the fat body (see Figure 2) at different times of day (A–D). Expression values are normalized to α-tubulin. Each experiment was performed at least twice. Error bars represent standard deviation.

Figure 4. The effect of restricted feeding on the rhythm and phase of cycling genes is mediated by both Clk-dependent and Clk-independent pathways

Quantitative PCR analysis in fat bodies of control Clk^Jrk flies (black) and Clk^Jrk flies fed between CT9 and CT15 (red). These experiments were repeated at least twice, and error bars represent the S.E.M. In Clk^Jrk flies, restricted feeding has less of an effect on the cyclic expression of some genes such as (A) per, (B) tim, (C) cyp6a21, (D) cyp6a20, and (E) CG31189, but can drive the cycling of pli (F) and Jhe (G) in a similar manner as in WT flies. Error bars represent standard deviation.

Figure 5. The egg-laying rhythm does not require clocks in the male or the female fat body

Flies were entrained to LD for 3 days, and from the first day in DD, the egg-laying profiles of (A) iso³¹ flies, (B) Clk^Jrk flies (C) control female flies mated with control males, (D) control female flies mated with male flies lacking a fat body clock, (E) female flies lacking a fat body clock mated with control males, and (F) female flies lacking a fat body clock mated with male flies also lacking a fat body clock, were recorded for 5 consecutive days. For (A) and (B), the experiments were done twice with 10 vials, of 10 flies each, for each genotype per experiment. The final data for each time point represent the average of 20 recorded values, and the data are presented as mean±SEM. For (C)–(F), the data for each time point were from an average of 10 recorded values for each genotype, and the data are presented as mean±SEM. The data were analyzed for a rhythm using JTK_Cycle. A ~24 hour rhythm was detected in all samples except (B) and (C). Since transgenes present in C are also present in some of the other conditions, lack of a rhythm in this control condition probably arises from a spurious data point.

Figure 6. The effect of restricted feeding on the timing and number of eggs laid

Flies were maintained on a restricted feeding paradigm for six days, and then transferred to ad lib food. (A) On the first day after RF, the egg-laying rhythm was measured, as described in Figure 4, in wildtype iso³¹ flies maintained on RF CT9-15. (B) The total number of eggs laid on the first day following RF was measured in wildtype and Clk^Jrk flies maintained on RF CT9-15 or RF CT21-3. Iso³¹ flies fed between CT9 and CT15 lay fewer eggs than those fed between CT21 and CT3. This difference is not seen in Clk^Jrk flies. The experiments were performed twice with 10 vials for each genotype per experiment. For (B) since eggs were collected over a 24-hour period, only five flies were housed per vial. Since the survival rate was different from vial to vial, the data are plotted per fly, with the error bars indicating variability across vials. Statistical significance was determined by two-tailed Student’s t-test with unequal variance. Error bars represent S.E.M for parts A and B.