Egg laying rhythm in drosophila melanogaster

Abstract

Many behavioral and physiological processes in the fruit fly Drosophila melanogaster show robust circadian oscillation. Some of the best studied circadian rhythms include those in adult emergence, activity/rest, olfaction, mating and egg laying. Unlike most other circadian rhythms egg laying rhythm is unique and relatively less understood; it persists under constant light (LL), and also in the absence of the ventral lateral neurons which are known to be the circadian pacemakers for several other rhythms. Further, the expression of core clock genes such as period (per) and timeless (tim) in the ovaries do not show any oscillation. In this thesis, I have discussed the findings of some of my studies aimed at probing behavioural, neuronal and genetic mechanisms underlying egg laying rhythm in Drosophila. We began by studying egg laying rhythm under temperature cycles with the objective of assessing whether it can synchronize to temperature cycles. We studied egg laying behavior in temperature cycles imposed under constant darkness (DD) and LL conditions. Results of our study suggest that temperature cycles synchronize circadian egg laying rhythm with the phase of oviposition peak occurring close to the onset of low temperature phase of temperature cycle. Also, the percentage entrainment of egg laying rhythm is significantly greater under temperature cycles compared to light/dark (LD) cycles suggesting that temperature cycle is a stronger Zeitgeber. Next we tested whether electrical silencing of the pigment dispersing factor (PDF) expressing LNv has any effect on the persistence and entrainability of egg laying rhythm. For this we genetically manipulated the electrical properties of PDFvi expressing ventral lateral neurons (LNv) in flies by using pdf-GAL4 driver to express ion channels dORKΔ-C1 and Kir2.1 in a tissue-specific manner, and studied its effect on the egg laying rhythm in DD and LD. We found that while electrical silencing of LNv neurons abolished adult emergence and activity/rest rhythms, egg laying rhythm continued unabated under DD. However, electrical silencing of the LNv neurons significantly lengthens the circadian periodicity of egg laying rhythm. This suggest that although the electrical output from the LNv neurons may not be required for the persistence of circadian egg laying rhythm in DD, it is required for maintaining the clock periodicity close to 24 hr. We also studied the role of mating in the regulation of egg laying rhythm to determine whether mating patterns have any effect on circadian egg laying rhythm. We used per0w mutants (arrhythmic for mating behaviour) and its control w (rhythmic for mating behavior) to assay egg laying behavior. Our results indicate that presence of rhythmic female (w) invariably enhanced the percentage of flies that showed rhythmic egg laying behavior in DD, and percentage of flies that entrained to LD cycles in all the male-female combinations compared to the case when arrhythmic females (per0w) were used. This suggests that the robustness in the persistence of egg laying rhythm in DD and its entrainability in LD is primarily driven by females. Finally, we studied the expression of logjam (loj), a gene essential for oviposition in Drosophila. The objective of this study was to find out whether expression pattern of this gene oscillates in ovaries. We used female CantonS flies and performed quantitative real-time PCR to quantify the mRNA levels of loj using primers which amplify a region of the gene which is common to all known transcripts of loj. Results of our studies showed that the mRNA expression of loj differs in a time vii dependent manner; its expression level is significantly greater at Zeitgeber Time 0 (ZT0) compared to ZT12. This suggests that loj may have some role to play in the regulation of circadian egg laying rhythm in Drosophila apart from its role in oviposition behaviour. Based on the results of our studies we conclude that egg laying rhythm in Drosophila is a complex circadian phenomenon whose underlying molecular mechanisms seem to be independent of the core clock genes and the circadian neural network that have been implicated in the regulation of other behavioural rhythms. It is likely that circadian egg laying rhythm in Drosophila is regulated by molecular mechanisms involving post-translational regulations of core clock genes and/or novel molecular mechanisms involving the gene loj. The circadian pacemaker for egg laying rhythm could be the peripheral oscillators in the ovaries.