Circadian coordination of life functions is believed to contribute to an organism's fitness; however, such contributions have not been convincingly demonstrated in any animal. The most significant measure of fitness is the reproductive output of the individual and species. Here we examined the consequences of loss of clock function on reproductive fitness in Drosophila melanogaster with mutated period (per 0 ), timeless (tim 0 ), cycle (cyc 0 ), and Clock (Clk Jrk ) genes. Single mating among couples with clock-deficient phenotypes resulted in Ϸ40% fewer progeny compared with wild-type flies, because of a decreased number of eggs laid and a greater rate of unfertilized eggs. Male contribution to this phenotype was demonstrated by a decrease in reproductive capacity among per 0 and tim 0 males mated with wild-type females. The important role of clock genes for reproductive fitness was confirmed by reversal of the low-fertility phenotype in flies with rescued per or tim function. Males lacking a functional clock showed a significant decline in the quantity of sperm released from the testes to seminal vesicles, and these tissues displayed rhythmic and autonomous expression of clock genes. By combining molecular and physiological approaches, we identified a circadian clock in the reproductive system and defined its role in the sperm release that promotes reproductive fitness in D. melanogaster.M any life functions, from cellular activities to behavior, display daily (circadian) rhythms. These rhythms are generated by cell-autonomous circadian clocks and involve several genes encoding transcriptional regulators, which are substantially conserved among animals ranging from fruit flies to humans (1). The core clock mechanism in Drosophila melanogaster involves rhythmic transcription of the period (per) and timeless (tim) genes followed by nuclear accumulation of their proteins, PER and TIM. Another gene, Clock (Clk), shows mRNA oscillations that are out of phase with the oscillations of per and tim mRNAs. Protein encoded by Clk, together with protein encoded by the gene cycle (cyc), activates transcription of per and tim. These four genes are essential for the function of the brain clock, and a null mutation in any of them renders the fly behaviorally arrhythmic (2).Although the importance of circadian clocks in controlling behavior is well documented, their functional significance for physiology is less understood. Clock genes are rhythmically and autonomously expressed in peripheral tissues of D. melanogaster (3-5), zebrafish (6), and mammals (7), suggesting that peripheral clocks may coordinate physiological processes ultimately affecting fitness. Few studies have compared physiological parameters between individuals with a normal or disrupted circadian clock in any species. A study in cyanobacteria demonstrated that colonies in which free-running circadian periods are in resonance with environmental 12-h light͞12-h dark cycles (LD) out-compete colonies in which internal and external periodicities are out of synchro...
Release of mature sperm from the testis into seminal ducts of the gypsy moth exhibits a circadian rhythm. The rhythm of sperm release was shown to persist in vitro, in isolated complexes of testis and seminal ducts cultured in light-dark cycles or in constant darkness. The phase of the rhythm was also reset in vitro by exposure to shifted light-dark cycles. Therefore, the testis-seminal ducts complex from the gypsy moth is photosensitive and contains a circadian pacemaker, which controls the rhythm of sperm movement. This finding extends the range of structures in multicellular organisms that are known to contain circadian oscillators and provides a new model system in which circadian mechanisms may be studied.
Females of gypsy moth, Lyrnantria dispar L. (Lepidoptera: Lymantriidae), mated to males kept in constant light (LL) as pharate adults fail to oviposit. In males, a rhythm of sperm release from the testis that occurs in light-dark (LD) cycles is abolished in LL, and the total amount of sperm released from the testis is approximately half of that of LD males. Moreover, any sperm that may be released from the testis of LL males tend to remain in the vasa deferentia instead of moving into the duplex as in LD males. Consequently, in LL very few sperm bundles are transferred to the bursa copulatrix during mating; furthermore, these bundles fail to disperse into spermatozoa and sperm do not reach the spermatheca. The presence of a spermatheca filled with sperm must play an important role in controlling oviposition because their removal from mated females prevents egg-laying. Our results indicate that the rhythm of sperm release from the testis is essential for the ability of sperm to migrate in male and female reproductive tracts. The rhythms may help to synchronize final stages of sperm development with the activity of phagocytic and secretory cells lining the reproductive tract.
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