Vivian Meyer

• Rhythmic oscillations that repeat every 24 hours can be found in numerous behavioral and physiological functions. Beside the endogenous master clock in the suprachiasmatic nucleus (SCN), peripheral oscillators exist that can disengage from the master clock rhythm by different mechanisms. The fact that core clock genes in peripheral tissues do not always have the same characteristics as in the SCN suggests that their function may vary in different organs. One main function of the mammalian testis is the production of sperm cells. The duration of the cycle of the seminiferous epithelium (DCSE) is species-specific and has a very low inter-individual variation. Various substances and environmental conditions failed to alter this duration which suggests that the exact timing of male germ cell development is mainly determined by the genetic background of the cell itself. The aim of this study was to search for a possible link between the DCSE and daily clock gene expression patterns in the testis. Therefore, daily expression patterns of the clock genes Bmal1, Period1, Period2, Clock, Cryptochrome1 and Cryptochrome2 were recorded at each of eight time points during a 24 hour period in the testis, kidney, liver, spleen and heart of three hamster species (Phodopus sungorus, Phodopus roborovskii and Cricetulus griseus; family: Cricetidae). Additionally, the DCSE of P. roborovskii and C. griseus was estimated, whereas the DCSE of P. sungorus has already been published. Clock gene expression was found to be rhythmic in all investigated organs, however with inconsistent results in the testis. Complex cosinor analysis revealed species differences in temporal gene expression patterns regarding their orthophase, number of peaks, and amplitude for all genes and organs with most pronounced differences in the testis. In this organ, all six gene expression patterns were more similar between P. roborovskii and C. griseus than between them and P. sungorus. However, the DCSE of P. roborovskii (9.2 days) was closer to that of P. sungorus (7.9 days, Van Haaster and De Rooij 1993a) than to that of C. griseus (17.1 days). The main hypothesis of this study could not be confirmed, since there was no hint for a link between clock gene expression patterns in the testis and the DCSE in the investigated species. The results of this study strongly indicate that clock gene expression is species-specific and that their function in peripheral tissues might be at least partly connected to clock-unrelated traits that vary between the investigated species.