ADJUSTABILITY OF THE CIRCADIAN CLOCK IN THE COCKROACHES: A COMPARATIVE STUDY OF TWO CLOSELY,RELATED SPECIES,BLATTELLA GERMANICA AND BLATTELLA BISIGNATA

A single 2h light pulse (250 lux) was given at various times to phase shift the locomotor circadian rhythm of two species of closely related cockroaches, Blattella bisignata and Blatella germanica. The phase-response curve (PRC) of both species showed a similar pattern. Phase delays and advances were induced by light pulse during the early and late subjective night, respectively, while no clear phase shifting was elicited during the subjective day. However, the magnitude of the phase delay (1.89h ± 0.66h) and advance (0.69h ± 0.36h) of B. bisignata was significantly larger than that of B. germanica (0.78h ± 0.38h and 0.35h ± 0.18h, respectively). This result indicates the superior adjustability of the circadian clock in B. bisignata. The periodresponse curve (PdRC) was also constructed for both species. Although both species did not show great flexibility in circadian period changes, the phase shifts were significantly correlated with the period changes in the advance zone of B. bisignata (r = 0.72, P <. 1). This allowed the circadian clock of B. bisignata to display better entrainability since the phase advance adjustment was significantly more difficult than that of phase delay. The results indicate the overall adjustability of the circadian clock of B. germanica is inferior to that of B. bisignata. The significance of this finding is discussed from an ecological perspective. (Chronobiology International, 18(5), 767– 780, 2001)