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 cockroach es, Blattella bisignata and Blatella germanica. The phase-response curve (P RC) of both species showed a similar pattern. Phase delays and advances wer e induced by light pulse during the early and late subjective night, respec tively, while no clear phase shifting was elicited during the subjective da y. 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 period-response curve (PdRC) was also constructed for both species. A lthough both species did not show great flexibility in circadian period cha nges, the phase shifts were significantly correlated with the period change s 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 phas e delay. The results indicate the overall adjustability of the circadian cl ock of B. germanica is inferior to that of B. bisignata. The significance o f this finding is discussed from an ecological perspective.