Abstract
Natural plasticity in overt circadian rhythms can be observed in various animals. Little is known about how this phenomenon help Euglena gracilis adapt to environmental stimuli. We used four groups of strain Z. Two groups were from our laboratory, ZObihiro1 and ZObihiro2; Third group was from the National Institute for Environmental Studies, Japan (ZNIES-48) and the other was from Osaka Prefecture University (ZOsaka). The latter two were grown photoautotrophically at a light intensity of 84 μmol m-2 s-1 (day-white type lamps) at 25°C with air bubbling, as were ours, for two months prior to experiments. Results showed that ZObihiro2 and ZOsaka grew faster than ZObihiro1 and ZNIES-48. Upon transferring from light to darkness, population growth ceased within 8-10 h with the cell number increase in the dark of 41% in ZObihiro1 and ZObihiro2, 35% in ZOsaka and remarkably low 22% in ZNIES-48. Timing of cell division bursts in the circadian rhythm of cell population growth in 24 h light-dark cycles was the same in all four groups. Magnitudes of the rhythm were different: both ZObihiro1 and ZObihiro2 completely doubled, but ZNIES-48 multiplied by 1.9, and ZOsaka multiplied feebly by 1.7. The photoinduction of commitment to cell division in DD followed a circadian rhythm. All four showed the same peak at subjective dusk, but the amplitudes differed in the order, ZObihiro2 > ZOsaka > ZObihiro1 >> ZNIES-48. The resistance to photosensitization against Rose-Bengal follows a clear circadian rhythm in all substrains except in ZNIES-48. ZObihiro1 and ZOsaka showed the phasing similar to UV resistance rhythm, but ZObihiro2 did not. These results suggest the plasticity of circadian rhythms within a species, if not within a strain. Moreover, it is also apparent that different substrains/ecotypes present within the same Z strain.
 Int. J. Appl. Sci. Biotechnol. Vol 7(2): 207-216
Highlights
Circadian rhythms are a ubiquitous occurrence in the natural world
We examined the circadian rhythms related to cell population growth, photo-inductive capacity of commitment to cell division and resistance to oxidative stress in four different Z strain collections
The GT was remarkably different among substrains; ZObihiro1 and ZOsaka grew with a GT of 15 and 15.6 h respectively, whereas the cell populations of both ZObihiro1 and ZNIES-48 doubled every 16.6 h
Summary
Circadian rhythms are a ubiquitous occurrence in the natural world. Most of the physiological processes and behavioral functions in many diverse organisms are expressed rhythmically according to the day and night cycles. These circadian rhythms are controlled and maintained by self-sustaining biological oscillators, and provide the organisms with survival advantages by optimizing the organisms’ responses to its environment and enhancing its fitness (Green et al, 2002). Studies on circadian rhythms in diverse organisms have led to an appreciation of the similarities and differences that exist between the organization of clocks in these organisms.
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