Evolution of time-keeping mechanisms: early emergence and adaptation to photoperiod

Abstract

Virtually all species have developed cellular oscillations and mechanisms that synchronize these cellular oscillations to environmental cycles. Such environmental cycles in biotic (e.g. food availability and predation risk) or abiotic (e.g. temperature and light) factors may occur on a daily, annual or tidal time scale. Internal timing mechanisms may facilitate behavioural or physiological adaptation to such changes in environmental conditions. These timing mechanisms commonly involve an internal molecular oscillator (a 'clock') that is synchronized ('entrained') to the environmental cycle by receptor mechanisms responding to relevant environmental signals ('Zeitgeber', i.e. German for time-giver). To understand the evolution of such timing mechanisms, we have to understand the mechanisms leading to selective advantage. Although major advances have been made in our understanding of the physiological and molecular mechanisms driving internal cycles (proximate questions), studies identifying mechanisms of natural selection on clock systems (ultimate questions) are rather limited. Here, we discuss the selective advantage of a circadian system and how its adaptation to day length variation may have a functional role in optimizing seasonal timing. We discuss various cases where selective advantages of circadian timing mechanisms have been shown and cases where temporarily loss of circadian timing may cause selective advantage. We suggest an explanation for why a circadian timing system has emerged in primitive life forms like cyanobacteria and we evaluate a possible molecular mechanism that enabled these bacteria to adapt to seasonal variation in day length. We further discuss how the role of the circadian system in photoperiodic time measurement may explain differential selection pressures on circadian period when species are exposed to changing climatic conditions (e.g. global warming) or when they expand their geographical range to different latitudes or altitudes.

Circadian rhythms are not involved in the regulation of circannual reproductive cycles in a sub-tropical bird, the spotted munia

“…A T21 or T27 day, falling within the range of the circadian entrainment (T=24±3 h), is expected to synchronize the circadian oscillator, as does a T24 day (Hut and Beersma, 2011). Thus, compared with a T24 day, which results in a 365 day annual cycle, the T21 and T27 days should result in the faster (short) and slower (long) circannual cycles, respectively.…”

Clock-Controlled and FLOWERING LOCUS T (FT)-Dependent Photoperiodic Pathway in Lotus japonicus I: Verification of the Flowering-Associated Function of an FT Homolog

“…3,4) It is generally accepted that the molecular bases of clock-controlled flowering time are highly conserved throughout the plant kingdom. [5][6][7][8] However, it is also assumed that the mechanisms by which the clock regulates flowering time differs considerably in detail between species. [9][10][11] In this respect, comparative genomics approach might be effective in addressing these issues.…”

Rhythmicity of the intestinal microbiota is regulated by gender and the host circadian clock

“…Circadian rhythms also exist in fungi and cyanobacteria (16). For example, a pacemaker in cyanobacteria transduces the oscillating daylight signal to regulate gene expression and to time cell division (17,18).…”