Abstract
Summary While angiosperm clocks can be described as an intricate network of interlocked transcriptional feedback loops, clocks of green algae have been modelled as a loop of only two genes. To investigate the transition from a simple clock in algae to a complex one in angiosperms, we performed an inventory of circadian clock genes in bryophytes and charophytes. Additionally, we performed functional characterization of putative core clock genes in the liverwort Marchantia polymorpha and the hornwort Anthoceros agrestis.Phylogenetic construction was combined with studies of spatiotemporal expression patterns and analysis of M. polymorpha clock gene mutants.Homologues to core clock genes identified in Arabidopsis were found not only in bryophytes but also in charophytes, albeit in fewer copies. Circadian rhythms were detected for most identified genes in M. polymorpha and A. agrestis, and mutant analysis supports a role for putative clock genes in M. polymorpha.Our data are in line with a recent hypothesis that adaptation to terrestrial life occurred earlier than previously expected in the evolutionary history of charophyte algae. Both gene duplication and acquisition of new genes was important in the evolution of the plant circadian clock, but gene loss has also contributed to shaping the clock of bryophytes.
Highlights
Adaptation to changing environments is critical to all life
To trace the origin of the plant circadian clock, we performed an inventory of homologues to known plant circadian clock genes in available bryophyte and charophyte genomes (Table 1; Methods S1)
Two subgroups have been named in this family, one comprising CLOCK-ASSOCIATED 1 (CCA1), LATE ELONGATED HYPOCOTYL (LHY), RVE1, 2, 7 and RVE7-like and the other RVE3, 4, 5, 6 and 8
Summary
Adaptation to changing environments is critical to all life. Some of these changes are predictable, such as day–night cycles and the ever-changing seasons. Organisms from all kingdoms of life have developed mechanisms to anticipate such predictable changes. Intrinsic clocks that generate circadian rhythms are present in most organisms, from cyanobacteria to land plants and animals. The overall architecture is generally conserved, the key genes involved are generally not, suggesting multiple independent origins of circadian clocks (Dunlap, 1999; Young & Kay, 2001; McClung, 2013). The clock has to cope with unpredictable variations in sunlight and temperature and is generally thought to have evolved towards a more complex and thereby flexible and robust architecture (Rand et al, 2004; Tsai et al, 2008)
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