ADAPTIVE FITNESS CONFERRED BY CIRCADIAN CLOCKS
It has long been presumed that the ability to anticipate light/dark cycles gives organisms a fitness advantage. One long-standing idea, termed the escape from light hypothesis, posits that organisms would accrue advantage from phasing light-sensitive processes, such as DNA replication, to the dark portion of the daily cycle (Pittendrigh, 1993).
In cyanobacteria, competitive ability depends on the correspondence between a strain's free-running period and ambient daylength; wild-type strains outcompete either long- or short-period mutants when grown in 24-h days (12 h light/12 h dark). This does not reflect a competitive advantage to the wild type under all conditions because long-period (30-h period) mutants outcompete the wild type (25-h period) when grown in long cycles (15 h light/15 h dark) (Johnson, 2005).
Early studies in tomato showed that growth improved on light/dark cycles of 24 h rather than short (6 h light/6 h dark) or long (24 h light/24 h dark) cycles or continuous light (Withrow and Withrow, 1949; Highkin and Hanson, 1954; Hillman, 1956),
although this work only indirectly implicates the circadian clock in the growth response. More direct testing has come in recent years. Arabidopsisclock mutants with longer than normal periods (28 h) have lower biomass than those with short periods (20 h) when grown under short cycles (10 h light/10 h dark), and these differences in size are largely attributable to impaired physiological function, including lower rates of chlorophyll production and carbon fixation (Green et al., 2002; Dodd et al., 2005).
In Arabidopsis, there is considerable circadian variation among natural genotypes (for examples, see Swarup et al., 1999; Michael et al., 2003; Edwards et al., 2005). There is a positive correlation between period length of a set of natural accessions and daylength encountered at latitude of origin of the accessions (Michael et al., 2003), which may indicate a selection of altered clock function under differing environmental conditions (temperature and daylength covary with latitude). In addition to the effects of period length on carbon fixation, biomass, and survival described above (Dodd et al., 2005), period length may also affect the flowering timing response (for example, see Yanovsky and Kay, 2002). Under the entraining conditions of a light/dark cycle, the period is 24 h. The effects of a long endogenous circadian period are seen as a lagging phase under entraining conditions. Thus, lengthening the period would delay the accumulation of COmRNA, which would increase the critical daylength required for the accumulation of CO protein. This would delay flowering until the longer days encountered later in the season, which could be advantageous at higher latitudes where daylength increases rapidly and precedes the cessation of freezing weather. Altered temperature compensation might also underlie this latitudinal cline; there is a similar latitudinal cline in a polymorphism in the Drosophila per gene that is thought to be related to altered temperature compensation properties conferred by the per alleles (Costa et al., 1992; Sawyer et al., 1997).
It has long been presumed that the ability to anticipate light/dark cycles gives organisms a fitness advantage. One long-standing idea, termed the escape from light hypothesis, posits that organisms would accrue advantage from phasing light-sensitive processes, such as DNA replication, to the dark portion of the daily cycle (Pittendrigh, 1993).
In cyanobacteria, competitive ability depends on the correspondence between a strain's free-running period and ambient daylength; wild-type strains outcompete either long- or short-period mutants when grown in 24-h days (12 h light/12 h dark). This does not reflect a competitive advantage to the wild type under all conditions because long-period (30-h period) mutants outcompete the wild type (25-h period) when grown in long cycles (15 h light/15 h dark) (Johnson, 2005).
Early studies in tomato showed that growth improved on light/dark cycles of 24 h rather than short (6 h light/6 h dark) or long (24 h light/24 h dark) cycles or continuous light (Withrow and Withrow, 1949; Highkin and Hanson, 1954; Hillman, 1956),
although this work only indirectly implicates the circadian clock in the growth response. More direct testing has come in recent years. Arabidopsisclock mutants with longer than normal periods (28 h) have lower biomass than those with short periods (20 h) when grown under short cycles (10 h light/10 h dark), and these differences in size are largely attributable to impaired physiological function, including lower rates of chlorophyll production and carbon fixation (Green et al., 2002; Dodd et al., 2005).
In Arabidopsis, there is considerable circadian variation among natural genotypes (for examples, see Swarup et al., 1999; Michael et al., 2003; Edwards et al., 2005). There is a positive correlation between period length of a set of natural accessions and daylength encountered at latitude of origin of the accessions (Michael et al., 2003), which may indicate a selection of altered clock function under differing environmental conditions (temperature and daylength covary with latitude). In addition to the effects of period length on carbon fixation, biomass, and survival described above (Dodd et al., 2005), period length may also affect the flowering timing response (for example, see Yanovsky and Kay, 2002). Under the entraining conditions of a light/dark cycle, the period is 24 h. The effects of a long endogenous circadian period are seen as a lagging phase under entraining conditions. Thus, lengthening the period would delay the accumulation of COmRNA, which would increase the critical daylength required for the accumulation of CO protein. This would delay flowering until the longer days encountered later in the season, which could be advantageous at higher latitudes where daylength increases rapidly and precedes the cessation of freezing weather. Altered temperature compensation might also underlie this latitudinal cline; there is a similar latitudinal cline in a polymorphism in the Drosophila per gene that is thought to be related to altered temperature compensation properties conferred by the per alleles (Costa et al., 1992; Sawyer et al., 1997).
