Developmental patterns of C elegans cell cycles

C. elegans embryonic development (Fig. 1), like that of many animals embryos, involves a series of rapid cleavage cell divisions which generate the various cells of the newly-hatched L1 larva (Sulston et al 1983). Postembryonic development differs substantially from embryonic development, chiefly in that postembryonic cell cycle progression requires growth. In the absence of food, the hatchling does not develop and postembryonic cell cycles are suspended in G1 until food is

FIG. 1. Phases of C. elegans embryonic and postembryonic development. Embryonic development involves cleavage cell cycles and no increase in mass. Embryogenesis is separated from larval development by an extended G1 phase that spans hatching. After hatching, subsequent cell cycle progression depends on, in addition to the intrinsic cell cycle machinery, other factors such as cellular growth and developmental signals. Postembryonic cell divisions occur during all four larval stages (L1—L4). Animals can develop continuously, or they can arrest as newly-hatched L1 larvae if food is absent. Starvation and crowding induces L2 animals to molt to the developmentally arrested dauer larva (Riddle & Albert 1997). Arrested L1 larvae or dauer larvae will resume development if transferred to favourable culture conditions.

FIG. 1. Phases of C. elegans embryonic and postembryonic development. Embryonic development involves cleavage cell cycles and no increase in mass. Embryogenesis is separated from larval development by an extended G1 phase that spans hatching. After hatching, subsequent cell cycle progression depends on, in addition to the intrinsic cell cycle machinery, other factors such as cellular growth and developmental signals. Postembryonic cell divisions occur during all four larval stages (L1—L4). Animals can develop continuously, or they can arrest as newly-hatched L1 larvae if food is absent. Starvation and crowding induces L2 animals to molt to the developmentally arrested dauer larva (Riddle & Albert 1997). Arrested L1 larvae or dauer larvae will resume development if transferred to favourable culture conditions.

encountered (Hong et al 1998). Postembryonic development can also be temporarily suspended at the end of the second larval stage, when harsh environmental conditions can cause the animal to arrest development as the so-called 'dauer larva' (Riddle & Albert 1997). Dauer larva cell cycles also seem to be arrested in G1 (Hong et al 1998). When dauer larvae are returned to favourable conditions, they resume development through the L3 and L4 stages to the adult. Although dauer larvae can only arrest at the second moult in the wild type, in certain mutants, the arrest can occur at the L1 or L3 moults (Liu & Ambros 1989), indicating that cells throughout larval development may have the capacity to suspend cell cycling. Many postembryonic cell lineages exhibit lengthy G1 phases during continuous development (Hedgecock & White 1985), suggesting that cell cycle progression in the C. elegans larva is chiefly regulated via the G1 cell cycle machinery.

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