THE ARCHITECTURE OF THE LIFE-CYCLE IN SMALL ORGANISMS

The life cycle of eukaryotes has a dual nature, composed of a vegetative cycle of growth and reproduction, and a sexual cycle of fusion and reduction, linked by the spore. Large size is often favoured through interactions with other organisms, or as a means of exploiting locally or temporarily abundant resources, despite the metabolic penalty of size increase. Beyond a certain point, large organisms must be multicellular (or multinucleate) because of the requirement for more deoxyribonucleic acid (DNA) to service larger quantities of cytoplasm. Multicellularity evolves in some lineages but not in others because its evolution is constrained by the pattern of spore development, being favoured, for example by the occurrence of multiple fission as the consequence of possessing a rigid cell wall. The separation of soma from germ is also the outcome of a developmental constraint, in this case the inability of cells to divide while flagellated, and also the necessity of remaining in motion. Once achieved, a general physiological advantage is realized through the specialization of soma as source and germ as sink. Large, complex multicellular organisms are fragile constructs that can only persist through deploying sophisticated devices for maintenance. Thus two crucial, and related, properties of life cycles are repair and repeatability. The dual life cycle achieves exogenous repair through spore production in the vegetative cycle and through outcrossing and recombination in the sexual cycle. Repeatability is enhanced by developmental mechanisms such as maternal control and germ-line sequestration, which by restricting the occurrence or the heritability of somatic mutations promote their own replication.