XII.6 Multicellular organisms exhibit a complicated life cycle, typically including a single-cell phase, a phase of growth and differentiation of the multicellular organism and a phase of sexual maturity
An individual multicellular organism can basically be formed in three different ways:1) through the combination of a group of originally independent cells, 2) through division of a parent multicellular organism and 3) through repeated reproduction of a single cell.The first way is not very frequent as genetically unrelated cells will primarily compete, especially for transition to germinal tissue (Strassmann, Zhu, & Queller 2000; Velicer, Kroos, & Lenski 2000; Hudson et al. 2002).This will substantially reduce the potential for cooperation and thus also the attainable degree of complexity and integrity of the multicellular organism.The second method also entails certain disadvantages and cannot function well in all types of organisms (see XII.6.2).The third way is most common in nature, i.e. the creation of an organism by repeated reproduction of a single cell.There are a number of specific variants of this type of development, from the simplest, similar to the formation of colonies of bacteria or protozoa, to highly complicated, in which first the cell nucleus divides many times, the volume of the cell increases many times and the final multinuclear unit is frequently complexly differentiated in shape and function before the individual nuclei and individual cytoplasm sections are separated, i.e. compartmentation of the originally continuous, single-cell body into a multicellular body occurs.
Thus, a new life cycle appears in multicellular organisms, the alternation of single-cell and multi-cell phases.Simultaneously, this cycle can occur quite independently of the cycle of alternation of diploid and haploid phases, i.e. on the metagenetic cycle.The two cycles are, of course, closely connected in specific biological species or rather individual higher taxons; however, this connection apparently is not based on functional causes, but rather historical or rather phylogenetic causes.In some taxons, the body of the multicellular organism is formed of diploid cells, in others by haploid cells and, in others, two multicellular, frequently very different phases in shape and function, one haploid and the other diploid, can appear in the life cycle (Mable & Otto 1998).