ONE-CELL, 2 CELL, RED-CELL, BLUE CELL - THE PERSISTENCE OF A UNICELLULAR STAGE IN MULTICELLULAR LIFE-HISTORIES

As developmental biologists come closer to understanding at the molecu lar and genetic levels how a zygote becomes an adult, it is easy to fo rget that the very phenomenon that gives them an occupation remains a vexing problem to evolutionary biologists: why do unicellular stages p ersist in life histories of multicellular organisms? There are two exp lanatory hypotheses. One is that a unicellular stage purges multicellu lar organisms of deleterious mutations by exposing offspring that are each uniformly of one genotype to selection. Another is that a one-cel l stage reduces conflicts of interest among genetically different repl icators within an organism.