One cell, two cell, red cell, blue cell: The persistence of a unicellular stage in multicellular life histories.
As developmental biologists come closer to understanding at the molecular and genetic levels how a zygote becomes an adult, it is easy to forget that the very phenomenon that gives them an occupation remains a vexing problem to evolutionary biologists: why do unicellular stages persist in life histories of multicellular organisms? There are two explanatory hypotheses. One is that a unicellular stage purges multicellular organisms of deleterious mutations by exposing offspring that are each uniformly of one genotype to selection. Another is that a one-cell stage reduces conflicts of interest among genetically different replicators within an organism.
Center for Population Biology, University of California, Davis, CA 95616, USA.
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