Maximal indirect development, set-aside cells, and levels of selection

The evolution of metazoan development as described by Davidson et al. (1995 . Science 270:1319-1325) is readily interpretable in terms of levels-of-sel ection conflicts, for instance, as recently modeled by Michod (1999. Darwin ian Dynamics, Princeton, NJ: Princeton University Press). Davidson et al. p ropose certain features of early bilaterians including small size, a small and fixed number of cell divisions during and subsequent to cleavage, and s pecification of cell fates prior to cell movement. These features suggest c onstraints on certain parameters of Michod's model, specifically t (the tim e available for cell division) and b (the benefit to cells of not cooperati ng in terms of their rate of replication). Such constraints clearly enhance between-cell cooperation and allow multicellularity to more easily evolve and be maintained. Nevertheless, these constraints are completely abrogated by the phenomenon of "set-aside cells, that is, undifferentiated cells tha t retain indefinite division potential. Levels-of-selection theory predicts that the evolution of these set-aside cells must be accompanied by feature s which alleviate cell-cell competition, and indeed the results of Ransick et al. (1996. Proc Natl Acad Sci USA 93:6759-6763) support this prediction: the evolution of "set-aside cells" in metazoans was accompanied by the evo lution of the sequestration of the germ line. (C) 2000 Wiley-Liss, Inc.