The Evloution of Multicellularity: A Minor Major Transition?

摘要

Benefits of increased size and functional specialization of qells have repeatedly promoted the evolution of multicellular organisms from unicellular ancestors. Many requirements for multicellulajjjorganization (cell adhesion, cell-cell communication and coorj|lna-tion, programmed cell death) likely evolved in ancestral unicellular organisms. However, the evolution of multicellular organisms from unicellular ancestors may be opposed by genetic conflicts that arise when mutant cell lineages promote their own increase at the expense of the integrity of the multicellular organism. Numerous defenses limit such genetic conflicts, perhaps the most important being development from a unicell, which minimizes conflicts from selection among cell lineages, and redistributes genetic variation arising within multicellular individuals between individuals. With a unicellular bottleneck, defecting cell lineages rarely succeed beyond the life span of the multicellular individual. When multicellularity arises through aggregation of scattered cells or when multicellulai organisms fuse to form genetic chimeras, there are more opportunities foi propagation of defector cell lineages. Intraorganismal competition may partly explain why multicellular organisms that develop by aggregation generally exhibit less differentiation than organisms that develop clonally.