Why does a nematode have an invariant cell lineage?
C. elegans is renowned for its invariant embryogenesis and functions as a major paradigm for a mode of development coupled to an invariant lineage. Recent work, however, suggests that the embryogenesis of the nematode is much more flexible than anticipated. The invariant premorphogenetic stage is formed from variable earlier stages through a sorting of cells. Cells do not act as individuals but already early in embryogenesis a regionalization of the embryo occurs. Cells are diversified by a binary specification of 'abstract' blastomere (regional) identities. The determination of tissues may thus be a very late event. It appears that C. elegans, although assigning cell fates in an invariant lineage pattern, uses the same strategies and mechanisms for embryogenesis as organisms with variable lineages.
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Max-Planck-Institute für Biochemie, D-82125, Martinsried, Germany.