Plectus - a stepping stone in embryonic cell lineage evolution of nematodes

The study was conducted using 4-D lineage analysis, 3-D modeling of developing embryos and laser-induced ablation of individual blastomeres.Detailed cell lineage studies of several Plectus species reveal that pattern formation and cell fate assignment differ markedly from C. elegans. Descendants of the first somatic founder cell S1 (AB) - but not the progeny of other founder cells - demonstrate extremely variable spatial arrangements illustrating that here distinct early cell-cell interactions between invariant partners, as found in C. elegans, cannot take place. Different from C. elegans, in Plectus alternative positional variations among early S1 blastomeres resulting in a ‘situs inversus’ pattern, nevertheless give rise to adults with normal left-right asymmetries. In addition, laser ablations of early blastomeres uncover inductions between variable cell partners.Our results suggest that embryonic cell specification in Plectus is not correlated with cell lineage but with position. With this peculiarity, Plectus appears to occupy an intermediate position between basal nematodes displaying a variable early development and the C. elegans-like invariant pattern. We suggest that indeterminate pattern formation associated with late, position-dependent fate assignment represents a plesiomorphic character among nematodes predominant in certain basal clades but lost in derived clades. Thus, the behavior of S1 cells in Plectus can be considered an evolutionary relict in a transition phase between two different developmental strategies.The phylum Nematoda is considered a very ancient phylum reaching back into the Precambrian [1-3]. Well-preserved fossils of nematodes from the early Devonian very much resemble recent basal representatives [4]. The number of extant nematode species is large with estimates ranging from tens of thousands to several millions [5-8]. Their adaptation to nearly all environmental conditions would predict a high plasticity on the morphological and ph