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Abstract Background Recent studies have challenged the widespread view that the pattern of embryogenesis found in Caenorhabditis elegans (clade 9) is characteristic of nematodes in general. To understand this still largely unexplored landscape of developmental events, we set out to examine more distantly related nematodes in detail for temporospatial differences in pattern formation and cell specification. Members of the genus Plectus (clade 6) seem to be suitable candidates to show variety, with certain idiosyncratic features during early development and the convenient availability of cultivatable species. Methods The study was conducted using 4-D lineage analysis, 3-D modeling of developing embryos and laser-induced ablation of individual blastomeres. Results 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. Conclusions 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.

Abstract Background Nematodes can be subdivided into basal Enoplea (clades 1 and 2) and more derived Chromadorea (clades 3 to 12). Embryogenesis of Caenorhabditis elegans (clade 9) has been analyzed in most detail. Their establishment of polarity and asymmetric cleavage requires the differential localization of PAR proteins. Earlier studies on selected other nematodes revealed that embryonic development of nematodes is more diverse than the essentially invariant development of C. elegans and the classic study object Ascaris had suggested. To obtain a more detailed picture of variations and evolutionary trends we compared embryonic cell lineages and pattern formation in embryos of all 12 nematode clades. Methods The study was conducted using 4-D microscopy and 3-D modeling of developing embryos. Results We found dramatic differences compared to C. elegans in Enoplea but also considerable variations among Chromadorea. We discovered 'Polarity Organizing Centers' (POCs) that orient cleavage spindles along the anterior-posterior axis in distinct cells over consecutive cell generations. The resulting lineally arranged blastomeres represent a starting point for the establishment of bilateral symmetry within individual lineages. We can discern six different early cleavage types and suggest that these variations are due to modifications in the activity of the POCs in conjunction with changes in the distribution of PAR proteins. In addition, our studies indicate that lineage complexity advanced considerably during evolution, that is we observe trends towards an increase of somatic founder cells, from monoclonal to polyclonal lineages and from a variable (position-dependent) to an invariable (lineage-dependent) way of cell fate specification. In contrast to the early phase of embryogenesis, the second half ('morphogenesis') appears similar in all studied nematodes. Comparison of early cleavage between the basal nematode Tobrilus stefanskii and the tardigrade Hypsibius dujardini revealed surprising similarities indicating that the presence of POCs is not restricted to nematode embryos. Conclusions The pattern of cleavage, spatial arrangement and differentiation of cells diverged dramatically during the history of the phylum Nematoda without corresponding changes in the phenotype. While in all studied representatives the same distinctive developmental steps need to be taken, cell behavior leading to these is not conserved.

In autumn 2011, 11 illegally imported animals were seized from a farm in southern Norway. These included four raccoon dogs (Nyctereutes procyonoides), four raccoons (Procyon lotor), and three South American coatis (Nasua nasua), all considered alien species in Norway. An additional two raccoons had escaped from the farm prior to seizure. The seized animals were euthanized and postmortem examination revealed that the four raccoons had moderate to high numbers of the zoonotic nematode Baylisascaris procyonis in their intestines, ranging from 11 to 115 nematodes per small intestine, with a mean of 53. The identity of the nematodes was confirmed using molecular analysis of ITS-1, ITS-2, cytochrome C oxidase 1, and 18S. Echinococcus multilocularis was not detected in any of the 11 animals. Toxocara and Toxascaris sp. eggs were detected in the feces of two raccoons, and two coatis had coccidia oocysts (80 and 360 oocysts per gram). Domestic dogs and other wildlife on the farm had potential access to the animal pens. Given that the eggs can remain infective for years in the environment, local veterinary and health authorities will need to remain vigilant for symptoms relating to infection with B. procyonis.