Developmental processes are thought to be highly complex, but there have been few attempts to measure and compare such complexity across different groups of organisms. Here we introduce a measure of biological complexity based on the similarity between developmental and computer programs. We define the algorithmic complexity of a cell lineage as the length of the shortest description of the lineage based on its constituent sublineages. We then use this measure to estimate the complexity of the embryonic lineages of four metazoan species from two different phyla. We find that these cell lineages are significantly simpler than would be expected by chance. Furthermore, evolutionary simulations show that the complexity of the embryonic lineages surveyed is near that of the simplest lineages evolvable, assuming strong developmental constraints on the spatial positions of cells and stabilizing selection on cell number. We propose that selection for decreased complexity has played a major role in moulding metazoan cell lineages.

We describe the complete embryonic cell lineage of the marine nematode Pellioditis marina (Rhabditidae) up to somatic muscle contraction, resulting in the formation of 638 cells, of which 67 undergo programmed cell death. In comparison with Caenorhabditis elegans, the overall lineage homology is 95.5%; fate homology, however, is only 76.4%. The majority of the differences in fate homology concern nervous, epidermal, and pharyngeal tissues. Gut and, remarkably, somatic muscle is highly conserved in number and position. Partial lineage data from the slower developing Halicephalobus sp. (Panagrolaimidae) reveal a lineage largely, but not exclusively, built up of monoclonal sublineage blocs with identical fates, unlike the polyclonal fate distribution in C. elegans and P. marina. The fate distribution pattern in a cell lineage could be a compromise between minimizing the number of specification events by monoclonal specification and minimizing the need for migrations by forming the cells close at their final position. The latter could contribute to a faster embryonic development. These results reveal that there is more than one way to build a nematode.

Filenchus fungivorus n. sp., isolated from autumn bellflower (Gentiana scrabra Bunge) in Japan and maintained on a fungus culture, is described based on LM, SEM and molecular data. The new species is characterised by combination of the following features: two lateral lines, short body (284-331 mu m), short stylet (7.0-8.7 mu m), amphid aperture associated with characteristic depression, short ovary, undifferentiated spermatheca, post-vulval sac shorter than corresponding body diam. and short (59-80 mu m) nonfiliform, almost straight tail and absence of males. According to a principal component analysis, a small tail/ vulva-anus (1.3-1.8) and V' ratio have the highest morphometrical power to separate F. fungivorus n. sp. from the remaining analysed Filenchus spp. (with the exception of F. misellus). Molecular analyses based on SSU rDNA placed F. fungivorus n. sp. within a clade that contained Filenchus sp. (heathland, Belgium), F. (Ottolenchus) discrepans, Malenchus andrassyi and F. misellus. However, the position of this clade could not be resolved within the Tylenchomorpha. Although the phylogenetic analyses, confirmed by Shimodaira-Hasegawa tests, significantly rejected the monophyly of Filenchus (P < 0.001), F. fungivorus n. sp. is placed within Filenchus in anticipation of a review of the Tylenchida

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The development of macrostomid flatworms is of interest for evolutionary developmental biology research because these taxa combine characteristics of the canonical spiral cleavage pattern with significant deviations from this pattern. One such deviation is the formation of hull cells, which surround the remaining embryonic primordium during early development. Using live observations with a 4D microscope system, histology, and 3D reconstructions, we analyzed the ontogeny of these hull cells in the macrostomid model organism Macrostomum lignano. Our cell lineage analysis allowed us to find the precursors of the hull cells in this species. We discuss the relation between macrostomid development and the development of other spiralians and the question of whether hull cells are homologous within rhabditophoran flatworms.