Development and evolution in nematodes

Embryogenesis of Caenorhabditis elegans has been studied in much detail on a cellular (Fig. 1) and molecular level (see overview at: www.wormbook.org). However, findings from model systems can result in misleading generalizations and impede our comprehension of developmental variations within a taxon as reflections of distinct evolutionary histories.

Fig. 1 : Embryogenesis of C. elegans . Left: After zygote formation (fusion of sperm and oocyte pronucleus; a-d) 5 somatic founder cells (AB, MS, E, C, D) are generated via asymmetric divisions of the germline (P0-P3). After formation of the primordial germ cell P4 gastrulation starts with immigration of the 2 gut precursor cells (E, yellow). Bar, 10 µm. Right: Within 12 h a juvenile is generated consisting of 558 cells. Multiplication of gut precursor cells (yellow) result in an intestine consisting of 20 cells at hatching. © E. Schierenberg

We are interested in the evolution of embryonic processes in the phylum Nematoda and therefore study nematodes from various clades (Fig. 2) in comparison to C. elegans. In general, we found that members of the taxon Chromadorea have many aspects of development in common while represenatives of the Enoplea, which are positioned closer to the root of the phylogenetic tree express fundamental differences.

Fig. 2 : Phylogenetic tree of nematodes based on DNA sequence data. Some species whose embryogenesis are being studied in the lab are shown. After: Schulze and Schierenberg, 2009

Our experimental data show that in the C. elegans embryo cells do not regulate, i.e adjust their developmental program according to the number of available blastomeres. However, on the level of unferztilized oocytes that is possible. Therefore, giant worms can be generated from fused oocytes (Fig. 3).

Fig. 3: a) Giant C. elegans adult (top) adjacent to a normal sized speciemen derived from a giant egg (bottom, adjacent to a normal sized egg) which was generated via laser-induced oocyte fusion (b). After: Irle and Schierenberg (1994)

In contrast to C. elegans we found that for instance in Acrobeloides nanus (clade 11; Fig. 2 ), cells can compensate for the loss of eliminated cells in that the remaining blastomeres replace these in a hierarchical manner (Fig. 4). Other species we tested show considerable differences with respect to the use of maternal gene products, establishment of polarity, pattern formation, gastrulation and cell specification. This suggests that during evolution different developmental strategies have been established among nematodes.

Fig. 4 : Cell regulation in A. nanus. After ablation of the AB cell, remaining blastomeres change their differentiation program and compensate the loss. From Wiegner and Schierenberg (1998)

As representatives of the Enoplea we have started to investigate Tobrilus (clade 1; Fig. 2) with its unusual gastrulation ( Fig. 5), resembling that in sea urchin or primitive vertebrates, and Romanomermis (clade 2) with its segregation of colored cytoplasm ( Fig. 6). The latter allowed us for the first time to perform an in-depth comparison of cell lineages and cell differentiation between the standard C. elegans and a basal representative of nematodes.

Fig. 5 : Gastrulation in Tobrilus (a-c) differs considerably from C. elegans (A-C) and resembles canonical gastrulation as found e.g. in sea urchins (a'-c'). From: Schierenberg and Schulze, 2008

Fig. 6 : Segregation of colored cytoplasm in Romanomermis (left) can be experimentally modified (right) without affecting normal development . From: Schulze and Schierenberg, 2008

Besides the analysis on a cellular level we are interested in variations of the gene expression pattern dependent on phylogenetic position (basal or derived species) and mode of reproduction (dioecious, hermaphroditic or parthenogenetic). For this we have cloned homologs of genes in other nematodes which play a central role during early embryogenesis in C. elegans. Expression studies on the level of mRNA (in-situs) and protein (antibodies) reveal that even between closely related species differences in time and space exist ( Fig. 7) suggesting considerable modifications of how cells are specified.

Fig. 7 : Expression pattern of mex-3 in C. elegans (left) differs considerably from that in Diploscapter coronatus (right) a close relative which, however, reproduces parthenogenetically.

By extending our comparative studies to a larger number of species including outgroups from neighboring phyla like nematomorphs or tardigrades and by identifying relevant genes, we should learn more about the intrinsic prerequisites for the implementation of embryonic novelty. In addition, we may better understand to what extent the interplay between the genetic program and external conditions (inside or outside of the organism) determines the chance for deviations from an original developmental pattern to arise and to succeed. Finally, the question can be addressed whether the establishment of modified embryonic cell behavior follows the same Darwinian rules of variation and selection as claimed for morphological and physiological traits.