Canonical and non-canonical Wnt signaling pathways in Caenorhabditis elegans: variations on a common signaling theme

Involvement of Wingless/Armadillo signaling in the posterior sequential segmentation in the cricket, Gryllus bimaculatus (Orthoptera), as revealed by RNAi analysis

In insects, there are two different modes of segmentation. In the higher dipteran insects (like Drosophila), their segmentation takes place almost simultaneously in the syncytial blastoderm. By contrast, in the orthopteran insects (like Schistocerca (grasshopper)), the anterior segments form almost simultaneously in the cellular blastoderm and then the remaining posterior part elongates to form segments sequentially from the posterior proliferative zone. Although most of their orthologues of the Drosophila segmentation genes may be involved in their segmentation, little is known about their roles. We have investigated segmentation processes of Gryllus bimaculatus, focusing on its orthologues of the Drosophila segment-polarity genes, G. bimaculatus wingless (Gbwg), armadillo (Gbarm) and hedgehog (Gbhh). Gbhh and Gbwg were observed to be expressed in the each anterior segment and the posterior proliferative zone. In order to know their roles, we used RNA interference (RNAi). We could not observed any significant effects of RNAi for Gbwg and Gbhh on segmentation, probably due to functional replacement by another member of the corresponding gene families. Embryos obtained by RNAi for Gbarm exhibited abnormal anterior segments and lack of the abdomen. Our results suggest that GbWg/GbArm signaling is involved in the posterior sequential segmentation in the G. bimaculatus embryos, while Gbwg, Gbarm and Gbhh are likely to act as the segment-polarity genes in the anterior segmentation similarly as in Drosophila.

Establishment of POP-1 asymmetry in early C. elegans embryos

In Caenhorabtidis elegans embryos, the nuclei of sister cells that are born from anterior/posterior divisions show an invariant high/low asymmetry, respectively, in their level of the transcription factor POP-1. Previous studies have shown that POP-1 asymmetry between the daughters of an embryonic cell called EMS results in part from a Wnt-like signal provided by a neighboring cell, called P(2). We identify here additional signaling cells that play a role in POP-1 asymmetry for other early embryonic cells. Some of these cells have signaling properties similar to P(2), whereas other cells use apparently distinct signaling pathways. Although cell signaling plays a critical role in POP-1 asymmetry during the first few cell divisions, later embryonic cells have an ability to generate POP-1 asymmetry that appears to be independent of prior Wnt signaling.

The Ciona intestinalis genome: when the constraints are off

The recent genome sequencing of a non-vertebrate deuterostome, the ascidian tunicate Ciona intestinalis, makes a substantial contribution to the fields of evolutionary and developmental biology.1 Tunicates have some of the smallest bilaterian genomes, embryos with relatively few cells, fixed lineages and early determination of cell fates. Initial analyses of the C. intestinalis genome indicate that it has been evolving rapidly. Comparisons with other bilaterians show that C. intestinalis has lost a number of genes, and that many genes linked together in most other bilaterians have become uncoupled. In addition, a number of independent, lineage-specific gene duplications have been detected. These new results, although interesting in themselves, will take on a deeper significance once the genomes of additional invertebrate deuterostomes (e.g. echinoderms, hemichordates and amphioxus) have been sequenced. With such a broadened database, comparative genomics can begin to ask pointed questions about the relationship between the evolution of genomes and the evolution of body plans.

Polycomb group regulation of Hox gene expression in C. elegans

Polycomb group (PcG) chromatin proteins regulate homeotic genes in both animals and plants. In Drosophila and vertebrates, PcG proteins form complexes and maintain early patterns of Hox gene repression, ensuring fidelity of developmental patterning. PcG proteins in C. elegans form a complex and mediate transcriptional silencing in the germline, but no role for the C. elegans PcG homologs in somatic Hox gene regulation has been demonstrated. Surprisingly, we find that the PcG homologs MES-2 [E(Z)] and MES-6 (ESC), along with MES-3, a protein without known homologs, do repress Hox expression in C. elegans. mes mutations cause anteroposterior transformations and disrupt Hox-dependent neuroblast migration. Thus, as in Drosophila, vertebrates, and plants, C. elegans PcG proteins regulate key developmental patterning genes to establish positional identity.