Developmental evolution: the unbearable likeness of beings

Comparative studies have revealed a remarkable range of genetic changes in the mechanisms that pattern the nematode vulva. Two new studies identify genetic variation within nematode species that affects cell division and competence in vulval precursors.     

PAR-1 is required for morphogenesis of the Caenorhabditis elegans vulva

The Caenorhabditis elegans vulva provides a simple model for the genetic analysis of pattern formation and organ morphogenesis during metazoan development. We have discovered an essential role for the polarity protein PAR-1 in the development of the vulva. Postembryonic RNA interference of PAR-1 causes a protruding vulva phenotype. We found that depleting PAR-1 during the development of the vulva has no detectable effect on fate specification or precursor proliferation, but instead seems to specifically alter morphogenesis. Using an apical junction-associated GFP marker, we discovered that PAR-1 depletion causes a failure of the two mirror-symmetric halves of the vulva to join into a single, coherent organ. The cells that normally form the ventral vulval rings fail to make contact or adhere and consequently form incomplete toroids, and dorsal rings adopt variably abnormal morphologies. We also found that PAR-1 undergoes a redistribution from apical junctions to basolateral domains during morphogenesis. Despite a known role for PAR-1 in cell polarity, we have observed no detectable differences in the distribution of various markers of epithelial cell polarity. We propose that PAR-1 activity at the cell cortex is critical for mediating cell shape changes, cell surface composition, or cell signaling during vulval morphogenesis.     

Nematode development: an evolutionary fugue

Recent studies of vulva development in the nematode Pristionchus pacificus have identified cell interactions that do not appear to occur in Caenorhabditis elegans. The new results underscore the diversity of patterning mechanisms that can produce structures with similar cellular morphology.     

The nematode even-skipped homolog vab-7 regulates gonad and vulva position in Pristionchus pacificus

In free-living nematodes, developmental processes like the formation of the vulva, can be studied at a cellular level. Cell lineage and ablation studies have been carried out in various nematode species and multiple changes in vulval patterning have been identified. In Pristionchus pacificus, vulva formation differs from Caenorhabditis elegans with respect to several autonomous and conditional aspects of cell fate specification. To understand the molecular basis of these evolutionary changes, we have performed a genetic analysis of vulva formation in P. pacificus. Here, we describe two mutants where the vulva is shifted posteriorly, affecting which precursor cells will form vulval tissue in P. pacificus. Mutant animals show a concomitant posterior displacement of the gonadal anchor cell, indicating that the gonad and the vulva are affected in a similar way. We show that mutations in the even-skipped homolog of nematodes, vab-7, cause these posterior displacements. In addition, cell ablation studies in the vab-7 mutant indicate that the altered position of the gonad not only changes the cell fate pattern but also the developmental competence of vulval precursor cells. Investigation of Cel-vab-7 mutant animals showed a similar but weaker vulva defective phenotype to the one described for Ppa-vab-7.     

Vulva formation in Pristionchus pacificus relies on continuous gonadal induction

One of the best known features of vulva development in Caenorhabditis elegans is the induction of vulval precursor cells by the gonadal anchor cell. Induction is crucial for the initiation of pattern formation within the C. elegans vulva equivalence group, and it is therefore surprising to find that this aspect of vulva formation, in particular, varies greatly among nematodes. In some species which form vulvae in the posterior body region, no gonadal signal is necessary for vulva induction. In other nematodes, such as Panagrolaimus, Oscheius, and Rhabditella, vulva formation depends on two temporally distinct gonadal inductions which specify the different cell fates. Here we report our analysis of vulva induction in Pristionchus pacificus, a specieswhich has recently been used as a genetic system to analyze the evolution of vulva development. Cell ablation studies in P. pacificus show that another mode of vulva induction exists. P. pacificus vulva formation depends on a continuous gonadal induction that starts several hours after hatching and continues until the birth of the anchor cell, some 20 h later. Mutations defective in gonadal induction result in the absence of vulva differentiation, suggesting that only one signaling system is involved in the gonadal-epidermal interaction. This new mode adds further to the great variety of gonadal inductions among nematode species. Received: 25 February 1999 / Accepted: 20 April 1999     

Wnt signaling induces vulva development in the nematode Pristionchus pacificus

The Caenorhabditis elegans vulva is induced by a member of the epidermal growth factor (EGF) family that is expressed in the gonadal anchor cell, representing a prime example of signaling processes in animal development. Comparative studies indicated that vulva induction has changed rapidly during evolution. However, nothing was known about the molecular mechanisms underlying these differences. By analyzing deletion mutants in five Wnt pathway genes, we show that Wnt signaling induces vulva formation in Pristionchus pacificus. A Ppa-bar-1/beta-catenin deletion is completely vulvaless. Several Wnt ligands and receptors act redundantly in vulva induction, and Ppa-egl-20/Wnt; Ppa-mom-2/Wnt; Ppa-lin-18/Ryk triple mutants are strongly vulvaless. Wnt ligands are differentially expressed in the somatic gonad, the anchor cell, and the posterior body region, respectively. In contrast, previous studies indicated that Ppa-lin-17, one of the Frizzled-type receptors, has a negative role in vulva formation. We found that mutations in Ppa-bar-1 and Ppa-egl-20 suppress the phenotype of Ppa-lin-17. Thus, an unexpected complexity of Wnt signaling is involved in vulva induction and vulva repression in P. pacificus. This study provides the first molecular identification of the inductive vulva signal in a nematode other than Caenorhabditis.     

Intersex,a temperature-sensitive mutant of the nematode Caenorhabditis elegans

A temperature-sensitive mutation, isx-1(hc17), is reported in the nematode Caenorhabditis elegans which alters the sexual phenotypes of both genotypic sexes. At the restrictive temperature, XX animals are functionally female rather than hermaphroditic due to the absence of spermatogenesis, and XO animals develop as intersexes. These intersexes have normal male head and tail structures and exhibit some mating behavior, but possess hermaphrodite-like gonads which produce no sperm and usually contain a few oocytes. An abortive vulva is usually present and evidence is presented which suggests that the formation of the vulva by the hypodermis is induced by the underlying gonad. The direct effects of the mutation are confined to the descendants of four primordial gonad cells. Gametogenesis and gonad sheath development do not seem to be tightly coupled and are shown to differ in their responses to X-chromosome dosage. The interaction of the intersex mutation with mutant alleles of two transformer genes tra-1 and tra-2 is discussed and a model for the action of these genes in gonad development and sex determination is proposed.     

外阴硬化性苔癣组织P53、PCNA表达、DNA含量及与细胞增殖的关系

探讨外阴硬化性苔癣组织中的 P5 3、PCNA表达 ,DNA含量与细胞增殖的关系。免疫组化方法测定 2 0例外阴硬化性苔癣组织和 10例正常外阴皮肤中 P5 3、 PCNA蛋白表达 ;图像分析技术检测两组基底层细胞核形态及 DNA含量。结果显示 ,外阴硬化苔癣组 P5 3阳性表达率为 40 % ,与正常皮肤比较 P<0 .0 5 ,PCNA阳性表达率为 70 % ,与正常皮肤比较 P>0 .0 5 ,阳性表达主要分布于棘层、颗粒层 ;基底细胞核显著变小和 DNA含量降低 (P<0 .0 5 )。结果表明外阴硬化性苔癣组织中存在细胞增殖异常     

The plant apoplasm is an important recipient compartment for nematode secreted proteins

Similarly to microbial pathogens, plant-parasitic nematodes secrete into their host plants proteins that are essential to establish a functional interaction. Identifying the destination of nematode secreted proteins within plant cell compartment(s) will provide compelling clues on their molecular functions. Here the fine localization of five nematode secreted proteins was analysed throughout parasitism in Arabidopsis thaliana. An immunocytochemical method was developed that preserves both the host and the pathogen tissues, allowing the localization of nematode secreted proteins within both organisms. One secreted protein from the amphids and three secreted proteins from the subventral oesophageal glands involved in protein degradation and cell wall modification were secreted in the apoplasm during intercellular migration and to a lower extent by early sedentary stages during giant cell formation. Conversely, another protein produced by both subventral and dorsal oesophageal glands in parasitic stages accumulated profusely at the cell wall of young and mature giant cells. In addition, secretion of cell wall-modifying proteins by the vulva of adult females suggested a role in egg laying. The study shows that the plant apoplasm acts as an important destination compartment for proteins secreted during migration and during sedentary stages of the nematode.     

sem-4/spalt and egl-17/FGF have a conserved role in sex myoblast specification and migration in P. pacificus and C. elegans

Evolutionary comparisons between Caenorhabditis elegans and the satellite organism Pristionchus pacificus revealed major differences in the regulation of nematode vulva development. For example, Wnt signaling is part of a negative signaling system that prevents vulva formation in P. pacificus, whereas it plays a positive role in C. elegans. We wondered if the genetic control of the second major part of the nematode egg-laying system, the sex muscles, has diverged similarly between P. pacificus and C. elegans. The sex muscles derive from the mesoblast M, which has an identical lineage in both species. Here, we describe a large-scale mutagenesis screen for mutations that disrupt the M lineage and the sex myoblast (SM) sublineage. We isolated and characterized mutations that result in a failure of proper SM fate specification and SM migration and showed that the corresponding genes encode Ppa-sem-4 and Ppa-egl-17, respectively. Ppa-sem-4 mutants have additional defects in the specification of the vulva precursor cells P(5, 7).p and experimental studies in the Ppa-egl-17 mutant background indicate a complex set of gonad-dependent and gonad-independent mechanisms required for SM migration. Mutations in Cel-sem-4 and Cel-egl-17 cause similar defects. Thus, the molecular mechanisms of SM cell specification and migration are conserved between P. pacificus and C. elegans.     

Evolution of vulva development in the Cephalobina (Nematoda)

Ventral cord and vulva development are analyzed in a large sample of nematode species of the suborder Cephalobina. We find a specific range of evolutionary variations at distinct developmental steps. (1) Unlike Caenorhabditis elegans and relatives, the vulva is formed from the four precursor cells P(5-8).p or, exceptionally, from P(6, 7).p only. (2) The vulval competence group is restricted to these four cells or is larger. (3) The fates of more anterior and posterior Pn.p cells vary between closely related species (mostly cell death versus epidermal fate). (4) The mechanism of vulval cell fate patterning varies within a single genus, even between strains of the same species. (5) We describe the first example of a vulval cell lineage that is asymmetric between the anterior and the posterior sides of the vulva. For a selection of the investigated taxa, phylogenetic trees were constructed in order to map vulval characters and infer evolutionary polarities. We can conclude that in this group, death of the Pn.p cells probably constitutes a derived character state compared to a syncytial fate. Rhabditophanes sp. and Strongyloides ratti are placed as sister taxa, probably sharing an exclusive common ancestor in which the number of precursor cells forming the vulva was reduced from four to two.     

Cell interactions coordinate the development of the C. elegans egg-laying system

Egg laying by the nematode Caenorhabditis elegans requires the functioning of the vulva, the gonad, the egg-laying muscles, and the two HSN neurons, which innervate these muscles. By analyzing a newly isolated mutant (dig-1) that displaces the gonad, we discovered that cell interactions coordinate the spatial relationships among the different components of the egg-laying system. First, the gonad induces the formation of the vulva, and vulval induction by dorsal gonads strongly suggests that the inductive signal can act at a distance. Second, the gonad acts at a distance to regulate the migrations of the sex myoblasts that generate the egg-laying musculature. Third, the positions of the axonal branch and synapses of each HSN neuron are displaced correspondingly with the rest of the egg-laying system in dig-1 animals, which suggests that cell interactions also control aspects of HSN development.     

Formation of <Emphasis Type="Italic">Yersinia pseudotuberculosis</Emphasis> biofilms on multiple surfaces on <Emphasis Type="Italic">Caenorhabditis elegans</Emphasis>

Summary A liquid-based assay was used to evaluate the ability of Yersinia pseudotuberculosis to form a bacterial biofilm on the nematode Caenorhabditis elegans. After 3 days of incubation in the liquid assay a biofilm was clearly visible by light microscopy on both the head and vulva region of the worms. At times, the biofilm formation on the vulva appeared to prevent the laying of eggs by the adult hermaphrodite; the eggs would later hatch inside of the worm. One possible explanation for the biofilm formation observed on the vulva may be the increased motion of the cuticle surrounding the vulva when the worm is immersed in a liquid culture. This is the first report of biofilm formation on the vulva of C. elegans.     

Pristionchus pacificus vulva formation: polarized division, cell migration, cell fusion, and evolution of invagination

Tube formation is a widespread process during organogenesis. Specific cellular behaviors participate in the invagination of epithelial monolayers that form tubes. However, little is known about the evolutionary mechanisms of cell assembly into tubes during development. In Caenorhabditis elegans, the detailed step-to-step process of vulva formation has been studied in wild type and in several mutants. Here we show that cellular processes during vulva development, which involve toroidal cell formation and stacking of rings, are conserved between C. elegans and Pristionchus pacificus, two species of nematodes that diverged approximately 100 million years ago. These cellular behaviors are divided into phases of cell proliferation, short-range migration, and cell fusion that are temporally distinct in C. elegans but not in P. pacificus. Thus, we identify heterochronic changes in the cellular events of vulva development between these two species. We find that alterations in the division axes of two equivalent vulval cells from Left-Right cleavage in C. elegans to Anterior-Posterior division in P. pacificus can cause the formation of an additional eighth ring. Thus, orthogonal changes in cell division axes with alterations in the number and sequence of cell fusion events result in dramatic differences in vulval shape and in the number of rings in the species studied. Our characterization of vulva formation in P. pacificus compared to C. elegans provides an evolutionary-developmental foundation for molecular genetic analyses of organogenesis in different species within the phylum Nematoda.     

lin-17/Frizzled and lin-18 regulate POP-1/TCF-1 localization and cell type specification during C. elegans vulval development

The Caenorhabditis elegans vulva is comprised of highly similar anterior and posterior halves that are arranged in a mirror symmetric pattern. The cell lineages that form each half of the vulva are identical, except that they occur in opposite orientations with respect to the anterior/posterior axis. We show that most vulval cell divisions produce sister cells that have asymmetric levels of POP-1 and that the asymmetry has opposite orientations in the two halves of the vulva. We demonstrate that lin-17 (Frizzled type Wnt receptor) and lin-18 (Ryk) regulate the pattern of POP-1 localization and cell type specification in the posterior half of the vulva. In the absence of lin-17 and lin-18, posterior lineages are reversed and resemble anterior lineages. These experiments suggest that Wnt signaling pathways reorient cell lineages in the posterior half of the vulva from a default orientation displayed in the anterior half of the vulva.