Bacterial filamentation as a mechanism for cell-to-cell spread within an animal host.

Intracellular pathogens are challenged with limited space and resources while replicating in a single host cell. Mechanisms for direct invasion of neighboring host cells have been discovered in cell culture, but we lack an understanding of how bacteria directly spread between host cells in vivo. Here, we describe the discovery of intracellular bacteria that use filamentation for spreading between the intestinal epithelial cells of a natural host, the rhabditid nematode Oscheius tipulae. The bacteria, which belong to the new species Bordetella atropi, can infect the nematodes following a fecal-oral route, and reduce host life span and fecundity. Filamentation requires UDP-glucose biosynthesis and sensing, a highly conserved pathway that is used by other bacteria to detect rich conditions and inhibit cell division. Our results indicate that B. atropi uses a pathway that normally regulates bacterial cell size to trigger filamentation inside host cells, thus facilitating cell-to-cell dissemination.

Asymmetry is defined during meiosis in the oocyte of the parthenogenetic nematode Diploscapter pachys.

Asymmetric cell division is an essential feature of normal development and certain pathologies. The process and its regulation have been studied extensively in the Caenorhabditis elegans embryo, particularly how symmetry of the actomyosin cortical cytoskeleton is broken by a sperm-derived signal at fertilization, upstream of polarity establishment. Diploscapter pachys is the closest parthenogenetic relative to C. elegans, and D. pachys one-cell embryos also divide asymmetrically. However how polarity is triggered in the absence of sperm remains unknown. In post-meiotic embryos, we find that the nucleus inhabits principally one embryo hemisphere, the future posterior pole. When forced to one pole by centrifugation, the nucleus returns to its preferred pole, although poles appear identical as concerns cortical ruffling and actin cytoskeleton. The location of the meiotic spindle also correlates with the future posterior pole and slight actin enrichment is observed at that pole in some early embryos along with microtubule structures emanating from the meiotic spindle. Polarized location of the nucleus is not observed in pre-meiotic D. pachys oocytes. All together our results are consistent with the idea that polarity of the D. pachys embryo is attained during meiosis, seemingly based on the location of the meiotic spindle, by a mechanism that may be present but suppressed in C. elegans.

Differences in embryonic pattern formation between Caenorhabditis elegans and its close parthenogenetic relative Diploscapter coronatus.

In order to evaluate the evolutionary preservation of developmental programs during nematode embryogenesis, we searched for close relatives of the model system Caenorhabditis elegans with deviant patterns. The parthenogenetically reproducing species Diploscapter coronatus shows prominent differences to C. elegans. While in the 2-cell stage of C. elegans a rotation of the nuclear/centrosome complex is found only in the posterior P1 cell, in D. coronatus cell isolation indicates that rotation takes place in a cell-autonomous manner in both blastomeres, resulting in a linear 4-cell array. In C. elegans, the ABp cell becomes different from its ABa sister via a germline-induced induction. In D. coronatus, AB daughters do not touch the germline but nevertheless execute different fates, suggesting a cell-autonomous mechanism or signaling over distance. Laser ablation experiments revealed that active migration of the EMS cell is required to transform the linearly ordered blastomeres into a 3-dimensional embryo, and the difference can be most easily explained with a heterochronic shift with respect to cell mobility. In D. coronatus, reversal of cleavage polarity in the germline, typical for C. elegans, is absent. This results in four different transient variants of posterior blastomeres which eventually merge into a single pattern prior to the onset of gastrulation. This merging includes primordial germ cell migrations of variable extent toward the gut precursor cell and suggests a specific cell-cell recognition mechanism. Cell distribution in advanced embryos is essentially indistinguishable between both species.

Nematodes, bacteria, and flies: a tripartite model for nematode parasitism.

More than a quarter of the world's population is infected with nematode parasites, and more than a hundred species of nematodes are parasites of humans [1-3]. Despite extensive morbidity and mortality caused by nematode parasites, the biological mechanisms of host-parasite interactions are poorly understood, largely because of the lack of genetically tractable model systems. We have demonstrated that the insect parasitic nematode Heterorhabditis bacteriophora, its bacterial symbiont Photorhabdus luminescens, and the fruit fly Drosophila melanogaster constitute a tripartite model for nematode parasitism and parasitic infection. We find that infective juveniles (IJs) of Heterorhabditis, which contain Photorhabdus in their gut, can infect and kill Drosophila larvae. We show that infection activates an immune response in Drosophila that results in the temporally dynamic expression of a subset of antimicrobial peptide (AMP) genes, and that this immune response is induced specifically by Photorhabdus. We also investigated the cellular and molecular mechanisms underlying IJ recovery, the developmental process that occurs in parasitic nematodes upon host invasion and that is necessary for successful parasitism. We find that the chemosensory neurons and signaling pathways that control dauer recovery in Caenorhabditis elegans also control IJ recovery in Heterorhabditis, suggesting conservation of these developmental processes across free-living and parasitic nematodes.

The simplicity of metazoan cell lineages.

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.

Control of vulval cell division number in the nematode Oscheius/Dolichorhabditis sp. CEW1.

Spatial patterning of vulval precursor cell fates is achieved through a different two-stage induction mechanism in the nematode Oscheius/Dolichorhabditis sp. CEW1 compared with Caenorhabditis elegans. We therefore performed a genetic screen for vulva mutants in Oscheius sp. CEW1. Most mutants display phenotypes unknown in C. elegans. Here we present the largest mutant category, which affects division number of the vulva precursors P(4-8).p without changing their fate. Among these mutations, some reduce the number of divisions of P4.p and P8.p specifically. Two mutants omit the second cell cycle of all vulval lineages. A large subset of mutants undergo additional rounds of vulval divisions. We also found precocious and retarded heterochronic mutants. Whereas the C. elegans vulval lineage mutants can be interpreted as overall (homeotic) changes in precursor cell fates with concomitant cell cycle changes, the mutants described in Oscheius sp. CEW1 do not affect overall precursor fate and thereby dissociate the genetic mechanisms controlling vulval cell cycle and fate. Laser ablation experiments in these mutants reveal that the two first vulval divisions in Oscheius sp. CEW1 appear to be redundantly controlled by a gonad-independent mechanism and by a gonadal signal that operates partially independently of vulval fate induction.

Papillomatous pastern dermatitis with spirochetes and Pelodera strongyloides in a Tennessee Walking Horse.

Papillomatous digital dermatitis is a common disease in cattle. The pastern dermatitis observed in a horse shared many of the gross characteristics of papillomatous digital dermatitis in cattle. Lesions included a mixture of proliferative and erosive changes, with a verrucose appearance in some areas. Microscopic similarities included pseudoepitheliomatous and papillomatous epidermal hyperplasia with hyperkeratosis, spongiosis of the epidermis, and intraepidermal spirochetes. The horse was also concurrently infected with Pelodera strongyloides. Papillomatous digital dermatitis in cattle is associated with poor husbandry practices. The environment of the affected horse was heavily contaminated with urine, manure, and other organic debris. Verrucous pododermatitis of horses may be the same as or similar to bovine papillomatous digital dermatitis, and these conditions have similar etiologies.

Evolution of sperm size in nematodes: sperm competition favours larger sperm.

In the free-living rhabditid nematode Caenorhabditis elegans, sperm size is a determinant of sperm competitiveness. Larger sperm crawl faster and physically displace smaller sperm to take fertilization priority, but not without a cost: larger sperm are produced at a slower rate. Here, we investigate the evolution of sperm size in the family Rhabditidae by comparing sperm among 19 species, seven of which are hermaphroditic (self-fertile hermaphrodites and males), the rest being gonochoristic (females and males). We found that sperm size differed significantly with reproductive mode: males of gonochoristic species had significantly larger sperm than did males of the hermaphroditic species. Because males compose 50% of the populations of gonochoristic species but are rare in hermaphroditic species, the risk of male-male sperm competition is greater in gonochoristic species. Larger sperm have thus evolved in species with a greater risk of sperm competition. Our results support recent studies contending that sperm size may increase in response to sperm competition.

Variable cell positions and cell contacts underlie morphological evolution of the rays in the male tails of nematodes related to Caenorhabditis elegans.

As a first step toward understanding their mechanism of morphological evolution, we compare the morphology and development of the male genitalia in 10 species of Rhabditidae, the family of nematodes that includes Caenorhabditis elegans. We describe a number of variable morphological characteristics and focus in particular on the differing arrangements of the caudal papillae or rays within the acellular fan. We analyze the development of the ray cells within the epidermis of the last larval stage and identify changes in cell positions and cell contacts that underlie evolutionary changes in the arrangement of the rays. Epidermal cell positions were determined by means of indirect immunofluorescence staining with a monoclonal antibody directed towards adherens junctions. Similarities between the species in the cellular arrangements during the earliest developmental stages allow us to propose homologies between the rays in different species. Evolutionary changes in the positions and order of homologous rays are correlated with shifts in cell positions during development. The results suggest that genes for cell recognition or adhesion proteins, or pattern formation genes that regulate cell recognition or adhesion proteins, may be important foci of evolutionary change affecting morphology.

Changes of induction and competence during the evolution of vulva development in nematodes.

In Caenorhabditis, the vulva is formed in the central body region from three of six equivalent cells and is induced by the gonad. In some nematodes, however, the vulva is located in the posterior body region. Vulval development has been analyzed in three such genera. The same precursor cells give rise to the vulva in Caenorhabditis and in the posterior vulva species, but in the latter the cells first migrate posteriorly. In two such species, the vulva is not induced by the gonad, but instead relies on intrinsic properties of precursor cells. Thus, evolution of organ position involves changes in induction and competence.