Operon Conservation and the Evolution of trans-Splicing in the Phylum Nematoda

The nematode Caenorhabditis elegans is unique among model animals in that many of its genes are cotranscribed as polycistronic pre-mRNAs from operons. The mechanism by which these operonic transcripts are resolved into mature mRNAs includes trans-splicing to a family of SL2-like spliced leader exons. SL2-like spliced leaders are distinct from SL1, the major spliced leader in C. elegans and other nematode species. We surveyed five additional nematode species, representing three of the five major clades of the phylum Nematoda, for the presence of operons and the use of trans-spliced leaders in resolution of polycistronic pre-mRNAs. Conserved operons were found in Pristionchus pacificus, Nippostrongylus brasiliensis, Strongyloides ratti, Brugia malayi, and Ascaris suum. In nematodes closely related to the rhabditine C. elegans, a related family of SL2-like spliced leaders is used for operonic transcript resolution. However, in the tylenchine S. ratti operonic transcripts are resolved using a family of spliced leaders related to SL1. Non-operonic genes in S. ratti may also receive these SL1 variants. In the spirurine nematodes B. malayi and A. suum operonic transcripts are resolved using SL1. Mapping these phenotypes onto the robust molecular phylogeny for the Nematoda suggests that operons evolved before SL2-like spliced leaders, which are an evolutionary invention of the rhabditine lineage

The Wolbachia endosymbionts of filarial nematodes

Filarial nematodes harbour intracellular, Gram-negative bacteria belonging to the genus Wolbachia. These bacteria have been observed in various species of filariae, including the main filariasis agents of humans and animals. It has been suggested that Wolbachia could play an important role in the biology of filarial nematodes and could be implicated in the pathogenesis of filarial diseases. Wolbachia could thus represent a target for the control of filariasis and a key to the understanding of these diseases. Indeed, in various species of filariae, tetracycline treatments have been shown both to reduce/eliminate the Wolbachia population and to determine detrimental effects on the nematodes. In addition, proteins of Wolbachia have been shown to determine specific IgG responses in animals infected by filariae and some Wolbachia molecules (e.g. LPS) have been shown to stimulate innate-immunity responses (e.g. production of cytokines such as IL1, IL6, IL10, TNF-\u3b1 and IFN-\u3b3 by macrophages)

Conservation of long-range synteny and microsynteny between the genomes of two distantly related nematodes

Background Comparisons between the genomes of the closely related nematodes Caenorhabditis elegans and Caenorhabditis briggsae reveal high rates of rearrangement, with a bias towards within-chromosome events. To assess whether this pattern is true of nematodes in general, we have used genome sequence to compare two nematode species that last shared a common ancestor approximately 300 million years ago: the model C. elegans and the filarial parasite Brugia malayi. Results An 83 kb region flanking the gene for Bm-mif-1 (macrophage migration inhibitory factor, a B. malayi homolog of a human cytokine) was sequenced. When compared to the complete genome of C. elegans, evidence for conservation of long-range synteny and microsynteny was found. Potential C. elegans orthologs for II of the 12 protein-coding genes predicted in the B. malayi sequence were identified. Ten of these orthologs were located on chromosome I, with eight clustered in a 2.3 Mb region. While several, relatively local, intrachromosomal rearrangements have occurred, the order, composition, and configuration of two gene clusters, each containing three genes, was conserved. Comparison of B. malayi BAC-end genome survey sequence to C. elegans also revealed a bias towards intrachromosome rearrangements. Conclusions We suggest that intrachromosomal rearrangement is a major force driving chromosomal organization in nematodes, but is constrained by the interdigitation of functional elements of neighboring genes

Genes expressed in Brugia malayi infective third stage larvae

We have used a tag sequencing approach to survey genes expressed in the third stage infective larvae of the human filarial nematode parasite Brugia malayi. RNA was isolated from late vector-stage L3 larvae after days 9 or 10 of infection in mosquitos, and converted to cDNA by reverse transcriptase. Double-stranded cDNA was produced by either conventional methods (non-SL cDNA library) or by PCR using the nematode spliced leader (SL1) and oligo(dT) primers (SL cDNA library). Two clone libraries (one from SL and one from non-SL cDNAs) were constructed in lambda ZapII. A set of these full-length clones was selected and 596 inserts were sequenced from the 5′ end. We have identified 364 B. malayi genes (the majority of which are new) that encode housekeeping proteins, structural proteins, proteins of immediate immunological or drug-discovery interest as well as a large class of novel sequences which may prove to have significant involvement in host invasion. Extensive, genome-wide approaches to the analysis of larval gene expression are now possible for B. malayi. We present several examples of this approach

Most of the Response Elicited against Wolbachia Surface Protein in Filarial Nematode Infection Is Due to the Infective Larval Stage

Immune responses to the intracellular Wolbachia bacteria of filarial nematodes are thought to contribute to the pathologic process of filarial infection. Here, we compare antibody responses of subjects living in an area where lymphatic filariasis is endemic with antibody responses elicited in a murine model of filarial infection, to provide evidence that the infective larval stage (L3), not adult nematodes, are the primary inducer of responses against Wolbachia. In human subjects, antibody responses to Brugia malayi Wolbachia surface protein (WSP) are most often correlated with antibody responses to the L3 stage of B. malayi. Analysis of anti-WSP responses induced in mice by different stages of the rodent filariae Litomosoides sigmodontis shows that the strongest anti-WSP response is elicited by the L3 stage. Although adult filarial nematode death may play a role in the generation of an anti-WSP response, it is the L3 stage that is the major source of immunogenic material, and incoming L3 provide a continual boosting of the anti-WSP response. Significant exposure to the endosymbiotic bacteria may occur earlier in nematode infection than previously thought, and the level of exposure to infective insect bites may be a key determinant of disease progression