|The ciliate protozoan Ichthyophthirius multifiliis (Ich) is an important parasite of freshwater fish that causes 'white spot’ disease. Ich is a major contributor of fish mortalities and economic loss to both the ornamental and edible fish stocks around the world. Despite its global importance, very little genetic information is available for Ich. The focus of this study is to create a large-scale genetic resource for Ich and to utilize those resources in a microarray platform to examine global gene expression in the parasite. The first goal was to generate expressed sequence tags (ESTs) for Ich. Toward this goal, a total of 10,368 EST clones were sequenced using a normalized cDNA library made from pooled RNA samples of the trophont, tomont, and theront life-cycle stages, leading to 8,432 high quality sequences. Clustering analysis of these ESTs allowed identification of 4,706 unique sequences containing 976 contigs and 3,730 singletons. This set of ESTs represents a significant proportion of the Ich transcriptome, and provides a material basis for the development of microarrays useful for gene expression studies concerning Ich development, pathogenesis, and virulence.
While analyzing the ESTs, a particular sequence cluster was over-represented in the library. The sequence was chose for further analysis. Upon first inspection, the sequence appeared to be a putative senescence-related transcript. Upon further inspection, the transcript was of ribosomal origin. However, the genetic library was created by the use of poly (A) tails; conventionally, ribosomal RNAs (rRNA) are not polyadenylated. Reported here is the discovery of polyadenylated rRNAs in Ich. Analysis using multiple sequence alignments revealed four potential polyadenylation sites including three internal regions and the 3′ end of the large subunit rRNA. While the functions of polyadenylation of rRNA in this organism are largely unknown at present, the presence of internal polyadenylation sites, along with the presence of truncated segments of the rRNA, may suggest a role of the polyadenylation in a degradation pathway.
The major goal of this study was to build and utilize a microarray platform for the analysis of gene expression in Ich. An oligo microarray containing all publicly available Ich ESTs representing 9,129 unique genes was developed. In addition to oligo features representing the 9,129 Ich genes, gene coding sequences from two related protozoa, Tetrahymena thermophila and Plasmodium falciparum, were also included to increase gene content through cross-hybridization. The microarray was used to examine gene expression patterns of the three life stages of Ich: infective theront, parasitic trophont, and reproductive tomont. A total of 173 putative genes were found to be differentially regulated among all three life-stages. Examples of differentially expressed transcripts included immobilization antigens, annexins, and epiplasmin, as well as various other transcripts involved in developmental regulation and host-parasite interactions. The microarray platform was further used to assess differential regulation of genes between early and late serial passages, as the parasite becomes senescent and losses some of its infectivity. A total of 215 transcripts were found to be differentially expressed and common to both tomont and trophont between passage 1 to passage 100, including surface proteins and other likely immunogens potentially involved in host-pathogen interactions. Genes that are differentially expressed during steady-state Ich development, as well as potential candidate genes involved in Ich infectivity will be presented and discussed.