Transcriptome Analysis of the Pathogenic Oomycete Pythium insidiosum

Oomycetes form a unique group of microorganisms that share hyphal morphology with fungi. Most of pathogenic oomycetes infect plants, while some species are capable of infecting animals. Pythium insidiosum is the only oomycete that can infect both humans and animals, and causes a life-threatening infectious disease, called “pythiosis”. Controlling an infection caused by P. insidiosum is problematic because effective antimicrobial drugs are not available. Information on the biology and pathogenesis of P. insidiosum is limited. We generated a P. insidiosum transcriptome of 26,735 unigenes, using the 454 sequencing platform. As adaptations to increased temperature inside human hosts are required for a successful pathogen, we generated P. insidiosum transcriptomes at 28°C and 37°C and identified 625 up-regulated and 449 down-regulated genes at 37°C. Comparing the proteomes of oomycetes, fungi, and parasites provided clues on the evolutionary history of P. insidiosum. Potential virulence factors of P. insidiosum, including putative RXLR and elicitin effectors, were indentified. P. insidiosum harbored an extensive repertoire of ~300 elicitin effectors that is at least 5 times more than that found in other oomycetes. Fifteen putative RXLR translocation effectors were identified in P. insidiosum, whereas none are found in the closely-related oomycete Pythium ultimum. Evolution, through gene gain, gene loss, and gene modification has produced a wide variety of effectors in oomycetes. Such evolutionary events are likely required for the specialization of lifestyle, host specificity, and virulence strategies of a particular oomycete species. The transcriptome, presented herein, is an invaluable resource for exploring P. insidiosum’s biology, pathogenesis, and evolution.