Stable transfection of the diplomonad parasite Spironucleus salmonicida

Eukaryotic microbes are highly diverse, and many lineages remain poorly studied. One such lineage, the diplomonads, a group of binucleate heterotrophic flagellates, has been studied mainly due to the impact of Giardia intestinalis, an intestinal, diarrhea-causing parasite in humans and animals. Here we describe the development of a stable transfection system for use in Spironucleus salmonicida, a diplomonad that causes systemic spironucleosis in salmonid fish. We designed vectors in cassette format carrying epitope tags for localization (3×HA [where HA is hemagglutinin], 2× Escherichia coli OmpF linker and mouse langerin fusion sequence [2×OLLAS], 3×MYC) and purification of proteins (2× Strep-Tag II-FLAG tandem-affinity purification tag or streptavidin binding peptide-glutathione S-transferase [SBP-GST]) under the control of native or constitutive promoters. Three selectable gene markers, puromycin acetyltransferase (pac), blasticidin S-deaminase (bsr), and neomycin phosphotransferase (nptII), were successfully applied for the generation of stable transfectants. Site-specific integration on the S. salmonicida chromosome was shown to be possible using the bsr resistance gene. We epitope tagged six proteins and confirmed their expression by Western blotting. Next, we demonstrated the utility of these vectors by recording the subcellular localizations of the six proteins by laser scanning confocal microscopy. Finally, we described the creation of an S. salmonicida double transfectant suitable for colocalization studies. The transfection system described herein and the imminent completion of the S. salmonicida genome will make it possible to use comparative genomics as an investigative tool to explore specific, as well as general, diplomonad traits, benefiting research on both Giardia and Spironucleus.