High-efficiency transformation of the chlorarachniophyte Amorphochlora amoebiformis by electroporation

Abstract Chlorarachniophytes are marine unicellular algae that acquired a complex plastid through an endosymbiosis with a green alga. As such, they have attracted attention especially from evolutionary biologists as a model for studying organellogenesis. A transient transformation system by particle bombardment was previously established for an amoeboid species of chlorarachniophytes, Amorphochlora amoebiformis, and this system has been utilized for expressing fluorescent protein tags, such as GFP. However, the transformation efficiency of this approach is likely too low to generate stable transformants, limiting its utility. Recently, we succeeded in transfecting A. amoebiformis cells by electroporation, and here we optimized electroporation conditions to achieve high transformation efficiencies. Interestingly, successful conditions for A. amoebiformis were similar to those for mammalian cells, despite their distant relationship. The maximum transformation efficiency was calculated to be over 1000 per 1 × 106 cells, which is 1000-fold higher compared to the previous bombardment system. Furthermore, we succeeded in generating a stably transformed line by culturing isolated transformants. We believe that this genetic tool will contribute to our understanding of cell biology in chlorarachniophyte algae and provide insights into the evolution of secondary endosymbioses.