Vestigial chloroplasts in heterotrophic stramenopiles Pteridomonas danica and Ciliophrys infusionum (Dictyochophyceae)

Two heterotrophic members of the Dictyochophyceae (stramenopiles), Pteridomonas danica and Ciliophrys infusionum, were investigated. An undescribed organelle bounded by four membranes and closely associated with the nucleus was detected in P. danica. The outermost membrane was continuous with the outer nuclear membrane. These features strongly suggested that this organelle was a vestigial chloroplast. A photosynthetic gene, rbcL, was successfully amplified by polymerase chain reaction (PCR) from P. danica and C. infusionum. These sequences were readily and well aligned with those of photosynthetic stramenopiles. Phylogenetic trees of 18S rDNA and rbcL were constructed. In all the trees obtained, P. danica and C. infusionum appeared in two different clades, the Pedinellales clade and the Ciliophryales/Rhizochromulinales clade, each of which contained photosynthetic members as well as heterotrophic members. The results indicated that the loss of photosynthetic ability occurred independently in P. danica and C. infusionum. This is the first report of the presence of a vestigial chloroplast (leucoplast) in colorless dictyochophytes.