Lineage differentiation in the cosmopolitan molluscan protozoan parasite Perkinsus olseni based on metabolic, molecular and morphological traits

The protozoan parasite Perkinsus olseni actively proliferates within marine molluscan hosts and causes serious negative effects on mollusk production worldwide. Recent research revealed that one Japanese isolate requires exogenous glycine betaine (GB) for vegetative proliferation, whereas isolates from some congeneric species do not, suggesting that this requirement is a derived characteristic of this genus. In the present study, we established various isolates from Japan, Europe, and Oceania, and showed that P. olseni isolates from Japan and Europe commonly require exogenous GB, whereas Oceanian isolates do not. This suggests that Oceanian isolates may retain ancestral-like characteristics compared to Japanese and European isolates. Analyses of the non-transcribed spacer region of ribosomal DNA further support the above interpretation that the Oceanian isolates represent a more basal split, and additionally suggest that the Japanese and European isolates belong to lineages that have undergone relatively recent geographic expansion. Morphological comparisons among isolates showed that the Oceanian isolates possessed larger schizonts and trophozoites, producing fewer daughter cells, but formed larger hypnospores capable of producing a greater number of zoospores, although the association between GB requirements and morphological differences remains unknown. The present study demonstrates the existence of distinct lineages of this parasite and provides novel insights into the evolution, geographical dispersion, and shifts in life-history strategies of P. olseni.