The First Ultrastructural Description of Appendicularians (Chordata: Tunicata) Infected by Microsporidia-Like Protists

Abstract

Intracellular parasitism in appendicularians has been identified for the first time. Using transmission electron microscopy, cells of microsporidia-like protists have been found in the digestive epithelium of the appendicularian Oikopleura gracilis that externally do not exhibit signs of infection: two morphotypes of cells, interpreted as pre-spore stages, and spores. Cells of the first morphotype are ultrastructurally similar to microsporidial sporonts, with vesicles of the Golgi apparatus visible in their cytoplasm. In cells of the second morphotype, presumably sporoblasts, the envelope is noticeably thicker and specific organelles can be seen in the cytoplasm: three coils of the polar filament and the vacuole. The detected spores (0.7 × 0.9 μm), in a similar manner to microsporidial spores, are covered with a two-layered envelope over the cytoplasmic membrane and contain a polar filament and a polaroplast. In the digestive epithelium of appendicularians, protists cause hypertrophy of invaded enterocytes, deformation of their nuclei, and an increase in the size of mitochondria.