Ultrastructure of the intracellular parasite Parvilucifera sinerae (Alveolata, Myzozoa) infecting the marine toxic planktonic dinoflagellate Alexandrium minutum (Dinophyceae)

Abstract

This ultrastructural study follows the infection by a parasite, Parvilucifera sinerae, of the marine toxic dinoflagellate Alexandrium minutum. In clonal cultures, infective zoospores, which are the infectious stage, entered host cells where they developed into sporangia. Mature, dark-colored sporangia were released from the dead dinoflagellate cells by the break-up of the host theca. The sporangial cytoplasm ultimately differentiated into many minute zoospores that escaped one host cell and subsequently infected healthy cells. The zoospores are 1.2–1.8 μm in diameter and possess an anterior (transverse) and shorter posterior flagellum in addition to alveoli, a refractile body, a mitochondrion with tubular cristae, micronemes, rhoptries, and a pseudoconoid. As in the other Parvilucifera species, the zoospore has a heteromorphic pair of central microtubules in the anterior axoneme and a reduced pseudoconoid. The micronemes have bulbous posterior ends and, as in other known species of the genus, there are (pseudo)conoid-associated micronemes. A comparison of these ultrastructural data with those available from the other two Parvilucifera species showed that, as in Parvilucifera infectans, the periphery of the nucleus is filled with thin fibers, in contrast to the syndinean-like nucleus of Parvilucifera prorocentri. Also, as in P. infectans, the transition region of the flagellar axoneme contains a dense structure, and the basal body a black globule; however, both structures are absent in P. prorocentri. There are no morphological differences of the zoospores from P. sinerae and P. infectans. The main difference between P. infectans and P. sinerae relay on the sporangium wall and ornamentation. The sporangium wall in P. sinerae consisted of three layers instead of two described layers in P. infectans. Therefore, morphologically, P. sinerae shows greater similarity to P. infectans than to P. prorocentri.