We investigated the development of the macronucleus of Cryptocaryon irritans, and the ingestion and digestion of host cells by the parasite. All developmental stages of the parasite except the theront (trophont, protomont and tomont) were examined with histological staining and/or whole-mount staining. The sections were also subjected to in situ hybridization targeting the 18S rRNA gene of the host fish. The macronucleus developed negligibly, maintaining its four-segmented shape in trophonts, elongated and formed a massive nucleus in tomonts before cell division, and subsequently underwent repeated divisions to generate theronts. Denatured host cells, examined in situ hybridization, were accumulated in trophonts, and disappeared in tomonts by the beginning of cell division. Denatured host cells were also observed in the host tissue surrounding trophonts. They had condensed nuclei. Protomonts filled with denatured host cells presented a 180-bp DNA ladder in gel electrophoresis, suggesting apoptosis of the host cells. These results indicate that DNA synthesis occurs exclusively in the early stage of the tomont and that host cells are fed and accumulated, probably as apoptotic cells, in the trophont and are digested in the early tomont stage.
2016 The Japanese Society of Fish Pathology