title: X-cells in fish pseudotumors are parasitic protozoans.
abstract: Bottom-dwelling teleosts, particularly flatfishes or cod living in temperate to cold seawater, sometimes develop tumor-like lesions on the body surface or in the branchial cavity. These lesions usually contain masses of so called 'X-cells' of unknown origin. We amplified a gene for small subunit ribosomal RNA (18S rRNA) from X-cell lesions of the flathead flounder Hippoglossoides dubius. Phylogenetic analysis clearly classified the obtained sequence as a protozoan, although the organism had no clear affinity with any known protistan groups. In situ hybridization showed that probes specific for the protozoan 18S rRNA hybridized only with X-cells, and not with the host-fish cells, indicating that X-cells harbor the protozoan rRNA. On the other hand, a probe specific for vertebrate 18S rRNA hybridized with the host-fish cells, but not with X-cells. This is conclusive evidence that X-cells are parasitic protozoans.
authors.affiliation: Inland Station, National Research Institute of Aquaculture, Tamaki, Mie 519-0423, Japan. miwasat@affrc.go.jp
authors.forename: Satoshi
authors.initials: S
authors.lastname: Miwa
authors.forename: Chihaya
authors.initials: C
authors.lastname: Nakayasu
authors.forename: Takashi
authors.initials: T
authors.lastname: Kamaishi
authors.forename: Yasutoshi
authors.initials: Y
authors.lastname: Yoshiura
mesh_terms: D000818:Animals; D001483:Base Sequence; D002477:Cells / Q000166:cytology*; D016000:Cluster Analysis; D017931:DNA Primers; D056890:Eukaryota / Q000235:genetics*; D005393:Fish Diseases / Q000175:diagnosis* / Q000469:parasitology; D005432:Flounder / Q000469:parasitology*; D006652:Histological Techniques; D017403:In Situ Hybridization; D008969:Molecular Sequence Data; D010802:Phylogeny; D011529:Protozoan Infections, Animal / Q000175:diagnosis*; D012337:RNA, Ribosomal, 18S / Q000235:genetics; D017422:Sequence Analysis, DNA
chemical_list: D017931:DNA Primers; D012337:RNA, Ribosomal, 18S