Morphology, morphogenesis and gene sequence of a freshwater ciliate, Pseudourostyla cristata (Ciliophora, Urostyloidea) from the ancient Lake Biwa, Japan

Abstract

The urostyloid freshwater ciliate Pseudourostyla cristata was recorded for the first time from Lake Biwa, a 4-million-year-old lake located in Shiga Prefecture, Japan. Its morphology and morphogenesis were investigated using live observation and protargol impregnation, and the SSU ribosomal RNA gene was sequenced. Based on the current observations and previous descriptions, this species is readily recognized mainly by the following characters: body slender or broadly oval to elliptical, and dark grey in color; size in vivo about 170–400×40–150 μm; pellicle flexible and contractile, with extrusomes forming a hyaline seam underneath; ciliature comprising about 60–130 adoral membranelles, usually 1 buccal cirrus, 20–24 frontal, 2 frontoterminal, 17–26 pairs of midventral, and 5–16 transverse cirri, 4–6 left and 4–5 right marginal rows, and 8–10 dorsal kineties; 15–83 macronuclear nodules and 2–9 micronuclei; freshwater habitat. The main morphogenetic developments are: (1) the oral primordium for the proter originates de novo on the dorsal wall of the buccal cavity, and the dedifferentiated undulating membranes and some parental proximal membranelles join in the primordial development; the old adoral zone will be partly replaced by new structures; (2) the oral primordium for the opisthe occurs epiapokinetally left of the midventral complex between the adoral zone and the transverse cirri; (3) the fronto-midventral transverse cirral (FVT) anlagen develop separately in both dividers by dedifferentiation of most of the midventral cirri; (4) the single buccal cirrus is generated from the posterior end of FVT anlage II; (5) the leftmost frontal cirrus is derived from the anterior end of the undulating membranes anlage (FVT anlage I); (6) the marginal rows of each side are formed from a single anlage which arises within the rightmost row; (7) the dorsal kineties develop by intrakinetal basal body proliferation; and (8) the most posterior FVT anlage contributes the two fronto-terminal cirri at its anterior end. The present observations indicate that P. cristata has a wide geographic distribution, and possesses constant morphological and morphogenetic traits. Phylogenetic trees inferred from SSU rRNA gene sequences suggest paraphyly of the genus Pseudourostyla.