Fig. 1

a–f Light microscopic and ultrastructural morphology of sarcocysts of Sarcocystis sp. in SD rats 5 months after inoculation with sporocysts isolated from rat snakes in Thailand. a Typical sporocyst from a fecal sample of Coelognathus radiatus; sporocysts from C. flavolineatus were identical in size and appearance; asterisk indicates single sporozoite. b Live sarcocyst, freed from muscle tissue; note the broad, palisade-like villar protrusions that could at times resemble those of Sarcocystis singaporensis with which this species can co-occur; however, the protrusions lack the basal stalks typical for the former species; the arrow highlights the septated compartments in the interior of the sarcocyst, and the inset shows a micrograph of live cystozoites freshly released from a cyst (arrowheads). c Typical structure of a cyst wall protrusion (isolate from C. flavolineatus); the arrows point to the electron-dense, knob-like structures of the primary cyst wall, whereby the knobs could apparently fuse to form an electron-dense borderline in larger protrusions (inset: arrowhead); also note the electron-light, thin layer of ground substance (GS) underneath the protrusions. d Typical cystozoites of the new species, which contained only two rhoptries (arrowheads) among relatively few micronemes (asterisks); additionally, the cystozoites exhibited vesicle-like structures in the anterior third of the cell containing electron-light, reticulate matter (arrow); the inset shows such a vesicle-like compartment at higher magnification, which was apparently not bound by a membrane (white arrow) and often located near micronemes (white asterisk); dense granules were present but rarely observed. e Interior and cyst wall of a mature sarcocyst (isolate from C. radiatus); metrocytes (asterisks) exclusively divided by endodyogeny, producing only two cystozoites (CZ). f Full-length section through a 15-µm-long protrusion of the sarcocyst wall; note that larger protrusions often occurred close to the tips of a cyst and showed a base with folds. AP, apicoplast; MI, mitochondrion; NU, nucleus; PT, villar protrusions