SYSTEMATIC BIOLOGY
Opalina LIFE HISTORY

Life cycle of Opalina ranarum. 1 Cysts are excreted by the adult frog and orally ingested by a tadpole. 2 After hatching the young gamont migrates to the cloaca. 3, 4 Formation of micro- and macrogametes (meiosis). 5 Fusion of the heterogametes. 6 Encystation of the zygote and excretion via feces. 7, 8 After oral uptake of a cyst by another tadpole the trophozoite grows up in the cloaca (up to 0.5 mm). 8.1, 8.2 The small trophozoite may start division inside the tadpole, finally leading to formation and excretion of cysts (1) which give rise to new trophozoites (2 → 8) after ingestion by another tadpole. 9 When metamorphosis of tadpoles to frogs is completed, the trophozoites (agamonts, trophonts) grow up and form up to 2000 nuclei. 10 During the non-breeding season of the frog the trophozoites multiply by binary fission, the axis of which is either longitudinal or olique-transverse. 11, 12 During the breeding season hormones released by the frog induce rapid divisions of the trophozoites without compensatory nuclear divisions and growth. Thus the parasites (precystic forms) become successively smaller. These stages, finally having 2–12 nuclei, encyst (1), are set free with the feces of the host and become infectious for tadpoles. CI, cilia; CW, cyst wall; N, nucleus

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