Zoology · Ch 1 — Reproduction in Organisms
Multiple Fission, Sporulation and Strobilation
Multiple Fission, Sporulation and Strobilation
In multiple fission, the parent's nucleus divides repeatedly first, without any accompanying division of the cytoplasm; only once many daughter nuclei have formed does the cytoplasm itself divide, splitting into as many parts as there are nuclei, so that a single parent organism produces many smaller daughter individuals all at once. When all the daughter individuals form by equal division and stay together until the whole process is complete, the special case is called repeated fission, as in Vorticella. In the malarial parasite Plasmodium, multiple fission occurs at two distinct life-cycle stages: in the schizont stage it is called schizogony, producing daughter cells called merozoites, and in the oocyte stage it is called sporogony, producing daughter cells called sporozoites.
Amoeba shows two further variations built around multiple fission. Under unfavourable conditions (temperature extremes, food scarcity) it withdraws its pseudopodia and secretes a hard, three-layered chitinous cyst wall around itself - a process called encystment - and becomes inactive; once conditions improve, the encysted amoeba undergoes multiple fission to produce many minute amoebulae (pseudopodiospores), which are released when the cyst wall absorbs water and ruptures, each going on to feed, grow and lead an independent life. Separately, Amoeba can also reproduce by sporulation without ever forming a cyst: its nucleus fragments into several chromatin blocks, each of which develops its own nuclear membrane and becomes wrapped in cytoplasm and a protective spore case, and when the parent body eventually disintegrates these spores are released, each hatching into a fresh young amoeba once conditions turn favourable. …
What this figure shows. A diagram of a Plasmodium schizont undergoing multiple fission (schizogony) inside a host red blood cell, showing the single nucleus having divided repeatedly into many daughter nuclei clustered within the shared cytoplasm, each daughter nucleus becoming enclosed by its own share of cytoplasm to form a merozoite, and the schizont's outer wall rupturing to release the cluster of newly formed merozoites into the blood …
What this figure shows. A four-panel sequence (A-D) showing multiple fission in an encysted Amoeba: panel A shows the amoeba surrounded by its hard, three-layered protective chitinous cyst wall with the nucleus and cytoplasm inside; panel B shows the nucleus having divided repeatedly into many small daughter nuclei within the cyst; panel C shows each daughter nucleus becoming enclosed by cytoplasm to form separate pseudopodiospores (amoebulae) still packed inside the ruptured cyst wall; and panel D shows the young amoebae, each bearing a fine pseudopodium, newly liberated and beginning indep …
What this figure shows. A diagram of the jellyfish Aurelia's strobila stage, showing the stalked, sessile polyp form attached by a basal adhesive disc with a stolon at its base, its body stacked into a series of disc-like segments (an early young strobila progressing to an adult strobila) produced by repeated simultaneous transverse fissions, and the topmost, most mature segment shown detaching as a free-swimming ephyra larva - drawn from both oral and side views - with its marginal lappets, tentacles, manubrium, mouth and radial c …
What this figure shows. A labelled sequence (A-E) showing sporulation in Amoeba without prior encystment: the nucleus of the parent amoeba (A) breaking up into several small chromatin blocks scattered through the cytoplasm (B), each fragment then developing its own nuclear membrane to become a separate small nucleus (C) that becomes surrounded by cytoplasm and encased in a protective spore case (D), and finally the parent body disintegrating to liberate the individual spores, each of which hatches …