Q.Describe, with reference to each stage, the life cycle of Plasmodium vivax in the mosquito.

[!TLDR] In the mosquito, ingested gametocytes undergo exflagellation and maturation, fuse by anisogamy into a zygote/ookinete, and produce sporozoites by sporogony inside an oocyst, which then migrate to the salivary glands.

A female Anopheles ingests gametocytes with a blood meal; the microgametocyte undergoes exflagellation to release motile microgametes, while the macrogametocyte matures into a macrogamete; anisogamous fusion forms a zygote, which becomes a motile ookinete, penetrates the gut wall, and rounds off as an oocyst; sporogony inside the oocyst produces sporozoites that migrate to the salivar …

The female Anopheles mosquito is the definitive host of Plasmodium vivax, because it is inside the mosquito that the parasite's sexual phase of reproduction — gametogony's completion, fertilisation, and sporogony — takes place.

Stage 1: Ingestion of gametocytes

When a female Anopheles mosquito bites an infected person and takes a blood meal, it ingests, along with the blood, any circulating gametocytes present — the sexual forms of the parasite, comprising microgametocytes (male) and macrogametocytes (female), that had earlier differentiated from merozoites in the human bloodstream but could develop no further there.

Stage 2: Exflagellation of the microgametocyte

Once inside the mosquito's stomach, the microgametocyte undergoes a dramatic transformation called exflagellation: its nucleus divides repeatedly, and several slender, motile, flagellum-like microgametes are thrown outward from the surface of the cell into the surrounding fluid.

Stage 3: Maturation of the macrogametocyte

At the same time, the macrogametocyte matures directly, without dividing, into a single, larger, non-motile macrogamete.

Stage 4: Anisogamous fertilisation

A motile microgamete actively swims through the stomach fluid and fuses with the stationary macrogamete. Because the two gametes involved are visibly dissimilar in size and motility, this fusion is termed anisogamy (fusion of unlike gametes). The result of this fusion is a diploid zygote.

Stage 5: Formation of the motile ookinete

The zygote does not remain a simple rounded cell; it elongates and becomes actively motile, transforming into a worm-like structure called the ookinete. Using its own motility, the ookinete actively penetrates through the stomach (gut) wall of the mosquito, moving from the interior of the stomach lumen to a position just beneath the outer wall of the stomach.

Stage 6: The oocyst and sporogony

Having reached this position, the ookinete rounds off and becomes an oocyst. Inside the oocyst, the nucleus undergoes repeated division by a process called sporogony — distinct from the schizogony that occurs in man, since sporogony follows sexual fusion and produces a different kind of daughter cell. This division produces many thousands of thread-like, infective sporozoites packed within the oocyst.

Stage 7: Rupture of the oocyst and migration to the salivary glands

The mature oocyst eventually ruptures, releasing the sporozoites into the mosquito's general body cavity (haemocoel). From there, the sporozoites actively migrate through the body and come to concentrate specifically in the mosquito's salivary glands, which is what makes the mosquito infective to a new human host.

Stage 8: Completion — transmission to a new host …

• Describing schizogony as happening in the mosquito — schizogony is the asexual process that occurs only in MAN; the mosquito-stage process is SPOROGONY, following sexual fusion.
• Reversing which gametocyte exflagellates — only the MICROgametocyte (male) undergoes exflagellation to release motile microgametes; the macrogametocyte matures directly into a non-motile macrogamete. …