Sexual Cycle of Malarial Parasite Development

The question asks us to identify the correct order of development for the malarial parasite, Plasmodium, during its sexual cycle. This part of the life cycle takes place inside the mosquito vector.

Let's look at the stages provided:

• (i) Gametocyte: These are the sexual stages (male and female) of the parasite that are taken up by the mosquito when it feeds on an infected human.
• (ii) Sporozoites: These are the infective stage for humans. They are found in the mosquito's salivary glands and are inoculated into a human during a blood meal.
• (iii) Oocyst: This is a structure that develops on the outer wall of the mosquito's midgut. It contains many sporozoites.
• (iv) Ookinete: This is a motile, elongated form of the zygote. It penetrates the mosquito's gut wall.

The sexual cycle begins when the mosquito ingests gametocytes (i). Inside the mosquito's gut, the gametocytes develop into male and female gametes, which fuse to form a zygote. The zygote is initially non-motile but quickly transforms into a motile stage called the ookinete (iv).

The ookinete (iv) is able to penetrate the epithelial lining of the mosquito's midgut and migrates to the outer surface. On the outer gut wall, the ookinete rounds up and develops into an oocyst (iii).

Inside the oocyst (iii), the parasite undergoes multiple divisions (sporogony), producing thousands of sporozoites (ii). When the oocyst matures and ruptures, the sporozoites (ii) are released into the mosquito's body cavity (hemocoel). They then migrate to the mosquito's salivary glands, from where they can be transmitted to a new human host.

Therefore, the correct sequence of these stages in the sexual cycle within the mosquito is: Gametocyte (i) → Ookinete (iv) → Oocyst (iii) → Sporozoites (ii).

Let's list the sequence:

1. Gametocyte (i)
2. Ookinete (iv)
3. Oocyst (iii)
4. Sporozoites (ii)

This order corresponds to the sequence (i), (iv), (iii), (ii).

Revision Table: Malarial Parasite Sexual Cycle Stages

Additional Information: Malaria Life Cycle Overview

The complete life cycle of the malarial parasite, Plasmodium, involves two hosts: humans and mosquitoes (specifically, female Anopheles mosquitoes). The cycle has both asexual and sexual phases.

• Asexual Cycle (in humans): Starts with sporozoites injected by the mosquito. These infect liver cells (exo-erythrocytic schizogony), then red blood cells (erythrocytic schizogony), producing merozoites that infect more red blood cells, leading to malaria symptoms. Some parasites develop into gametocytes.
• Sexual Cycle (in mosquitoes): Starts when mosquitoes ingest gametocytes from infected human blood. Gametocytes mature into gametes, fertilize to form a zygote which becomes an ookinete. The ookinete develops into an oocyst on the gut wall. Oocysts produce sporozoites which migrate to the salivary glands, ready to infect a human. This is the cycle we analyzed in this question.

Understanding both phases is crucial to grasping how malaria is transmitted and how it affects the human body.