Alternation of generations: the malaria parasite and a fern
Some plants and animals reproduce asexually and sexually in turn in one life cycle. This gives them the advantages of both: quick multiplication and variety, which helps them spread and meet changes in the surroundings. In every such cycle the number of chromosomes is halved by meiosis and restored by fertilization, so a haploid generation that makes gametes alternates with a diploid generation that makes spores. The first example is Plasmodium, the parasite that causes malaria. It is a sporozoan protozoan that lives in humans and in the female Anopheles mosquito. The cycle starts when an infected mosquito bites and puts spindle-shaped sporozoites into human blood. They go to the liver and reproduce asexually by schizogony into merozoites. The merozoites enter red blood cells and repeat schizogony. The book says that the new merozoites are released together about every 2 days, when the infected cells burst and toxins are set free; fever comes with this. A person who has a fever after travelling should see a doctor, who will decide what is wrong. Some merozoites become gametocytes. When a mosquito takes up this blood, the gametes are formed in its stomach, fuse into a zygote, and the zygote crosses the stomach wall and divides by meiosis into an oocyst. The nucleus of the oocyst divides by mitosis (sporogony) into many sporozoites, which move to the mosquito’s salivary glands. So there is one sexual generation, in the mosquito, and asexual generations by sporogony in the mosquito and schizogony in humans. The second example is a fern. The sporophyte (2N) has sori on the lower side of its leaves, each with sporangia. Spore mother cells divide by meiosis into spores (N). Wind carries them, and on wet soil a spore grows into a small heart-shaped gametophyte (N) with rhizoids. On it grow antheridia (male), whose sperm swim through soil water, and archegonia (female), each with an egg. Fertilization gives a zygote (2N) that grows into a new sporophyte on the gametophyte, which then withers.
| Stage of Plasmodium | Where | How it reproduces |
|---|---|---|
| Sporozoites | human liver | schizogony, into merozoites |
| Merozoites | red blood cells | schizogony, repeated |
| Gametocytes to gametes | stomach of the female mosquito | sexual: gametes fuse into a zygote |
| Oocyst | wall of the mosquito’s stomach | sporogony, into sporozoites |
Draw a loop, with a grown-up. On a sheet draw a large circle with four boxes for the fern cycle, and put in each box the stage name and the letter N or 2N. Write meiosis on the arrow where the number halves and fertilization on the arrow where it is restored. Then draw a second, smaller sketch with only the words mosquito and human, and write which of them hosts schizogony and which hosts the sexual generation. This is a drawing exercise only.