Spore-Forming Protists: Characteristics & Examples

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll discuss a phylum of spore-forming protists, the Apicomplexa. We will discuss their structure and their life cycle, then talk about some apicomplexans that are important to humans.

Apicomplexan Definition

What group of organisms can either live in a mosquito and make you sick, or kill a mosquito and keep you safe? Meet the Apicomplexans!

When we talk about spore-forming protists, we're usually talking about the phylum Apicomplexa. Other groups, such as Microsporidia, used to be classified in the same group, but we now know that Microsporidia are highly reduced fungi and don't share an evolutionary background with protists.

Apicomplexans form spores because this is an important part of their parasitic life cycle. A parasite is an organism that lives in, with, or on another organism and benefits itself while harming its host. The only known apicomplexan that isn't a parasite is Nephromyces, a genus of mutualistic (beneficial) symbionts that live in sea squirts.

Apicomplexan Morphology

Apicomplexans are eukaryotic, meaning that apicomplexan cells contain a nucleus. Like most eukaryotic cells, apicomplexans have organelles such as mitochondria, a Golgi apparatus, and endoplasmic reticulum. They also have a pellicle, a structure of the cytoskeleton made up of a cellular membrane, alveoli, and microtubules. The apical complex and the apicoplast are structures specific to the apicomplexans.

Apicomplexans are named for the apical complex, a complex of microtubules in their apex. This cone-shaped organelle is used for invading host cells. Structures within the apical complex, rhoptries and micronemes, secrete enzymes and other proteins that circumvent a host cell's defenses and allow the parasite to enter.

The apicoplast is an organelle that appears to have evolved from an ancestral chloroplast that lost its photosynthetic function. For reasons that aren't clear, they are necessary to apicomplexan survival. Because only apicomplexans have apicoplasts, this means that apicoplasts are very interesting to scientists who would like to find better treatments for apicomplexan diseases. If we can manufacture a drug that targets apicoplasts, it could potentially be very effective against apicomplexans without harming us or our beneficial gut microbiota.

Apicomplexan Life Cycle

Apicomplexans have a complex life cycle, and there are large variations in the life cycle between different species. Generally speaking, the apicomplexan life cycle is said to start at the sporozoite, or infective, stage. For the next stage, sporozoites will invade a host cell then divide into many cells called merozoites. For sexual reproduction, merozoites then change into reproductive cells called gamonts. When two gamonts fuse, they form a gamontocyst, which then produces sex cells, gametes. Just like with most sexually reproducing organisms, two gametes fuse to form a zygote, which divides via meiosis and develops into a new generation of sporozoite.

Important Apicomplexans

The most famous apicomplexans are in the genus Plasmodium, better known as malaria. Malaria causes about 1.5-2.7 million deaths per year, and it is considered a major obstacle to economic development in many developing countries. Humans are just the intermediate hosts for these malaria-causing apicomplexans. This means that Plasmodium only reproduces asexually in humans; Plasmodium needs its definitive host, the mosquito, to complete its life cycle.

Mosquitoes can harbor malaria.
Mosquitoes can harbor malaria.

Other diseases caused by apicomplexans include toxoplasmosis, babesiosis, and isosporiasis. You may recognize toxoplasmosis as the disease that you can catch from your cat. Your chances of catching toxoplasmosis from a cat have been greatly exaggerated by the media. Cats don't carry the virus externally, and only shed toxoplasmosis-contaminated waste for a few days in their lives. In addition, cats that don't eat raw meat are very unlikely to contract the disease in the first place. So, no, toxoplasmosis isn't a good reason to get rid of the cat. Recently, scientists have also found toxoplasmosis in marine mammals like sea otters. No one knows why just yet.

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