Tag Archives: Parasites

Friday Fellow: Brown spot of maize

by Piter Kehoma Boll

I’ll continue the parasite trend from last week, but this time shifting from human parasite to maize parasite, and from a prokaryotic to a eukaryotic parasite. So let’s talk about Physoderma maydis, commonly known as the brown spot of maize or brown spot of corn.

The Brown spot of maize is a fungus of the division Blastocladiomycota that infects corn (or maize) plants. Its common name comes from the fact that it causes a series of brown spots on the leaves of an infected plant.

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The brown spots seen on this corn leaf are due to an infection by Physoderma maydis. Credits of the photo to Clemson University – USDA Cooperative Extension Slide Series.*

The life cycle of the brown spot of maize is as complex as that of many fungi. The infection of the plants occur through spores that remain in the soil during winter and are carried to the host by the wind, germinating in the rainy season. The germinated spores produce zoospores, flagellated spores able to swim. Swiming through the maize leaf, the zoospores infect single cells and produce zoosporangia at the surface of the leaf. The zoosporangia release new zoospores that infect new cells. In late spring and summer, the zoospores produce a thallus growing deep inside the maize leaf that infects many cells and produces thick-walled sporangia. After the plants dies and the leaves become dry and broke, the sporangia are released and reach the soil, where they wait for the next spring to restart the cycle.

The brown spot of maize is a considerable problem for maize crops in countries with abundant rainfall. Heavy infections may kill the maize plant or severely reduce its fitness before the ears are ready to be harvested. Although fungicides may help in slowing down the infectio throughout the crops, one of the most efficient ways to reduce the damage is to destroy, usually by fire, the remains of the last harvest.

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References:

Olson, L. W.; Lange, L. (1978) The meiospore of Physoderma maydis. The causal agent of Physoderma disease of maize. Protoplasma 97: 275–290. https://dx.doi.org/10.1007/BF01276699

Plantwise Knowledge Bank. Brown spot of corn (Physoderma maydis). Available at: < http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=40770&gt;. Access on Agust 7, 2017.

Robertson, A. E. (2015) Physoderma brown spot and stalk rot. Integrated Crop Management News: 679. http://lib.dr.iastate.edu/cropnews/679/

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Friday Fellow: Operculate Acrochaete

by Piter Kehoma Boll

Last week I introduced a red alga, the Irish moss. Today I’m bringing another alga, this time a green one, but this is not an ordinary green alga, but a parasite of the Irish moss! So let’s talk about Acrochaete operculata, or the operculate acrochaete as I decided to call it in English, since obviously there would be no common name for an alga parasite of another alga.

Discovered and named in 1988, the operculate acrochaete is an exclusive parasite of Chondrus crispus. The infection occurs when flagellate zoospores of the parasite settle on the outer cell wall of the Irish Moss, where they start their development and digest the cell wall, penetrating the tissues of the host. In sporophytes of the Irish moss, the operculate acrochaete digests the intercellular matrix and spreads through the frond, while in gametophytes the infections remains localized, forming papules. The damages caused by the green alga lead to secondary infections by other organisms, especially bacteria, and the infected fronds end up falling apart, completely degradated.

ccrispus

A frond of the host (Chondrus crispus) to the left and the parasitic Acrochaete operculata that infects its tissues to the right. Photo extracted from chemgeo.uni-jena.de

As mentioned last week, the sporophytes and gametophytes of the Irish Moss have different forms of the polysaccharide carrageenan and this seems to be the reason why the parasite infects both forms differently. The sporophytes have lambda-carrageenan, which seems to increase the virulence of the parasite, while the kappa-carrageenan of the gametophyte seems to limit the green alga’s spread.

Since its discovery, the operculate acrochaete and its interaction with the Irish moss has been studied as a way to both reduce its damage on cultivated crops of the red alga and as a model to understand the relationship of plants and their pathogens.

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References:

Bouarab, K.; Potin, P.; Weinberger, F.; Correa, J.; Kloareg, B. (2001) The Chondrus crispus-Acrochaete operculata host-pathogen association, a novel model in glycobiology and applied phycopathology. Journal of Applied Phycology 13(2): 185-193.

Correa, J. A.; McLachlan, J. L. (1993) Endophytic algae of Chondrus crispus (Rhodophyta). V. Fine structure of the infection by Acrochaete operculata (Chlorophyta). European Journal of Phycology 29(1): 33–47. http://dx.doi.org/10.1080/09670269400650461

Correa, J. A.; Nielsen, R.; Grund, D. W. (1988) Endophytic algae of Chondrus crispus (Rhodophyta). II. Acrochaete heteroclada sp. nov., A. operculata sp. nov., and Phaeophila dendroides (Chlorophyta). Journal of Phycology 24: 528–539. http://dx.doi.org/10.1111/j.1529-8817.1988.tb04258.x

 

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Friday Fellow: Gray Mold

by Piter Kehoma Boll

Today’s Friday Fellow will show you how beauty is only a matter of perspective. Being an ascomycete fungus, it is commonly known as gray mold and is usually found growing on decaying vegetables, especially fruits such as the strawberry in the photo below:

Botrytis_cinerea1

Gray mold growing on a strawberry. Most people would not see it as a beautiful image. Photo by Wikimedia user Rasbak.*

The gray mold has a controversial biological nomenclature, as many other fungi. The most common name is Botrytis cinerea used for its asexual stage (anamorph), which is the most common. Its sexual stage (teleomorph) is known as Botryotina fuckeliana. I guess this issue, which was common in naming fungi with rare or unknown occurrences of sexual stage, has already been settled, but as I’m not a taxonomist of fungus, I cannot speak much on the subject.

More than only having a controversial name, this fungus has also a controversial interaction with humans. It is a notable pest in wine grapes and may lead to two different infections on them. One of those is called “grey rot” and happens under wet conditions, leading to the loss of the grapes. The other is called “noble rot” and is a beneficial form of the infection that happens when the wet condition is followed by a dry one and produce a fine and sweet vine due to the concentration of sugars in the grape.

Out of the vine world, however, the gray mold is not something that you want growing on your crops. As as it attacks more than 200 species, many of them being important food crops, there is a big interest in developing strategies to reduce the damage it causes. And these strategies include the use of pesticides, plant essential oils or even other organisms that may parasitize the gray mold.

But one cannot deny that if you look closer, even the gray mold is beautiful:

Botrytis_cinerea

A beautiful tiny forest of gray mold on a strawberry. Photo by Macroscopic Solutions.**

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ResearchBlogging.orgReferences:

Wikipedia. Botrytis cinerea. Available at <https://en.wikipedia.org/wiki/Botrytis_cinerea&gt;. Access on June 2, 2017.

WILLIAMSON, B., TUDZYNSKI, B., TUDZYNSKI, P., & VAN KAN, J. (2007). Botrytis cinerea: the cause of grey mould disease Molecular Plant Pathology, 8 (5), 561-580 DOI: 10.1111/j.1364-3703.2007.00417.x

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Friday Fellow: Toxo

by Piter Kehoma Boll

If I had to bet on a parasite that you who are reading this probably have in your body, I’d go for today’s fellow, the protist Toxoplasma gondii, sometimes simply called toxo.

Found worldwide, the toxo is one of the most common parasites in humans, with estimations that about half of the world’s population is infected. Fortunately, this creature usually occurs in a latent form and does not offer great risks, but eventually it may develop into a more serious condition called toxoplasmosis, especially in people with weakened immunity.

But let’s take a closer look at this tiny fellow.

Toxoplasma_gondii_oocyst

Oocysts of Toxoplasma gondii. This is the form found in the environment and that can start an infection in your body.

The toxo is a protist belonging to the phylum Apicomplexa, a group of parasitic alveolates that also includes the agent that causes malaria. Although traditionally considered a protozoan, the apicomplexans are closely related to dinoflagellates (which are generally considered as a group of algae). They have a unique organelle called apicoplast, which they use to penetrate a host cell. The apicoplast is derived from a plastid (such as the chloropast), so in a certain way we can say that the apicomplexans are algae that evolved into intracellular parasites!

Toxoplasma_gondii_tachy

Tachyzoites of Toxoplasma gondii stained with Giesma from the peritoneal fluid of a mouse.

The life cycle of the toxo is kind of complex. Let’s start with the inactive form called oocyst, which may be found in the environment. If a warm-blooded animal ingests an oocyst, it will “burst” inside the gut of the animal and release several “quick-moving” forms called tachyzoites. The tachyzoites invade almost any cell of the body and multiply asexually inside it until the cell dies and release them, allowing them to infect more and more cells. When invading the brain, liver and muscles, the tachyzoites usually differentiate into cysts that become inactive. In this stage, the only thing that the toxo wants is that a cat (any species of the family Felidae) eats the host. It may even change the host’s behavior in order to make it bolder and more easily accessible to predators.

Toxoplasma_gondii_cyst

A cyst of Toxoplasma gondii that forms in the muscles, brain and liver of any warm-blooded anymal. All the cyst wants is to be eaten by a cat!

Now let’s assume that a cat ate the host (that was likely a bird or mouse). Inside the cat’s gut, the cyst burst and releases several “slow-moving” forms called bradyzoites. This form invades the epithelial cells of the cat’s intestine and multiply asexually inside them. Eventually, the bradyzoites differentiate into either tachyzoites or gametocytes (sperm- and egg-like cells). When two gametocytes fuse, they form a zygote that matures into an oocyst and is released into the environment, restarting the cycle.

Toxoplasma_life_cycle

The complex life cycle of Toxoplasma gondii. Credits to Mariana Ruiz Villarreal.

As always, the lifecycle of parasites is a wonderful adventure!

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ResearchBlogging.orgReferences:

Tenter, A., Heckeroth, A., & Weiss, L. (2000). Toxoplasma gondii: from animals to humans International Journal for Parasitology, 30 (12-13), 1217-1258 DOI: 10.1016/S0020-7519(00)00124-7

Wikipedia. Toxoplasma gondii. Available at <https://en.wikipedia.org/wiki/Toxoplasma_gondii&gt;. Access on March 6, 2017.

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Friday Fellow: Heartworm

by Piter Kehoma Boll

Life is not composed only by beautiful and cute creatures. Parasites form a big part of life. In fact, it is likely that there are more parasitic species than non-parasitic ones.

The heartworm (Dirofilaria immitis) is one of these not-so-cute species. A species of roundworm, it infects small mammals, especially dogs, and is spread by mosquitoes.

The name heartworm comes from the fact that this worm lives in the heart and pulmonary arteries of dogs during its adult stage. The result of the infection may be heart failure and damage on the heart and the arteries, but some infections may pass completely unnoticed, especially in sedentary dogs.

Not a pleasant view. Heartworms in a dog's heart. Photo by Alan R Walker*.

Not a pleasant view. Heartworms in a dog’s heart. Photo by Alan R Walker*.

After males and females mate in the heart of the dog, females give birth to live larvae called microfilariae. These are released in the bloodstream and await for being transfered to a bloodsucking mosquito during a bite. Over 60 species of mosquitoes are known to serve as intermediate hosts of microfilariae.

Inside the mosquito, the microfilariae grow from the larval stage L1 to the larval stage L3 and then migrate to the mosquito’s salivary glands and, once it bites another dog, they are transferred to it and develop under the skin at the site of the bite to the stage L4. Now the L4 larve migrate to the dog’s muscles and develop into the stage L5. Finally, they start to migrate through the bloodstream until they reach the heart and the pulmonary artery, where they mold into adults and the cycle is complete.

We may find such worms disgusting, but we must admit that they have a complex and amazing life.

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References:

Wikimedia. Dirofilaria immitis. Available at: < https://en.wikipedia.org/wiki/Dirofilaria_immitis >. Access on June 7, 2012.

Ludlam, K. W.; Jachowski, L. A.; Otto, G. G. 1970. Potential vectors of Dirofilaria imiitis. Journal of the American Veterinary Medical Association, 157: 1354-1359.

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