Biologists should never be bored! They have the opportunity to study many unusual situations that we would never have anticipated. One of these involves lifestyles of certain parasitic or disease-causing organisms. In several cases a parasite passes through two entirely different kinds of host/victim in order to continue its nasty parasitic existence. Often alternation of hosts is the only choice the parasite has to continue its existence. Not only are these situations fascinating, but biologists are mystified how these relationships (involving very different hosts) could have evolved.

Most fungi do not show such fancy lifestyles, but the rusts are infamous for their dependence on alternate hosts. Wheat, for example, is a monocot (grass) crop particularly important to Canada and the mid-western United States. Over the winter, wheat stubble may harbour black two-celled diploid spores of Puccinia graminis (wheat rust). In the spring these teleutospores send out a thin thread (mycelium) in which the diploid nucleus undergoes meiosis producing 4 haploid nuclei and four basidiospores are produced. Two of these haploid spores carry the plus mating strain, and two carry the minus strain. These tiny basidiospores are carried by the wind and infect barberry leaves [a broad leaved or dicot shrub]. The basiodiospores are not able to infect wheat.

The fungus on the barberry leaf produces a cluster of fungus threads which extend receptive threads into the air and tiny sex cells called spermatia. These are carried by insects to other receptive threads on other barberry leaves. Once a spermatium lands on a receptive thread, a dikaryon (cell with 2 nuclei is produced. This situation is a trademark of this kind of fungus.) The dikaryon growth develops a thick mat which emerges from the bottom of the barberry leaf. This growth produces dikaryon aeciospores. These are carried by the wind to infect wheat stems.

On the wheat stem the fungus produces rust coloured spores called uredospores (also dikaryon). These spores can infect other wheat stems. Late in the season black teleutospores (diploid) are produced after the two haploid nuclei in the dikaryon cell join together to form a diploid. These overwinter on the wheat stems. In the spring meiosis takes place and the haploid basidiospores infect barberry.

If barberry were eliminated in Canada, this would eliminate the fungus here entirely. Unfortunately, the uredospores are able to survive the winter in the southern US after which they blow north to Canada in the spring.

[Synopsis of wheat rust: haploid basidiospores are blown to barberry in spring. On this host, haploid spermatia reach receptive hyphae to form dikaryon or 2 haploid nuclei of opposite mating strains in the cell. The dikaryon produces aeciospores which infect wheat. Rust coloured dikaryon uredospores grow on wheat and can infect more wheat. In fall black teleutospores {diploid] overwinter on wheat. In spring following meiosis, haploid basidiospores infect barberry.]

There are other examples of this situation. For example, the alternate hosts of white pine blister rust are a conifer (white pine) and dicot shrub Ribes (currants and gooseberries). Another well-known example is the Gymnosporangium rust spp. which infect junipers or cedars as a conifer host alternating with Saskatooon (serviceberry) trees or other members of the rose family (dicot flowering plants).

Among animals, there are some examples of alternation of hosts found especially among flatworm parasites. These cause a lot of anguish and economic damage.

We start the life cycle of the flatworm Schistosomiasis with diploid eggs released in human sewage. If the sewage contaminates fresh water, the eggs hatch into slipper shaped many celled bodies called miracidia. These go looking for a particular kind of aquatic snail. Inside the snail, the parasite develops sporocysts which release lots more sporocysts. Eventually fork-tailed cercaria are produced. These leave the snail and look for humans. Inside human veins, larvae of opposite sex meet. They move together to blood vessels in the gut. The adults mate and release eggs into the intestine. The eggs make their way to fresh water. In this case when the snail is eliminated, then the disease is also gone. This is also a case of obligate alternation of hosts.

In schistosomiasis the parasite inside humans is diploid, but in malaria (Plasmodium)  the protozoan parasitic stage in people is haploid. The mosquito ingests haploid gametocytes from human blood. Inside the mosquito sexual reproduction takes place resulting in diploid ookinetes which produce diploid oocysts which via meiosis produce haploid sporozoites. This haploid stage of the parasite is injected by the mosquito into human blood. In the human blood the parasite goes through two stages the second of which invades the red blood cells. Eventually from destroyed red blood cells, gametocytes are released back into the blood stream, and ingested by mosquitos.  This is another case of obligate alternation of hosts. If the mosquito is eliminated, there is no further problem with malaria.

It is evident that there is a huge taxonomic gulf between the parasites and their hosts and a taxonomic gulf between the alternate hosts. It would not be too surprising however to see a worm or fungus or protozoan exploit a much more complex kind of host. But how could it become obligately dependent on two very different hosts? With critical but different stages of a parasite’s lifecycle confined to alternate hosts, evolutionary theorists have found it difficult to explain the origin of these relationships. This is yet another case where evolution theory could not predict what we see in nature.