Did the Malaria Parasite First Evolve in the Insect Vector or the Vertebrate Host?
By John P. Roche
Malaria is a terrible problem, killing nearly half a million people per year, mostly in sub-Saharan Africa. This serious disease is caused by a protozoan parasite of the genus Plasmodium, which is spread to humans by mosquitoes of the genus Anopheles.
In the Anopheles mosquito, the life cycle of the parasite begins as bodies called oocysts in the wall of the insect’s gut. In the next stage, cells called sporozoites emerge and go to the mosquito’s salivary ducts, where they are spread to their human hosts and cause illness when the mosquito bites to take a blood meal.
Malaria parasites have been around since the mid-Cretaceous period. Fossils from that period show parasites of the taxonomic genus Paleohaemoproteus within the bodies of biting midges of the genus Protoculicoides. The midges possessed physical features indicating that they bit reptiles, and thus reptiles are thought to be the first vertebrates infected with malarial parasites. The fossil record to date indicates that these extinct biting midges were the first insects to serve as vectors of malaria, and that reptiles were the first hosts.
Malaria was present in both insect vector and vertebrate host in the mid-Cretaceous (66 million to 145 million years ago). But where did the parasite first evolve? In the insect, or in the vertebrate? A new study by George Poinar, Jr., published in American Entomologist, provides new insights into this question.
For a long time, many scientists believed that malaria evolved in its vertebrate hosts. However, some scientists, beginning with Clay Huff in 1945, concluded that the parasite evolved in insects. In his current study, George Poinar examined the fossil record, in combination with data on the life cycle of malaria, to address this question.
Poinar lists several factors that point to the likelihood that insects were the organisms in which the malaria parasite first evolved. For example, sexual reproduction, which is the central step in the life cycle of the malaria parasite, is found only in insects. Asexual stages, on the other hand, are found only in vertebrate hosts. Also, there are two potential ancestors of malaria, parasites of the taxonomic group Coccidia and parasites of the group Gregarinida. If malaria evolved in vertebrates, scientists believe that coccidian parasites are the most likely origin. However, coccidian parasites are rare in insects, and they do not infect the insects that are known historically to spread malaria, like mosquitoes, biting midges, and their relatives. Gregarines, on the other hand, do not infect vertebrates, but they do infect many insects, including the insects that are known to spread malaria. In addition, gregarines and modern malaria parasites share a range of characteristics in common. Based on all of this evidence, Poinar concludes that malarial parasites most likely have evolved from gregarine parasites, and they most likely evolved first in insects.
Another key question of interest is where did malaria evolve? Previous work by Poinar found that malaria was present in an ancient mosquito known as Culex malariager, which was embedded in amber from the Dominican Republic from the mid-Tertiary period (2.6 million to 66 million years ago). This parasite, Plasmodium dominicana, was the first fossil record of Plasmodium. This finding means that Plasmodium was in the New World about 15 million years ago. Poinar explains that Plasmodium dominicana could have spread from mosquitoes to birds, then to mammals, and eventually to people when humans reached South America. Plasmodium dominicana could possibly have evolved into another species of malaria, Plasmodium brasilianum, which infects monkeys in South America. Plasmodium brasilianum may be the species that evolved into Plasmodium malariae, one of the malaria species that infects humans.
As a next step in his research on the evolution of malaria, Dr. Poinar said that he is interested in locating samples of mosquitoes and biting midges in amber that are infected by protozoan lineages showing developmental stages found in both gregarine parasites and Plasmodium.
The results of this study shed light on the probable animal group in which the malaria parasite evolved. Discoveries such as this on the evolution of malaria could help inform attempts at blocking the transmission of the parasite from the mosquito vector to humans.
When asked about possible blocking strategies, Dr. Poinar said, “Specific sites on the malarial genome might be altered. For instance, the site that controls movement of the sporozoite cells to the salivary glands of the mosquito. Or, alternately, loci on the mosquito genome that controls the thickness of the salivary gland walls could be altered, thus reducing penetration by sporozoite cells.”
Both of these strategies could potentially block transmission to humans.
Future research in this field will further refine our understanding of how malaria evolved, and could also shape our efforts at reducing transmission, and lessening the immense toll of this disease.
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John P. Roche is a science writer and author with a PhD and a postdoctoral fellowship in the biological sciences. He has served as editor-in-chief of periodicals at Indiana University and Boston College, as a senior scientist at Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 150 articles, and has written and taught extensively about science. Dr. Roche also directs Science View Productions™, which provides writing and editing services for clients in academia and business. For more information about Dr. Roche’s writing, visit http://authorjohnproche.com.