Origin of Parasitic Protozoa PDF

**[Origin of Parasitic Protozoa]** Entozoic protozoa may have been derived from ectoparasites, as various authors have suggested. It does seem logical to suppose that free-living forms first became associated with hosts as casual commensals loosely attached to the skin or gills, and then gradually...

**[Origin of Parasitic Protozoa]** Entozoic protozoa may have been derived from ectoparasites, as various authors have suggested. It does seem logical to suppose that free-living forms first became associated with hosts as casual commensals loosely attached to the skin or gills, and then gradually fortified their position by moving into the mouth, gill chambers, anus, and other openings. But ectozoic forms are mostly primitive ciliates and flagellates, and only a few genera, such as Trichodina and Hexamita, contain both ectozoic and entozoic species. Another logical guess as to origins of parasitic protozoa is that they were derived from species accidentally ingested by their future hosts. When we consider the large numbers of protozoa that are steadily ingested with the food of larger animals, and when we think of the nutritional benefits and the protection and moisture provided by the intestine, we appreciate the inescapable advantages for survival furnished by lodgings in a gut. Once established in the intestine, the parasite could migrate to all other parts of the body. Since the parasitic habit among protozoa is not limited to exclusively parasitic groups but is scattered among orders containing free-living species, the parasitic habit probably arose frequently and independently from different groups of free-living ancestors. Sporadic and temporary invasion by free-living species into hosts may be comparable with the initial step in the origin of endoparasitism. For example, species of the Euglenida are sporadically found in tadpoles and in millipedes, and *Tetrahymena* is occasionally found in such sites as the digestive tracts of slugs, the coelom of sea urchins, the hemocoel of insects, and the gills of the amphipod, *Gammarus pulex*. When the complete life cycle of a protozoon is known, a critical examination of the various stages will often lead to information suggesting phylogenetic relationships with neighboring groups. Such a study of the opalinids (parasitic in the large intestine of tailless Amphibia), with particular attention to the infraciliature and mode of fission, has led to the removal of this group from the ciliates and the placement of it in a separate subphylum. Baker has reviewed the literature on the evolution of parasitic protozoa, and he has combined some current theories with some of his own to propose the following hypothesis, which we feel is the most logical of those postulated. The **Sarcomastigophora**, known as ameboflagellates, are the least-changed modern descendents of the original protozoa. From this assemblage of ameboflagellates, specialization probably began along three main directions. The first was characterized by a suppression of both amoeboid and flagellar phases without completely losing either, giving rise to the **Sporozoa**. \"The **Ciliophora** presumably arose from the flagellate line of development by an increase in the number of locomotor organelles and, subsequently, the development of a very complex pellicular and subpellicular morphology.\" In 1935 Wenrich made a comprehensive study of the hypothetical origin of parasitism among protozoa, and we shall select samples from his study. A number of investigations have shown that, when free-living protozoa first become entozoic, they do not necessarily undergo any marked morphologic modification. If protozoa intermediate in behavior and habitat between free-living and parasitic can be found, they should present pres a clue to an answer to the question of origin raised above. Such intermediate forms are common. Wenrich described a **holotrichous** ciliate, *Amphileptus branchiarum*, found on the gills of tadpoles. The ciliate has a free-swimming stage that roams over the gills devouring ectozoic **Trichodina** or **Vorticella**. At other times, and more commonly, the \"parasite\" attaches itself to the tadpole gills by a thin membrane within which it gently rotates, pausing now and then to indulge its predacious tendencies to engulf masses of gill cells. The ciliate is, perhaps, in the process of changing over from a free-living predacious organism to a parasitic one. Other common species of Amphileptus are predacious. Wenrich discovered some colorless euglenoids, belonging to the genus Menoidium, in the gut of the millipede, *Spirobolus marginatus*. He attempted to infect the millipedes by feeding both *Menoidium sp*. and *Euglena gracilis* to them, and he found that both (the former more successfully) were able to survive within the host intestine for a few days, but neither was able to become established as a permanent entozoic flagellate. Wenrich concluded that the Menoidium \"displayed the facultative capacity of maintaining for a brief time, at least, an entozoic existence.\" This situation led Wenrich to conclude that the host, representing a special and limited environment, has not had a marked directive influence on the evolution of its parasites; other- wise there should be more evidence of convergence in evolutionary trends among the parasites. It should be recalled, however, that hosts markedly influence physiologic changes in their parasites. Such changes, as we have al- ready indicated, are the first to take place in evolutionary divergence and, indeed, are often the only significant modifications to occur. After all, the protozoa have had a longer time than any other animal phylum in which to evolve. They have invaded almost every possible eco- logic niche, and some genera, such as Hexamita, are to be found in a wide variety of unrelated hosts without exhibiting significant morphologic changes. The family **Trypanosomatidae** has stimulated much speculation on its evolution. The promastigote body form is generally considered to be the most primitive type of the family, and ancestral flagellates presumably were parasites of the gut of invertebrates. Another hypothesis is that Bodo-like flagellates became established in the intestines of vertebrate hosts, then invaded tissues and secondarily became transmitted by insects. A third suggestion¹¹ is that mammalian trypanosomes recently originated from leech-transmit- ted parasites of aquatic reptiles. This ancestral promastigote probably led, on the one hand, to the genera **Leptomonas** and **Phytomonas**, and on the other hand, to the genera ***Trypanosoma***. Methods of transmission offer clues as to the kinds of evolution experienced by the genus Trypanosoma. Thus the contaminative type of infection in *Trypanosoma cruzi* is evidently the more primitive. Hoare has suggested that the origin of inoculative transmission and its attendant form of parasite life cycle, as in *T. gambiense*, may be a secondary acquisition that originally developed in the hind-gut of the in- sect vector. Such trypanosomes may have been taken up by tsetse flies that began to transmit them mechanically to new vertebrate hosts, but when the flagellates adapted themselves to development in the proboscis and/or salivary glands, tsetse flies became their new obligatory transport hosts. Evidence for this hypothesis is presented by *T. vivax*, which develops only in the mouth parts of its insect vector, and by *T. congolense* (representing the next step in evolution), which develops in the mid-gut of the insect, and finally by the **Brucei**-group, which utilizes the mid-gut and then the salivary glands of the tsetse fly. A final bit of evidence for the described phylogenetic relations among trypanosomes is provided by the differences in susceptibility of their vectors to infection. Practically 100 per cent of triatomid bugs fed *T. cruzi* become infected, while less than one per cent of tsetse flies fed *T. brucei* become infected. The obvious conclusion is that *T. cruzi* and its bugs represent a much older and more stable association. Moreover, tsetse-borne trypanosomes easily lose the power to develop in the insect host and they may revert to mechanical trans- mission. Such a transformation is illustrated by *T. vivax* in cattle of South America and by *T. evansi*, which presumably originated in Africa from *T. brucei*. A final step in the evolutionary series is *T. equiperdum*, which has become completely emancipated from an insect vector, and is transmitted directly from horse to horse by contact during the sexual act. The evolution of the large and complex group of flagellates that inhabit the intestines of termites stemmed from the simple Monocercomonas or from a Monocercomonas-like form. This form has an uncomplicated parabasal body and axostyle, three free flagella, and an adherent or free-trailing flagellum. The **Trichomonadidae** have added a **Costa** and an undulating membrane in place of the recurrent flagellum. **Devescovina** is similar to **Monocercomonas**, but it possesses a triangular **cresta** and its parabasal body is coiled around the axostyle. The **Calonymphidae** are derived from the **Devescovinidae**. **Sarcodinians** have arisen from the Sarcomastigophora by a loss of the flagellar stage, and they appear to represent a polyphyletic group. Numerous examples of amoeboid flagellates lend ample support to these conclusions. **Tetramitus** has amoeboid stages but is usually classified as a flagellate. A line of evolution through this type of protozoon has lead to the ameba, **Vahlkampfia**. Another ameba, **Naegleria**, is strikingly similar to Vahlkampfia but unlike the latter it has flagellate stages. **Dientamoeba** has apparently arisen through another line involving the amoeboid flagellate **Histomonas**. Soil amebas (e.g., Naegleria) may enter the noses of mice and monkeys and migrate to the lungs or brain causing severe lesions. These amebas can be taken from the soil, grown in cultivation media, injected into laboratory animals and show immediate pathogenicity. Experiments like these prompt a re-evaluation of our concept that amebas have evolved from free-living forms slowly on their way to becoming parasites. Pre adaptation to parasitism plays a significant role and undoubtedly the length of time it has taken for free-living organisms to travel the road to parasitism has varied widely. **Apicomplexa**, **Myxospora**, and **Microspora** possess some structural characteristics suggestive of the flagellates (e.g., merozoites similar to promastigotes; flagellated microgametes similar to Bodo; sexual processes similar to those of phytomonads). However, an amoeboid method of locomotion is common among these parasites. They probably arose from flagellates or ameboflagellates possessing life cycles similar to those of present-day phytomonads. The sporozoans and **cnidosporans** probably first became adapted to parasitism in the intestines of aquatic invertebrates, and then moved to terrestrial and aquatic vertebrates. The gregarines today have retained the ancestral characteristic of inhabit- ing the intestinal lumen of invertebrates. The primitive Apicomplexa stock appears to have given rise to the gregarines, then to the **coccidia**. The **Haemosporina** (including the malarial parasites and their close relatives) arose from the coccidia, but there is a controversy as to whether the ancestral groups were coccidia of vertebrates or of invertebrates. In favor of the latter is the fact that the Haemosporina are less pathogenic to their invertebrate hosts than to their vertebrate hosts, and they are more host-specific to their invertebrate hosts. In favor of the view that they have evolved from coccidian parasites of vertebrates is the tendency of coccidia to be- come tissue parasites of vertebrates, and the fact that malaria occurs in birds and reptiles that probably originated before the advent of blood-sucking flies. Finally, as Bray pointed out, \"if the **haemosporidia** (=Haemosporina) are coccidian by nature and originally insect parasites\.... It is we who should be carrying the sporozoites or the sporogony stages and the mosquito whose gut should contain the exocoelomic schizogony stages and their haemocoelomic fluid which should suffer schizogony and gametogony stages.\" We are inclined to join those who believe that the Haemosporina have evolved from coccidian parasites of vertebrates, probably beginning with reptiles. Coatney and associates have summarized their own and others\' views on the evolution of the primate malarias. They suggest that \"the **nidus** (A place in which bacteria have multiplied) of the primate plasmodia universe lies somewhere in the jungles of Southeast or South Central Asia and that there, there has been a simultaneous development of non-human pri- mates and their malaria parasites.\" For thou- sands of year's man and non-human primates lived in close proximity and probably ex- changed malaria parasites. As man became more separated ecologically from his simian and anthropoid relatives the sharing of parasites became less pronounced. Many parasitologists believe that primate malarias reached the New World through Europeans and their West African slaves in the 16th century. The most comprehensive works on the phylogeny of the protozoa have been those concerned with the ciliates. A fruitful approach to phylogenetic problems of the ciliates utilizes the subpellicularly located basal granules, or kinetosomes, which are intimately and indispensably associated with all external ciliary systems. This infraciliature, as it is called, is present even in the absence of external ciliature. The approach also focuses attention on the ontogeny or morphogenetic aspects of ciliate development. From an unknown zooflagellate ancestry the **Gymnostomata**, a large subclass embracing a great variety of forms, is situated at the base of the ciliate evolutionary tree.