Parasite Diversity

Although it is difficult to date precisely the emergence of parasitism, paleontological data show that parasitism is a very old lifestyle. Several parasites have been identified in Carboniferous levels (300 Ma), and the older traces suggesting a parasitic lifestyle date back to Cambrian (570 Ma). The first parasites surely evolved from free-living organisms, simply because no one around means nobody to parasitize. One thing is for sure - there is today a considerable diversity of parasites which can be seen from different angles (taxonomy, life cycles, transmission modes).

Parasites are widespread among living organisms, as they are thought to represent more than 50% of all species on Earth. From the simplest viral particle to avian brood parasites, parasitic lifestyle has regularly arisen during evolutionary time. Not all phylogenetic groups have been equally prone to the evolution of parasitism since the proportion of parasitic species is very heterogeneous among taxa. Whereas all viruses are parasites, no parasite is known in echinoderms. Nearly all platyhelminthes are parasites, whereas a unique species of cnidarian is parasitic among the 11 000 species described in this phylum, so far. Finally, some groups contain roughly equal proportions of parasitic and free-living species, such as bacteria, fungi, and nematodes. This heterogeneity raises two main questions. The first one concerns the mechanisms responsible for the evolution of this taxonomic diversity, which may result from speciation events (formation of two different parasitic species from a single one) or transitions from free-living stages toward parasitism. Both mechanisms have played a role in parasite history, but they have different implications. Within the context of speciation, parasitic lifestyle is simply inherited from a common ancestor, whereas transition events mean independent acquisitions of parasitism by several species. How many transitions toward parasitism have punctuated the evolution of life? It is impossible to answer precisely to this question because the number of parasitic lineages which became extinct is unknown. However, phylogenetic analyses have shown no less than 63 independent transitions in the metazoan parasite phylogeny, which contains more than 100 000 described species (Table 1). Again, the frequency of these events varies among taxonomic groups. For example, only one transition has occurred in cnidarian, whereas parasitism has evolved several independent times in nematodes.

This leads to the second question: are some taxonomic groups more prone to evolve toward parasitism? The answer is probably yes. A mutation occurring in a free-living organism might enable it to exploit another organism (which needs to be frequently encountered for an intimate interaction to be established). If the mutation provides the individuals with a slight advantage in term of reproductive success, parasitic lifestyle will be favored by natural selection. However, it is unlikely that a single mutation would allow a free-living species to exploit a host without preadaptations for survival, feeding, or reproduction within the host. The evolution toward parasitism is like an inclined path starting with a stair which cannot be got over if too high. Once the progression is engaged, the new parasitic lineage is usually subject to morphoanatomical (such as reduction/loss of sense or

Table 1 Minimum numbers of evolutionary transitions to parasitism (sensu stricto) and numbers of living species in the major groups of metazoan parasites of metazoan hosts

Parasite taxon

Minimum number of transitions Minimum number of living species

Phylum Mesozoa Phylum Plathelminthes® Class Cercomeridea (subclasses Trematoda, Monogenea, and Cestoidea) Phylum Nemertinea® Phylum Acanthocephala Phylum Nematomorpha Phylum Nematoda® Phylum Mollusca® Class Bivalvia® Class Gastropoda® Phylum Annelida® Class Hirudinea® Class Polychaeta® Phylum Pentastomida Phylum Arthropoda® Subphylum Chelicerata® Class Arachnida® Subclass Ixodida Subclass Acari® Subphylum Crustacea® Class Branchiura Class Copepoda® Class Cirripedia® Subclass Ascothoracida Subclass Rhizocephala Class Malacostraca® Order Isopoda® Order Amphipoda® Subphylum Uniramia® Class Insecta® Order Diptera® Order Phthiraptera

(suborders Ischnocera, Amblycera, and Anoplura) Order Siphonaptera

4 17

>40000

>400

>100

>800 >30000

aDenotes taxa containing free-living species.

Reproduced from Poulin R and Morand S (2000) The diversity of parasites. The Quarterly Review of Biology 75: 277-293.

digestive organs), functional, or physiological changes, leading to a stronger dependence of the parasite upon its host. It seems, however, that this process is not irreversible because the phylogeny of Diplomonadida (a group of protozoans) suggests that reversal to free-living stages has occurred twice.

Parasitism is also characterized by a great diversity of life cycles. In the simplest case, the parasite only needs one host to complete its life cycle. Several fungi, such as mildew, are plant parasites. Once a spore reaches a leaf, the fungus penetrates the plant and matures to become infective by starting to produce and release spores in the environment. These spores have to encounter another leaf to begin a new cycle. Complex life cycles involve two or more (up to four) hosts, each host housing a different developmental stage of the parasite (Figure 1). Adults of the trematode Halipegus ovocaudatus live under the tongue of green frogs where they reproduce sexually. Eggs are released in the water and the emerging parasite needs to pass through three intermediate hosts (a mollusk, a

Figure 1 A complex life cycle. The life cycle of the digenean Clinostomum marginatum involves three hosts. Adult worms parasitize the intestine of egrets and other fish-eating birds, where they produce eggs that are dropped into water with the bird's feces. A miracidium hatches out of the egg and swims until it finds a snail and infects it. The miracidium sheds its cilia and develops into a sporocyst, which then produces multiple redia. The redia produce multiple cercaria, which leave the snail and swim until they find a fish to infect, and then develop into metacercaria. Predation of fish by birds facilitates the completion of the parasites life cycle. Reproduced from Thomas F, Renaud F, and Guegan J-F (2005) Parasitism and Ecosystems. New York: Oxford University Press.

Figure 1 A complex life cycle. The life cycle of the digenean Clinostomum marginatum involves three hosts. Adult worms parasitize the intestine of egrets and other fish-eating birds, where they produce eggs that are dropped into water with the bird's feces. A miracidium hatches out of the egg and swims until it finds a snail and infects it. The miracidium sheds its cilia and develops into a sporocyst, which then produces multiple redia. The redia produce multiple cercaria, which leave the snail and swim until they find a fish to infect, and then develop into metacercaria. Predation of fish by birds facilitates the completion of the parasites life cycle. Reproduced from Thomas F, Renaud F, and Guegan J-F (2005) Parasitism and Ecosystems. New York: Oxford University Press.

copepod, and a dragonfly). If a parasitized dragonfly is eaten by a frog, the cycle is eventually completed.

It seems parsimonious to assume that early evolutionary stages involved simple life cycles, with direct transmission between hosts of the same species. Understanding the evolution of complex, multispecies life cycles has been more puzzling, as adding intermediate hosts might increase the risk of failing to encounter the right host. Recent theoretical work has explored the conditions that might have led to the evolution of complex life cycles in helminth parasites with no penetrative infectious stages. Two scenarios, and the benefits associated with each of them, have been put forward. According to the first scenario, transition from a single- to a multihost life cycle can be attributed to upward incorporation of a new host which preys upon the original host (Figure 2). In this scenario, benefits for the parasites are avoidance of mortality when the host is eaten by the predator, and greater body size at maturity and fecundity. In the second scenario, incorporation comes downward by adding a host at a lower trophic level (Figure 2). This new host can initially be a paratenic (facultative) host, which becomes later an obligate host, if this enhances transmission rate to the definitive host. Although the addition of a paratenic host in a life cycle is an accidental event, complex life cycles of parasites are probably adaptive responses to the two main difficulties set by their environment, namely the transmission from one host to another one, and the compatibility between the parasite and the host(s). Life cycles can also evolve toward simpler cycles when one intermediate host is lost. Simpler life cycles can be beneficial for the parasite if this accelerates its development (loss of one larval stage).

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