Fundamentals of Evolution - Korzh O.P. - 2006

Part III. PATHS OF LIFE'S DEVELOPMENT

Chapter 20. Main Stages in the Development of Multicellular Animals

20.3. The Transition to Parasitism

Parasitism, as a complex system of diverse biological relationships, originated from Symbiosis, predation, or a saprozoic lifestyle. In the course of evolution, parasites developed progressive adaptations to the host Organism and, in some cases, to environmental conditions as well, which contributed to their widespread distribution in nature. The ORIGIN AND EVOLUTION of parasitic relationships between parasite and host occurred over a long period of time. Moreover, evolution affected not individual parasites, but rather a group of individuals of a given species within a certain range.

The adaptation of Protozoans to a parasitic lifestyle necessitated adjusting to feeding and locomotion under new conditions. These forms have a more complex Structure when they are intracellular parasites; they do not exhibit the morphophysiological regression frequently observed in multicellular parasites. This can be explained by the fact that protozoans are single-celled organisms, meaning that the reduction of Organelles could disrupt essential PHYSIOLOGICAL AND BIOCHEMICAL Functions. In some forms, by contrast, a process of organelle polymerization is observed (such as in Opalina and others).

Intracellular parasites are constrained by the size of the host Cell, which dictates different pathways of their evolution. Neither an increase in body size, nor polymerization, nor an elaboration of Organization takes place in them. Often, intracellular parasites undergo profound morphophysiological degeneration, with some of their biochemical processes being carried out by the enzymatic systems of the host Cells. At the same time, forms with free-living stages retain a relatively complex organization (coccidia).

The evolution of Blood parasitism is a rather complex process. According to the majority of researchers, it is a secondary phenomenon that emerged from intestinal parasitism. Studies on the biology of individual species indicate that certain blood parasites of vertebrates evolved from the intestinal parasites of those same vertebrates (coccidia). In others, however, such as flagellates, the primary hosts were the intestinal parasites of invertebrates, predominantly insects.

In Multicellular Organisms (Metazoa), the transition to parasitism was accompanied by distinct changes in Morphology, feeding habits, METABOLISM, and reproductive features. At the same time, the modificational shifts in the biology of endo- and ectoparasites followed different trends. For instance, in most blood-sucking Diptera acting as ectoparasites, the overall appearance remained practically unchanged, but their feeding apparatus underwent major transformations. Specifically, their proboscis acquired The ability to pierce the Skin, and their Salivary Glands began producing an anticoagulant secretion that prevents blood from clotting. Endoparasites, during their adaptation to new living conditions, lost certain morphological structures. For example, cestodes lack an intestine and have a poorly developed muscular layer. However, helminths develop progressive morphophysiological adaptations that ensure their optimal existence as parasites. Such changes include The Development of the tegument (a protective covering) and the appearance of various Organs of attachment. In addition, parasites are characterized by high fecundity driven by the Progressive development of the Reproductive System.

Certain pathogens of parasitic diseases exhibit rapid growth rates: some cestodes can grow by 10 cm or more per day. This characteristic, much like their high fecundity, became possible due to more efficient Nutrition compared to free-living organisms. The parasite absorbs a greater quantity of nutrients from the host organism, which drives the growth of its Tissues. In intestinal parasites, the cuticle prevents them from being digested by the host's Enzymes—as soon as the parasite dies, it undergoes lysis.

THE ORIGIN OF ectoparasites is most likely traced back to free-living organisms, primarily predators. This transition can be traced with particular clarity in Arthropods, which progressed from temporary parasitism (such as bugs and mosquitoes) through a prolonged association with the host (ticks and fleas) to permanent ectoparasitism (lice).

A somewhat different pattern of transition to ectoparasitism occurred in feather-feeding mites (Sarcoptiformes) and chewing lice (Mallophaga). Most likely, they originated from inhabitants of nest litter, where they fed on PLANT AND ANIMAL debris. Subsequently, these organisms began moving onto the nest's occupants and feeding on the keratinized outgrowths of their integument, and in some cases, on blood.

Ectoparasitism could also have arisen from sessile forms. This is how the origin of parasitism in barnacles (Cirripedia) can be explained, some of which sink quite deeply into the host's skin (Coronula).

The establishment of endoparasitism in certain cases may be linked to a prior ectoparasitic lifestyle. An example of this phenomenon is the pelican louse *Tetrophthalmus*, which migrated from the plumage to the sublingual pouch, where it began feeding on blood. Even more interesting is the transition to ectoparasitism in the frog fluke (*Polistomum integerrimum*), which in its larval stage parasitizes the gills of tadpoles as an ectoparasite. Following the metamorphosis of the tadpole into a frog, as the gill slits atrophy, the parasite moves to the Urinary Bladder via the cloaca, crawling along the ventral surface of the host's body. Here, it transforms into an endoparasite and reaches sexual maturity.

Among other factors driving the evolution of endoparasitism is a shift in egg-laying instincts: instead of decaying organic matter, eggs might have accidentally begun to be deposited on the wounds and ulcers of still-living animals. Since these conditions proved more favorable, this accidental behavior became fixed through evolution. Over time, egg-laying within Body Cavities emerged (such as in the Wohlfahrtia fly and warble flies).

The vast majority of endoparasitism cases, particularly intestinal ones, are primary phenomena that developed As a result of free-living organisms' eggs, or other passive developmental stages, being introduced into the Digestive System. A striking example of such a transition is found in nematodes, a significant number of which are considered free-living forms, while simultaneously possessing a whole spectrum of transitional forms leading to obligate endoparasites.

The development of resistance to adverse environmental conditions is also a progressive adaptation in certain parasite species. For instance, the larvae of some helminths can withstand desiccation for prolonged periods; argasid ticks can survive without food for many years (up to 11). Another considered progressive adaptation is the migration of larvae to their permanent habitats within the host organism. As a result, they can localize in almost all tissues and organs.

All parasitic organisms are members of specific biogeocenoses, yet their relationships with the environment vary.

An ectoparasite is a member of an open biocenosis; it leads a free-living existence and uses the host temporarily for feeding (primarily on blood). An endoparasite is connected to the external environment mostly in a dormant state (such as cysts in geo-Protozoa or eggs in geo-helminths) or is not connected to it at all (bio-protozoa, bio-helminths). Nevertheless, most internal parasites possess certain

stages that come into contact with the external environment. Therefore, they can be divided into two groups:

1) those that exist in the external environment in an active state (predominantly larvae or sexually mature individuals);

2) those that exist externally in a passive state.

Migrating parasites develop specialized adaptations to penetrate the skin, intestinal walls, or Blood Vessels. For example, the oncosphere of cestodes contains an embryo equipped with hooks used to damage the intestinal epithelium, whereas the larvae of certain nematode species possess a pointed anterior end or Proteolytic Enzymes that facilitate tissue penetration. After crossing the intestinal or cutaneous barrier, the parasite migrates via the circulatory or Lymphatic system to its permanent habitat.

Endoparasites disperse primarily through the external environment; consequently, they must, firstly, counteract the harmful impacts of various environmental factors; secondly, advance their development to a stage capable of surviving within the host organism; and thirdly, locate and penetrate a host. The first two challenges are resolved mainly through adaptations that enhance the resistance of the free-living Stages of the parasite (eggs, cysts, larvae) against environmental factors. The solution to the latter involved the evolution of other specialized adaptations: increased fecundity, a more complex developmental cycle, and the appearance of ALTERNATION OF GENERATIONS and change of hosts. Naturally, these adaptations are interrelated.

The living conditions of parasites within their hosts are peculiar and diverse, differing in localization, The excretion of metabolic products by the parasites, and the biochemical Influence of the host on the parasite and vice versa. Furthermore, the host organism does not remain a qualitatively constant environment for the parasite; rather, it changes with age, diet, and The impact of various ecological factors.