By Acad. Vladimir SERGIEV (Russian Academy of Medical Sciences, RAMS), Director of the Ye. Martsinovsky Institute of Medical Parasitology and Tropical Medicine (I. Sechenov Moscow Medical Academy)
Pathogenic viruses and bacteria are the causative agents of infectious diseases responsible for many epidemics that have devastated whole continents time and again. But there is yet another aspect of the relationship between the microbial community and Homo sapiens traced to the genome level.
Natural selection is but one of the two "great engineers" of evolution; the other one is variability, or mutation. The diversity of the animal and plant kingdoms owes everything to the laborious work of these two "engineers" over millions and millions of years. The concepts of "the struggle for existence" and "survival of those best adapted to the environment" are the corner-stones of the Darwin-Wallace theory of natural selection. This theory, however, has exposed just one side of the evolutionary process by explaining how species that have acquired useful characters supplant those less adapted and fit. A theory advanced by the Dutch botanist Hugo de Vries (elected to the USSR Academy of Sciences as foreign member in 1932) has further eluci-
Articles in this rubric reflect the authors' opinion. -Ed.
Peter Bruegel l'ancien. FALL OF THE ANGELS, 1562.
dated the matter of acquired and hereditary characters by the occurrence of random variations of heredity, or mutations.
The fusion of the Darwinian and geneticist teachings gave birth to the theory of microevolution in the 1940s. In keeping with this theory gene mutations are unpredictable and inexplicable. But following in their wake is natural selection that turns random changes (mutations) into definitive, inherited characters that inaugurate changes in populational characteristics.
Ilya Prigogine, a Belgian physicist and physicochemist of Russian descent (Nobel Prize in chemistry, 1977) has provided a fresh impulse to theoretical thought by postulating what he described as bifurcational evolution. Today we know that along with mutations there are also other pathways of variability (e.g. hybridization, polyploidy). Besides, genetic material can be brought in from outside at certain stages of the life of a particular species.
Thus, evolution is not just a unilinear chain of sequential random mutations fixed by natural selection. To some extent it is predictable and obeys certain laws, as present-day evolutionists see it. The integration of foreign genetic fragments allows "to economize on time" though the use of ready-made elements and in this way cuts the time of the assembly of DNA and other super-complex biological structures. Yet another component part of this process: isolation of a populational segment under conditions when a newly acquired character could attain to selective significance.
Natural selection evolves as a stabilizing factor in a stable community. The weakening of competition (struggle for existence) during a crisis caused, say, by an epidemic* may trigger a dramatic increase in the scope of variability. Something like this occurs during artificial selection or domestication when species divorced from a natural ecological system and its stabilizing impact produce forms far beyond the natural polymorphism proper to a wild type.
Crises are conducive both to the birth of macromutants (liberation of latent variability) and to their conservation (diminished competition). Here of decisive significance is the fast reproduction rate making up for losses in the population numbers. Thereby protective barriers are eliminated, which keep the gene pools (genofonds) of
* See: A. Ginzburg, B. Boyev, "Computer-Aided Simulation of Epidemics", Science in Russia, No. 5, 2005. -Ed.
Giardia (UAMBLIA INTESTINAUS), protozoal parasites 8 to 30 pm in size causing inflammatory processes in the intestine and bile ducts of man and other mammals.
evolutionary lines from the intrusion of foreign hereditary material. Two channels are open in this case, and these are hybridization and the transfer of genes by viruses or bacterial plasmids.* Hybridization often accompanies changes in the customary conditions of habitation. A similar situation obtains under the action of medical preparations on a pathogenic agent's population within the human organism infected with one or several diseases simultaneously.
The mobile genetic elements identified in the last few decades are playing a substantial role in the exchange of genes both in closely and in distantly related bacteria alike. The entry of such structures into different sites of the genome touches off mutations and modifications of different types. The interspecies gene transfer brings about a fast evolution and stimulates the acquisition of new characteristics, pathogenic among them too, by other microorganisms. Although most of the islets of pathogenicity are sited in chromosomes, thy may also occur in bacterial plasmids and phages**. These elements may implicate groups of genes and evolve into toxicogenic agents resistant to antibiotics. This is how the polyresistant strains of the TB pathogene (i.e. resistant to many drugs) have appeared.
Mobile genetic elements are widespread both in prokaryotic microorganisms and in eukaryotes.*** This factor enforced the rethinking of many tenets of classical biology and problems related to the variability of living organisms.
PARASITES AS A FACTOR OF EVOLUTION
Interspecific competition (struggle), useful to one particular population but detrimental, harmful to another, is called parasitism. Its agents infiltrating the host organisms of other species in search of food or an adequate habitation medium are all, without exception, pathogens causing diseases in man. plants and animals. Parasites may persist in the host organism on a permanent or temporary basis; these are prions*, viruses, bacteria, parasitic fungi (mycose, trehalose), protests, helminths (parasitic worms) and many arthropod species.
Parasitism appeared at the earliest stages of life as a result of behavioral contacts among organisms of different forms, and it subsequently evolved with the adaptation of certain organisms to the existence within or on other organisms. In this fashion parasites were able to overcome the characteristic proper to any species and even cell-keeping off foreign organisms and prohibiting them from entering the host organism. Thus mutual adaptation occurred whereby the one-of-a-kind pair, "parasite/host", could coexist, with both acting upon each other.
Meanwhile, with the progress of man-caused modification and adaptation of the environment to human needs, the reverse trend set in: animals, arthropods, plants and parasites came to be adjusting to changed conditions. Zoonotic pathogens, the causative agents of animal diseases, came into being and in some cases infected man as well. Ever "new" zoonotic infections occur, causing deadly fevers in man, with the Hendra, Nipah or avian (bird) flu viruses as agents.
The competition between host organism and parasite communities is the main cause of their continued vari-
* Plasmids-genetic (DNA) elements in bacterial cells that are additional to and independent of the cell chromosome and dispensable to the cell, capable of autonomous replication. Plasmids are widely used in gene engineering. -Ed.
** Phages (bacteriophages)-simplest known viruses, with bacteria as hosts, capable of reproducing within a bacterial cell and destroying it. -Ed.
*** Eukaryote (Eukaryotic cell; Eukaryocyte)-an organism which, in contrast to prokaryotes (bacteria in particular), has cells with a discrete membrane to the nucleus, enclosing the genetic material. -Ed.
* Prions-submicroscopic infectious particles that, unlike viruses, built of protein and nucleic acid, are yet even smaller proteic structures devoid of hereditary material, the RNA. -Ed.
Judging by his very look, this native of ancient Egypt depicted on the ceramic plate (ca. 1400 B.C.) was afflicted with polio.
ability and that, in turn, shows the advantages of sexual development over asexual one. Now sexual development accounts for the faster adaptation and propagation of several characters within a population. The parasite/host interaction does not always cause the host's rapid death. For instance, the protozoan Nosema parasitizing in the meal worm larva (Tenebrio molitor) secretes an analog of the juvenile hormone that prevents the development of the maggot into an adult insect. Instead, it keeps growing apace to become a huge biotope for a great number of protozoa.
In the course of long coevolution and adaptation parasitism may develop into a parasite coenosis characterized by peaceful intercourse, or "peaceful coexistence". A classical example of such symbiosis is demonstrated by man and the colibacillus Escherichia coli (E. coli)-this obligate parasite is no longer able to live outside the biocenosis of the human intestine. In turn, man gets as a result of its vital activity the bulk of vitamins much above, quantitatively and qualitatively, their amount contained in food. In a similar may, bifido- and lactic-acid bacteria suppress the growth of pathogenic microflora.
The mechanisms of host/parasite interaction do not necessarily come into play as a pathological process. Some parasites have been detected only in animals, while others-in man. In the process of symbiosis ancient prokaryotes formed different varieties of eukaryotic cells when bacterial genomes could become linked up by means of nucleotide bonds, the introns. Cell mitochondria*, including those of man, are presumably symbiotic bacteria. In a similar mode, a multicellular organism still retains traces of protozoic associations.
Here in this country Drs Nikolai Vorontsov (N. Koltsov Institute of the Biology of Development, Russian Academy of Sciences) and Konstantin Umansky (M. Chumakov Institute of Poliomyelitis and Viral Encephalitis, Russian Academy of Medical Sciences) proposed a new approach a quarter century ago concerning the role of viruses in the biosphere as carriers of genetic information capable of being transmitted both sexually and asexually. In the latter case such exchanges are also possible among different species, genera, families and classes of organisms.
By now a large number of works have been published confirming the role played by viruses in the evolution of higher organisms owing to the wide propagation and integration of the viral genome into the host genome, a factor conducive to the capture and introduction of part of the hereditary information of one organism into the corresponding apparatus of another one. That is viruses are implicated in the mutation and recombination of the cell genetic apparatus.
Viral epizootics-and probably epidemics, too-rolling in waves through a host species area, may cause mass chromosomal mutations. Apparently this technique of hereditary information transmission has been instrumental in species transformations. Many viruses can insert themselves into the human genome and persist there for years without causing any visible aftereffects for the host organism. However, they may become activated with certain changes in habitation conditions, as seen in the example of oncoviruses from the group of retroviruses responsible for sarcomas and leukoses (leukemias).
Today we know of more than 500 retroviruses integrated within the human genome. Although we cannot tell about the exact sequels of such integration, it is not excluded that according to Dr. Joshua Lederberg, US scientist (Nobel Prize, 1958), these microorganisms have contributed to the divergence of Homo sapiens as an independent species from the line of the other Hominidae. Implicated in the formation of the human genome were bacterial elements, too. Current data from the available gene libraries show that by their nature about 65,000
* Mitochondrion - rod-shaped organelle (body) visible by light-microscopy in the cytoplasm of animal and plant cells. Mitochondria orchestrate redox reactions supplying cells with energy. Such reactions do not occur in prokaryotes-their function is performed by the cell membrane. -Ed.
That's what antiplague dresses looked like in the 17th century. As one of the descriptions of the day says, "the clothing of medics and other persons attending plague cases, is composed of a morocco cloak protecting against flea bites, a glass mask over the eyes and a long nose cap filled with scents."
Religious charity brotherhoods were obliged to pick up and bury the bodies of the dead during the second plague pandemic in Europe. The outbreak of this disease in the 14th century carried off nearly one-quarter of Europe's population.
sequences of expressed human genes have been acquired from microorganisms.
Infectious and parasit diseases have also been found to act as a selection factor which, in the long run, is responsible for explicit polymorphism in man. In this connection Dr. Konstantin Umansky proposed in 1981 that the increase in the acute respiratory infections observed throughout the 20th century was an adaptive response to environmental modification, first and foremost, to air pollution in conurbations. In this case corresponding viruses perform as adaptogenes implicated in genetic code recombinations.
Certain bacteria, pathogenic protozoa and helminths as well, can also modulate the incidence of human diseases and thereby increase or decrease their selection pressure. Thus, changes in the intestinal microflora have a direct effect on the outcome of virus diarrhea in children. Say, bifidobacteria activate, while E.coli inhibits, the action of viral immunoglobulin antibodies. In most children changes in the intestinal microflora of the dysbacteriosis type are induced by intestinal worms.
The available evidence shows that pathogenic microorganisms are a permanent factor of natural selection
An outbreak of plague in Avignon, France, anno 1348. As testified by the physician of Pope Clement VI, people fled in panic, leaving the sick even among their next of kin.
impacting the evolution of their hosts, Homo sapiens among them.
PATHOGENS AND DEVELOPMENT OF MAN
The most palpable result of interaction between parasite/host populations is manifest in selection as a consequence of the mass death of infected hosts. The killer epidemics of the past decimated the inhabitants of vast territories to the extent of depopulation. Today infectious diseases continue to be in the lead as to the socioeconomic damage inflicted on humankind. Seven diseases (TB, AIDS/HIV, malaria, diarrheas, respiratory infections, virus hepatitis and measles) account for half of the deaths of small children and adolescents worldwide.
In the United States, for instance, the downtrend in the mortality rate had held on until the 1980s. However, it went up 58 percent in the following thirteen years due to the incidence of a "new" disease, AIDS/HIV, and "re-emerging" TB. The same uptrend has been registered in Russia, too: the current growth in the death-rate began in 1991. The incidence of TB doubled in the seven subsequent years, and today it is killing twice as many people as all the other infections combined.
Dr. John Holdane, a British biologist (foreign honorary member of the USSR Academy of Sciences from 1942 to 1948) was the first to point at the cause-and-effect of such pathologies. He postulated in 1949 that infection as one of the basic selection pressures for evolution leads to genetic polymorphism in Homo sapiens. Hereby the organism inherits characters whose presence increases the survival rate or alleviates a disease. The latest discoveries in molecular biology demonstrate that the highest diversity is detected in genes responsible for protecting the organism against pathogenic agents. Thus, the histocompatibility system (an association of immune response genes) comprises about 2,000 allele variants.* In fact, not only interracial but also interethnic distinctions are conspicuous in the occurrence rate of particular specificities.
TB (tuberculosis) is thought to have been the main factor of genetic selection among Caucasoids (white men). The examination of remains found in burial tombs dating from the fifth to the third millennia B.C. reveals traces of bone TB-caused lesions. During the recent five centuries TB has triggered pandemics in Western Europe and North America. In the 17th and 18th centuries this disease killed 20 percent of white adults. The lethality rate continued high later on, too: over one billion people died of TB from 1850 to 1950. The TB incidence rate rose again from 1985 on after the TB causative agents had developed resistance to antibiotics. Today every year as many as 1.9 mn die of TB every year, largely due to the widespread circulation of strains with multiple drug resistance.
Another infectious disease, malaria, has played a major part in the development of man. According to some experts, blood groups antigens were formed in response to the malaria causative agents. The Negroid race was worst hit: living in the tropical belt, this race became the target of malaria-effected genetic selection. Yet specific anomalies have been identified among all races populating territories of the most violent outbreaks of the infection. Such characteristics are clearly distinct in the composition of blood. We know that oxygen takes hemoglobin to tissues from the respiratory organs, while carbon dioxide carries it from tissues to the respiratory organs (hemoglobin is part of erythrocytes, the red blood cells). But under the action of selection pressure, with malaria as agent, hereditary defects, the hemoglobinopathies, came into being-by way of protection against the disease.
* Allele - the changed form of a gene or locus occurring by mutation, in which deletions, substitutions or insertions have altered the original specific sequence of nucleotides. -Ed.
People of West and Central Africa have developed a specific mutation, the absence of the Duffy factor in erythrocytes. Its carriers-and this is more than 98 percent of the population-are immuneable to one form of the disease, tertian fever (benign tertian malaria) since the protozoan Plasmodium vivax is incapable of getting into red blood cells devoid of the above-mentioned factor. Thus far this is the only genetic anomaly caused by malaria pathogens, and its negative aftereffects have not been explained yet.
Thalassemias (Cooley's anemias) are yet another instance of mutation. This is a class of anemias caused by anomalies in genes coding for hemoglobin output. The higher incidence of this pathology in Europe has been found on the Mediterranean coast. It also occurs in Africa, in the Near and Middle East, on the Arabian Peninsula, in India, in Central and Southeast Asia, South China as well as on islands in the western Pacific all the way from the Philippines in the north to the Timor Sea in the south, and to New Guinea and Melanesia in the east. This hemoglobinopathy in homozygotes and heterozygotes alike halves the risk of malaria.*
The lack of the enzyme glucoso-6-phosphate-dehydrogenase (G6PD) was first identified as hemolysis (blood pathology characterized by the destruction of red blood cells with loss of hemoglobin) that sets in after the eating of legumes (French beans, peas and the like). This anomaly ensures 50 percent protection against the grave clinical form of tropical malaria. As shown by studies carried out at our research center, the percentage of cases afflicted with this pathology in the CIS countries (former Soviet republics) ranges from 3 percent in Moldova to 30 percent in Azerbaijan. The zone of G6PD deficiency concurs with the territory ravaged by tropical malaria in the past. The same peculiarities are shown in the other parts of the world.
Sickle-cell anemia (hemoglobin S disease, or S-hemoglobinosis) is still another pathology caused by gene mutation. Today its victims survive up to the middle age (formerly, before present-day healthcare, they usually died in childhood). Hemoglobin S disease occurs throughout the African continent with the exception of southern Africa, along the southern and eastern coasts of the Mediterranean, on the Arabian and Indian Peninsulas up to the eastern border of what is now Bangladesh. Children with hemoglobin S seldom die of tropical malaria, while those having the normal hemoglobin A perish ten times as often.
The connection between the occurrence rate of the hemoglobin S gene and the risk of the malaria infection is indirectly confirmed by comparative studies of Africa's aboriginals and Afro-Americans (Negroids) in the United States whose ancestors were brought to North
In 1945 British troops in Italy came up against mass episodes of malignant malaria in Cassino province. The road sign warns about the danger of contagion.
America in the slave trade times. African Blacks show a much higher presence of the hemoglobin S gene. However, the occurrence of this gene among the Afro-Americans of North America has been down, since there is no hazard of infection and selection pressure.
The above-cited examples of polymorphism are observed in populations hit hardest by malaria for centuries at least. As a consequence, even today 300 thousand to 1.5 million babies are born with bad forms of such congenital genetic anomalies. That is why the natives of northern Europe (except the Low Countries),
* Homozygote -'d cell or organism in which homologous (similar) chromosomes carry one and the same gene form. By contrast, a heterozygote has in its chromosome set (genotype) different forms (alleles) of this or that gene. -Ed.
of northern Asia, Australia, the Americas and islands of eastern Oceania, who have not been subjected to the pressure of this infection, are free from such anomalies.
Diseases to which African descendants show predisposition are also thought to be connected with the malaria-effected selection. For instance, Afro-Americans succumb oftener to hypertension and show a higher content of iron in their organism.
In recent years the implication of polymorphism has been established in the clinical picture and characteristics of many infections. Likewise detected have been hereditary changes of the human genome which are responsible for resistance to AIDS/HIV or affecting the clinical symptoms of these diseases. Thus, deletion (chromosomal mutation) controlling synthesis of the chemokine* receptor CCR5 (deletion designated as A32) is also assigned to mutations which in the homozygotic state can determine resistance to HIV infection. Among Caucasoids the mutant allele occurs at a rate of 12 to 18 percent in the heterozygotic state, and about 1 percent-in the homozygotic. This figure is significantly lower among other races (say, not above 2 percent for Afro-Americans). In our country the CCR5 A32 mutation is fairly common (among the ethnic Russians the heterozygotic genotype occurs in 17 - 24.4 percent of cases, and the homozygotic one-in 1 - 2 percent). This genotype provides 99.9 percent protection against the HIV infection in heterosexual contacts, transmission of the virus from mother to fetus or in blood transfusion.
The above facts confirm the hypotheses on the importance of the selection-mediated effects of microorganisms on the evolution of the human genome. However, the impact of pathogens is not only restricted to the modification of the biological characteristics of Homo sapiens. Acute infectious and parasitogenic diseases spilling over into epidemics affect the social and economic conditions and destinies of peoples and civilizations. Here are just a few examples. Tropical (falciparum) malaria that hit the northern Mediterranean in the first century A.D. devastated the southern tip of the Apennine Peninsula, a region that had been developing with much success for nearly 1,000 years. Because of the infection hazard these districts of Italy had persisted desolate up to the onset of the 20th century. Traveling to the European continent in the beginning of the first millennium A.D., tropical malaria led to the rapid decay of the Hellenic civilization.
The spread of the Justinian plague in the 6th century A.D. finished off what remained of the Roman Empire. Smallpox, carried by the Spanish conquistadors to America, undermined the Incan empire-in 1520 alone this disease killed more than 3 million American Indians. The import of syphilis by Columbus's sailors from the New World late in the 15th century and its propagation contributed to the twilight of the European Renaissance. The dysentery epidemic in the armies of Austria and Prussia advancing on the revolutionary Paris of 1794 compelled the troops incapacitated by diarrhea to fall back, which in no small measure helped save the French Revolution.
Just as dramatic are instances of social panic, as seen in the example of real or imaginary epidemics of the last fifty years. The outbreak of cholera caused by El Tor vibrio in Indonesia (1961) and in other regions of the globe resulted in an actual economic blockade of the countries involved. The first epidemic of the Ebola fever in Africa (Sudan, 1976) stalled air traffic there. Such kind of inadequate "anti-epidemic" measures were also taken in respect of a "pneumonic plague" in India (1994), though no cases of that disease were actually reported. The same is true of the atypical pneumonia SARS (Severe Acute Respiratory Syndrome) said to have hit China, Viet Nam and other countries in 2003.
It's a paradox: periodic epidemics excite but a fleeting "interest" among the public. Policy-makers and even top healthcare officials close their eyes to the socioeconomic role of infectious diseases as the prime cause of mass incapacitation and premature death of people the world over and do not allocate essential funds for combating this hazard jeopardizing the very existence of humanity.
Well aware of the primary role of causative pathogenic agents and the fact of interspecific competition between microorganisms and man, we cannot afford to pin hopes on consecutive adaptation of Homo sapiens as a biological species in the context of the Darwin-Wallace theory. Millions and millions of lives will have to be sacrificed in epidemics and in the struggle for survival, as it has happened time and again. Dr. Joshua Lederberg, a US geneticist (Nobel Prize, 1958), had a point when he said that in the race for survival with microbial genes our weapon should be human intelligence, not the natural selection of our genes.
Illustrations supplied by the author.
* Chemokines - surface proteins binding small protein molecules. -Ed.
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