It matters that we have a quantitatively meaningful sense of the complexity of biological systems.
N THE DIM PAST, when I was a graduate student and a very junior faculty member, parasitology was an important subject in the curriculum. The high regard in which it was held had little to do with management of the parasitic diseases of man and higher animals, although the practical significance of the subject was recognized. It was an important field for biology departments because of taxonomic and evolutionary puzzles implicit in the parasitic way of life. While a good many of those mysteries had faded by the time I was dozing through lectures on evolution, the original issue had lost none of its force: parasites have evolved to live, apparently cost-free, at the expense of some other species, and yet they seem much simplerbeings than their hosts or free-living relatives. A favorite case for demonstration was that of the barnacle, Sacculina carcini, parasitic on crabs. "The Nadir of Parasitism," Sacculina has no resemblance in its adult stage to anything like a barnacle. It can best be described as a disordered, ramifying tumor of the crab's abdomen.
Such reduction to simplicity was an undeniable opportunity for drawing moral conclusions, explicitly or by choice of words. Last week I rummaged up an example of the latter in Richard Swann Lull's distinguished textbook of evolution that was used in our course. On its yellowed and fragile pages, in a chapter entitled "Parasitism and Degeneracy," I found the following:
". . .Hence as a means of adaptation for survival, parasitism must be looked upon as a remarkably successful device, although the resultant evolution may often be one of retrogression and ends in greater or less degeneracy according to the degree of parasitism and the relative rank of the animal at the beginning of its degenerating career."
Strong words, those: retrogression; degeneracy; degenerating career. Words that can be taken to mean anything between two extremes: simple loss of one or more parts or functions; or a general decline of capacity, even of value. For it is a truism that complexity - information content - is a determiner of value, in everything from an omelette to a Faberge egg.
The derivation of moralities, personal or political, from the facts of biology has fallen into disfavor today, as well it should. But the reputed "degeneracy" of parasites was for a long time too good an opportunity for the moralists to miss. One distinguished parasitologist of the 1950's, for example, published an essay in Science that drew close and extended parallels between the state of parasitism and the Welfare State. The resulting correspondence rendered the Letters pages of Science livelier than they had been in years.
I needed, recently, to consult the plans for next summer's laboratory work in the MBL's influential course on the Biology of Parasitism. I found among the experiments that Drs. Englund, Sher, Donelson, Wassom, Snary, and Anders will oversee such ventures as: construction of gene ("genomic") libraries of Trichinella spiralis (agent of trichinosis); screening those libraries for genes conserved in evolution, such as those for histones and actin; determining their nucleotide sequences; preparing monoclonal antibodies to antigenic proteins from Schistosomes (parasitic flukes) infecting mice; demonstrating antigenic variation, using immunofluorescence, in the African trypanosome, Trypanosoma brucei.
These laboratory projects (which could hardly be duplicated in any university course) stand for discoveries of the past decade, and they are absolutely essential to understanding the parasitic way of life. Take, for example, antigenic variation. Those two words identify the most cunning device yet discovered for evading the immune defenses of a vertebrate victim. Each trypanosome has several hundred different genes encoding certain proteins of its cell surfaces. Only one of these proteins (antigens) is needed, and expressed, at a time. But with each cycle of parasite release into the host's bloodstream, a different antigenic protein appears. Just as the host has begun to mount an immune attack upon the invader, having learned to recognize it as non-self, the invader changes its identity - and escapes. The trick can be played, apparently, hundreds of times. The mechanism by which this is accomplished is astonishing: a particular variant gene to be expressed in a cycle is duplicated and moved to another part of the chromosome set, to a location that determines the "on," as opposed to an "off' condition of the gene. It is therefore copied into an RNA message, and that, in turn, is translated into the necessary cell-surface protein. The parasites survive, having frustrated the host's search-and-destroy system.
HE TRANSFORMATIONS parasites undergo in the course of their complex life cycles include some of the most dramatic, even bizarre, cellular changes known to biology. These changes are developmental processes, just as is the change of a spherical sea urchin blastula to a bilaterally- symmetrical larva, or the transformation of a formless inner cell mass, arising in the blastocyst, to the layered anlagen of the organ systems and the adult body shape in the human embryo.
Those who desire to assign values should bear in mind that nearly everything alive depends upon something else alive, one way or another.
Each planned exercise for the MBL
laboratory course represents a salient discovery or a new opportunity of modern parasitology. (These have been summarized recently in a research briefing on the Biology of Parasitism prepared for the Office of Science and Technology Policy.) What has been learned is that the parasite's developmental control program is not simpler than those of free- living animals. The books of code in which are written the life of a parasitic animal have the same number of bits of information as do the books of their foraging, struggling cousins on the phylogenetic tree. It would appear that for every loss of, say, a digestive function, or of organs with which to sniff out prey, the parasitic species have substituted a complex function that immobilizes the host's defense, or commands a sweeping change of body organization, rendering it resistant to hostile changes of environment that are the norm in a parasitic life cycle.
LL THIS IS, perhaps, taking the idea of "degeneracy," a relic of my educational past, too seriously. It cannot matter much that in classical times, i.e., the 1950's, the relative complexity of living things was measured by their external appearances and functions. But it does matter that we have a quantitatively meaningful sense of the complexity of biological systems. The information content of all the genes in an organism is the beginning of a better measure of complexity than the presence or absence of a digestive system, or the degree of dependence for food and locomotion upon another being. (It cannot be the end, because genes in development are a parsimonious system: small and repeti tive processes yield big and unique results.) Those who desire to assign values, meanwhile, should bear in mind that nearly everything alive depends upon something else alive, one way or another. The microbial exceptions are interesting, of course, but they too are dependent: upon sulfur, iron, oxygen - elements gathered up by this planet in its accretion from the solar nebula_ and upon the nuclear fires that burn in the heart of the sun.