T. M. Sonneborn

Recent advances in the genetics of Paramecium and Euplotes.

Publisher Summary The genetics of the ciliated Protozoa entered a new phase with the discovery of mating types in Paramecium aurelia (P. Aurelia). It became possible to cross-breed genetically diverse stocks as easily as in higher organisms, and thus pursues genetic studies by means of Mendelian methods. The genetic advances that have followed from this discovery, in every species in which mating types are now known, form the subject of this chapter. This chapter focuses on P. Aurelia that has been extensively investigated by both the older and the newer methods. The results on this species illustrate the genetic phenomena known in the earlier work on the Ciliates. Therefore, familiarity with the cytology and genetics of P. aurelia provide an excellent introduction to the cytogenetics of the ciliated Protozoa. The chapter also presents accounts of the mating types and breeding system, of the basic cytogenetic processes forming the indispensable groundwork for discussion of the genetics and of the genetic results themselves.

Gene Action in Development

This paper deals with processes occurring in the assembly of genic products into cell organelles and structures, especially with processes determining the initiation, location, orientation and number of cortical structures in the ciliated protozoan, Paramecium aurelia. The examples analysed experimentally are: ciliary basal bodies and their associated structures, the repeating unit territory of cortical structure, the pattern of path directions of the rows of basal bodies and unit territories, the distribution in the cortex of two different kinds of unit territories, and the ingestatory apparatus (vestibule and gullet). Among the processes that appear to occur are homonucleation, heteronucleation, and allosteric transitions, as found by others in the in vitro assembly of bacterial flagella, bacterial viruses, and the parts of ciliary microtubules and their appendages. In Paramecium, new basal bodies can arise only in one position and orientation, close and at right angles to an existing basal body at a specified spot. Unit territories reproduce by forming additional parts and subdividing. The information for the positioning and orientation of the developing parts of the unit territory is located within the unit itself and, when experimentally altered, reproduces in the altered orientation which cannot be corrected by genic action. This hereditary aspect of development is determined by an unbroken chain of self-reproducing arrangements of cortical parts. Search for DNA in the cortex gave negative results. Analysis of the hereditary determination of initiation, location, and orientation of the gullet gave results similar to those on the basal body and unit territory. Analysis of the hereditary determination of the path of the rows of unit territories and basal bodies and of the distribution of the two kinds of unit territories showed developmental and genetic control by spatial relations to the vestibule-gullet. Thus, for all corticaltra its examined, development is hereditarily determined by existing and self-reproducing cortical arrangements: the genes (or DNA) doubtless control synthesis of the molecular building blocks, but not their site of assembly or the position, orientation and number of assemblies. The flatworm Stenostomum also shows hereditary developmental control by self-perpetuating structural arrangements, two variations from normal (two kinds of doublet worms) reproducing true to type during asexual reproduction. Whether similar processes occur in the inheritance of the developmentally decisive organization of the amphibian egg is still an open question The processes described in this paper constitute but one of many options available for hereditary control of development.

Degeneracy of the Genetic Code: Extent, Nature, and Genetic Implications

SUMMARY Part I of this paper deals with the amount of degeneracy in the genetic code or, looked at in reverse, the amount of nonsense. Section A considers the experimental evidence and suggests (a) that technical difficulties have prevented assigning meaning to the still unassigned triplets and (b) that the validity of the earlier evidences for nonsense triplets may be questioned in the light of recent discoveries. Complete degeneracy of the code and total absence of nonsense have not yet been excluded. Section B comes to the conclusion, on the basis of general evolutionary considerations, that natural selection would be expected to establish and preserve a completely degenerate code. Section C points out that different nondegenerate codes differ greatly in the builtin frequency of nonsense mutations by single base substitutions. Part II deals with the nature of degeneracy in the existing code. The high frequency of shared doublets between “synonyms” (i.e., codons for the same amino acid) suggests that they are “connected,” that is, interconvertible by altering one base only. Among the sets of synonyms thus far reported, some must be completely connected, and, in the absence of knowledge of the order of the bases, the synonyms of each set could be ordered so as to be completely connected. Current knowledge leads to the expectation of either complete connectedness or a high degree of connectedness among synonyms. Part III explores the consequences of complete connectedness between synonyms; the consequences would be modified only quantitatively, not qualitatively, if connectedness is less than complete. Section A discusses “silent” mutations, i.e., single-base changes that yield a synonym. These must occur in the code as now known. Their frequency (if equal probability of all base substitutions is assumed) depends on the number of synonyms in a set, on the way in which the synonyms are connected, and on whether the various synonyms are used equally or unequally. By selecting one synonym of a set for almost exclusive usage, the minimal percentage of silent mutations stays constant at 11.1% in sets of two or more synonyms, but the maximum increases steadily with set size up to 100% in sets of ten or more. If the synonyms of a set are equally used, the pattern of connection that yields the highest frequency of silent mutations gives peaks for sets of four, seven, and ten synonyms. It would not be surprising if 20% or more of all single-base mutations were silent unless the frequency of trans versions is greatly restricted. Comparable analyses can be made of a quadruplet code. The frequency of mutations to synonyms is not enough to account fully for modulatory mutations, but it is enough to constitute a potential source of error in certain amino acid replacement studies. Section B points out that the degeneracy of the existing code implies as an evolutionary consequence a molecular orthogenesis, that is, marked built-in differences among the synonyms of a set in their possible amino acid replacements by single-base mutations. Section C shows the surprising fact that crossing over between synonyms must in certain cases yield recombinant codons for a different amino acid. Although likely to be scored as mutations, these recombinations would be much more frequent than true mutations. The advantage of connected synonymy is opposed by the disadvantage of these recombinational pseudomutations. Possible ways out of this paradox are discussed.

The Dependence of the Physiological Action of a Gene on a Primer and the Relation of Primer to Gene

THE determination of each hereditary difference thus far examined in certain varieties of the ciliated protozoan Paramwecium aitrelia involves a cytoplasmic factor. In the cases analyzed further, the cytoplasmic factor is not the sole determinant, but depends in a peculiar way upon a gene. In order to understand the action of such a gene as a phy siological agent, the interrelations between gene and cytoplasmic factor need to be examined in detail. The present paper, after briefly reviewing what has already been published (Sonneborn, 1943) on these interrelations in the most fully studied case, deals with a series of new experim-ents which carry the analysis further.

The Origin, Evolution, Nature, and Causes of Aging

Aging is here defined as inherent, progressive, irreversible impairments of function. It occurs not only in higher organisms but also as far down the scale of beings as some — but not all — unicellular organisms. What does the whole array of organisms tell us about aging? Is there a good identifiable reason for its existence? Are there any general bases or causes of aging? In searching for answers to these questions, I shall be guided by evolutionary considerations.

Inheritance in Ciliate Protozoa

HEREDITY ill ciliate Protozoa is too complex to be portrayed fully or adequately in the time at lily disposal. I have therefore selected for discussion what appears to me to be the most important and interesting problem of ciliate genetics-the problem of the basis of the observed hereditary diversities. In genetics of higher organisms we have learned to interpret all hereditary diversities as due to differences in the number, arrangement or kinds of genes present, except for a few instances of plastid and other cytoplasmic differences. In the Protozoa, on the other hand, there are many observations difficult to reconcile with genic or plastid interpretations; but the great weight of evidence from higher organisms and the impracticability of carrying out the necessary genetic analysis long prevented these observations from leading to any solidly founded alternative interpretation. In recent years, however, genetic analysis has been facilitated and this has resulted in a rapidly growing body of knowledge that may lead to radically new genetic principles. I shall attempt to portray current and earlier work primarily in their relation to this. The problem of the basis of hereditary diversities in ciliates may be studied ill two kinds of material: during purely vegetative reproduction, when the genie constitution should remain constant; and at fertilization and nuclear reorganizations, when new genic combinations mayr be formed. In vegetative reproduction, it has been known ever since the pioneer studies of Jennings (1908) that hereditary characters ordinarily remain constant. At first all the products of uniparental reproduction were

THE DELAYED OCCURRENCE AND TOTAL OMISSION OF ENDOMIXIS IN SELECTED LINES OF PARAMECIUM AURELIA

If daily isolation lines of P. aurelia are regularly discarded as soon as they go into endomixis, and if these are then replaced by sister lines that have not yet gone into endomixis, it is possible to maintain for long periods lines which have not been in endomixis since the start of this procedure. With this method there were obtained lines of race R which omitted endomixis for as long as 130 days and 350 fissions, and lines of race W which omitted endomixis for 163 days and 303 fissions. At the end of these long periods, all lines died.In the culture of groups of lines selected in this way, there is an initial period during which endomixis does not occur; this is followed by a period in which endomixis occurs in many of the lines (which have to be discarded); during the remaining history, endomixis occurs rarely and eventually not at all, but the death rate rises and results in the final extinction of the group. The details as to the duration of these periods and their characteristic death and endomixi...

Secondary School Preparation for Making Biological Decisions

Sonneborn discusses here the weighty biological decisions that must be faced by the generation now in high school. Believing that preparing students for making those decisions intelligently should be a high priority of the secondary school, he describes a way of teaching biology that opens up discussion of societal problems after giving students the hard facts for deal ing with them.