Inbreeding Degeneration in Tetrahymena.

NBREEDING degeneration has been observed in many higher plants and animals I and its physical basis and its bearing on the genetic economy of these organisms have been widely discussed (GOWEN 1952; LERNER 1954). While some genetic problems such as gene action, mutagenesis and genetic recombination have been actively and profitably pursued with microorganisms, relatively little work on inbreeding degeneration and its converse, hybrid vigor, has been undertaken with the smaller and more readily studied forms. The chief reason for this neglect is that most microorganisms are haploid for a major portion of their life cycles, and both heterosis and inbreeding degeneration are characteristic of diploids. Among the haploids only the inbreeding effects in yeast (WINGE and LAUSTSEN 1940) and heterocaryosis in ascomycetes (DODGE 1942; BEADLE and COONRADT 1944) bear a similarity to such phenomena in higher organisms and these similarities may be superficial. Even the organisms with predominantly diploid life cycles often undergo autogamy or some other form of intensive inbreeding which assures a high degree of homozygosity and for this reason have an “essentially haploid” economy free from the disadvantages of inbreeding and from certain of the advantages of crossbreeding observed in other diploids. Many, but not all, of the Ciliated Protozoa do, however, show a type of inbreeding degeneration similar to that in higher organisms (JENNINGS 1944a, 1944b). Recently work on Tetrahymena pyriformis has demonstrated that this Ciliate has a genetic system essentially like that in other crossbreeding diploids and that it may provide a useful model system for studying the effects of inbreeding. It undergoes conjugation, but autogamy has not been observed; it shows deterioration upon inbreeding and vigor is restored on outcrossing. The occurrence of inbreeding degeneration is not, of course, an unmixed blessing and presents problems for a genetic program in which homozygosis is a valuable tool. Fortunately, strains can be obtained which survive the various inbreeding crises and return to approximately the state of vitality observed in the original crossbred strains. The Ciliated Protozoa in general are useful in a comparative study of the types of genetic economy since closely related forms may show great differences in their breeding patterns and genetic structure (SONNEBORN 1956). The observations to be reported here have been acquired incidentally in the course of studying other traits in Tetrahymena and do not constitute a definitive analysis. A thorough evaluation of the phenomena reported must await more extensive study.