Mating Types in Paramecium
This chapter is placed here at the beginning of the book because the concept of mating types is basic for the analysis of genetic and epigenetic aspects of Paramecium.
When the early work on Paramecium was being done during the nineteenth century, it was not possible to obtain sexual processes of Paramecium at will in the laboratory, although conjugation was observed from time to time by several observers. Thus, Maupas1 remarked, very optimistically, “on peut s’en procurer des conjugaisons, autant que l’on désirera” [one is able to get conjugation whenever one wishes]. However, it was realized by early workers that without knowledge of how to get conjugation, genetic analysis by Mendelian methods was an impossibility with Paramecium. A major step forward was therefore taken by Sonneborn2 when he finally made Maupas’s observation a reality when he discovered at Johns Hopkins University how to obtain conjugation in Paramecium at will. It was considered by him (see Beale3) that the discovery of mating types in Paramecium was a major landmark in the progress of genetic research of ciliates. From that time, genetic analysis by Mendelian methods became possible, for one could then do what Sonneborn had previously considered his main task, namely, to find out to what extent a unicellular organism like Paramecium conformed with the rules of classical genetics, and to what extent other mechanisms operated. He had not forgotten what had been suggested to him when he was a student at Johns Hopkins University, that there might be alternative hereditary systems in lower organisms, different from those of genes and chromosomes. Now in 2007 it should be remembered that when such methods involved conjugation or autogamy in Paramecium, formation of new macronuclei from micronuclei takes place, and the macronuclei thus formed, which determine the phenotype, are not genic replicas of micronuclei (see Chapter 8). That would be a complication that inevitably arises when conjugation or autogamy is used as a mechanism for getting recombination in Paramecium, and possibly also other difficulties that were understandably not appreciated by Sonneborn in his early work. These will be discussed later.