Genetics of Paramecium Bursaria. I. Mating Types and Groups, Their Interrelations and Distribution; Mating Behavior and Self Sterility.

ARAMECIUM bursaria is one of the most widely distributed of P the species of Paramecium, though it does not as a rule occur in such large numbers in a given region as does sometimes Paramecium caudatum. It occurs commonly amid growing vegetation in the quiet parts of pools, ponds or streams. It does not flourish in regions in which the water is foul or there is much decaying vegetation. ’ The species is commonly at once recognizable by the algal cells which it contains, which give it a green color. The algal cells are usually packed closely, but a t times they are few and scattered or perhaps even nonexistent, so that the animals are nearly white. Such white individuals may readily be produced by allowing the animals to multiply rapidly in a rich culture medium. The ciliates then multiply more rapidly than do the algae, so that the animals are left with almost none of the algal cells. Figures and descriptions of this species in comparison with others are given by WENRICH (I 9 28) , KALMUS (I93 I) , and KAHL (I 93 I). Many diverse biotypes occur, which may differ greatly in form, size and physiological characteristics. The typical form is more flattened, “slipper shaped,” than in Paramecium caudatum or aurelia. The flattened body is slightly curved in a segment of a flat spiral, the broad oral groove extending on one surface from the anterior end to about the middle of the body. The anterior tip projects forward, to the right, and a little downward (toward the oral surface) ; in very pronounced cases this gives almost the appearance of a hook. Typical forms and sizes for certain biotypes are shown in outline in figure 7 on page 225. As there illustrated, the length ranges from about IOO microns (in the clone 1 figure 7, 2) to about 160 microns or a little more, in a clone of the type E from California (figure 7, 12). Paramecium bursaria has been much employed in genetic and physiological investigations. The earlier studies on it are resumed by MAUPAS (1889, pp. 228-238), who himself studied the nuclear processes in conjuga-