Factors Determining Conjugation in Paramecium Aurelia II. Genetic Diversities between Stocks or Races.

N TYPICAL biotypes of the Johns Hopkins stock R of P. aurelia, I SONNEBORN (1936) has shown that, when surplus animals from isolation lines are collected in a small mass culture and placed a t 31OC, conjugation invariably occurs if the animals have recently been through a nuclear reorganization (that is, endomixis or conjugation), but never a t other times. In the present paper we contrast the behavior of this stock with that of the Yale stock, cultivated since 1907 by WOODRUFF, which responds very differently to these same conditions. In P. aurelia, racial diversities in the tendency to conjugate have been reported by JENNINGS, WOODRUFF, HOPKINS, and BALL, but their work has been criticized by ENRIQUES, ZWEIBAUM, and the CHATTONS. JENNINGS (1910) found a race k, which was observed to go through 20 epidemics of conjugation in about two years; but during this period race C2 conjugated only a few times, and races c and i only twice. Long continued, systematic andvaried attempts to induce conjugation in races c and i invariably failed, but similar attempts in race k were always successful if two or three weeks had elapsed since the last conjugation. WOODRUFF (191 2) reported similar results; in some stocks conjugation occurred readily, but in his stock cultured since 1907 conjugation could be obtained only twice, once in 1913 (WOODRUFF 1914) and once in 1920 (WOODRUFF 1921). The most extensive study along these lines was made by HOPKINS (1921). He found marked differences among 13 races, in three respects. (I) Differences in the tendency to conjugate: under similar conditions, some races conjugated frequently, other races only rarely, some not a t all. (2) Differences in the conditions required for conjugation: some races conjugated in large stock cultures without special experimental treatment; other races would not conjugate under these conditions, but did so when transferred to small watch glasses with a rapidly diminishing food supply; still other races would not conjugate even under these conditions, but could be made to conjugate when this treatment was preceded by a longer or shorter period of “dormancy.” (3) Differences in the periodicity