SEXUALISOLATION, MATING TYPES, AND SEXUAL RESPONSES TO DIVERSE CONDITIONS IN VARIETY 4, PARAMECIUM AURELIA

¹ Department of Zoology, Indiana University, Bloomington

Among the 53 races of P. aurelia that have been investigated, four races (29, 32, 47 and 51) do not conjugate with any of the three previously described varieties. They constitute a fourth variety with two new interbreeding mating types, VII and VIII. Mating type VII occurs in all four of these races, but mating type VIII has appeared only in the two races 32 and 51.

The mating types VII and VIII give with each other the agglutinative mating reaction characteristic of Paramecium and proceed to conjugate. As in the other three varieties, agglutination and conjugation occur only when mixture is made between cultures of the two types that are neither well-fed nor starved, but are nearing the stage of nutritive exhaustion. Like variety 1, but unlike varieties 2 and 3, variety 4 shows no diurnal periodicity in sexual reactivity: cultures exposed to the natural alternation of daylight and night are capable of reacting sexually at any hour. Further, again like variety 1 and unlike varieties 2 and 3, variety 4 can react sexually throughout the range of temperatures from 9° to 36°, but not at 39°. At 16°, the sexual reactions are weak, leading to but a small proportion of conjugants. In mixtures made at higher temperatures and transferred at once to 9°, pairs begin to form but break apart without conjugating; however, if cultures are first adapted to 9° before they are mixed, a small proportion of true conjugation may occur at this temperature. In all these details, varieties 1 and 4 are alike; but they differ in behavior at the higher temperatures. The maximum optimum temperature for conjugation lies between 30° and 36° in variety 4, between 38° and 40° in variety 1. Thus at 36°, variety 1 gives a maximum sexual reaction, while variety 4 gives only .12 to 25 per cent of the optimum. Variety 4 shows a gradual falling off in sexual reactivity as temperature increases above the optimum, while variety 1 shows a sudden cessation of sexual reactivity at a temperature only 2° above the optimum.

It is thus possible to distinguish these four varieties of P. aurelia not only by their mating types, but also by the sexual responses to diverse conditions.