Alpha, an Infectious Macronuclear Symbiont of Paramecium aurelia

SYNOPSIS. A spiral, rod- or crescent-shaped symbiont here designated alpha, is present in the macronucleus of killer stock 562, syngen 2 of Paramecium aurelia. This stock has a cytoplasmic symbiont, kappa, as well as alpha. Lines were obtained which had only alpha, others which had only kappa, and some which had neither. It was possible to purify and separate both kinds of symbiont from homogenates of stock 562 using an ECTEOLA column. The killing action of this stock is due to kappa, not alpha. Observations on the structure of alpha with the electron microscope indicate that alpha, like the cytoplasmic symbionts in this species, is a bacterium. Alpha is never seen in the micronucleus, is rarely found in the cytoplasm, but abounds in the macronucleus. If paramecia are allowed to grow slowly after autogamy, alpha passes from the old macronuclear fragments, infects the new macronucleus, and all animals retain alpha. In exautogamous paramecia growing at maximum fission rate, however, alpha often does not infect the new macronucleus and is lost from many lines when the old macronuclear fragments disappear. In mixed cultures containing alpha-bearing and alphafree paramecia, it has been found that alpha readily invades the macronucleus of paramecia of susceptible stocks. Homogenates of alpha-bearing cultures are also infective. Infection is highly specific, occurring in only 6 of the 44 stocks of P. aurelia in which infection was attempted, and these 6 are all syngen 2. It is suggested that the short rod or crescent form of alpha is the reproductive form, while the elongated spiral form is probably the invasive motile form.