Amicronucleate infusoria

Abstract

It has generally been accepted that the dimorphic condition of the nucleus (macronucleus and micronucleus) is a diagnostic character of typical Infusoria, and, aside from a few primitive or aberrant species, the only apparent exceptions have revealed the micronucleus (or micronuclei) within the macronuclear membrane during vegetative stages. Recently, however, Dr. Dawson, working in this laboratory, described a race of Oxytricha hymenostoma Stokes which throughout several years of pedigree culture showed no indication of a morphological micronucleus. During the past year, the isolation for certain experiments of 14 “wild” lines representing 6 species of hypotrichous Ciliates revealed 7 lines (4 species) with micronuclei and 7 lines (2 species) without morphological micronuclei. Ten of the lines were all isolated from a “wild” mass culture of the same species, Urostyla grandis, found in a laboratory aquarium. Six of these lines were amicronucleate. All of the lines of all of the species have bred true with respect to the character in question and one amicronucleate line at present is at the 102d generation. Similarly a culture of Paramecium caudatum, which the present writer supplied a year ago to a course in protozoölogy for the study of the nucleus, failed to reveal a micronucleus, although in other races the micronucleus was readily demonstrated. The apparent conclusion is that a distinct morphological micronucleus is a variable character among different races of the common free-living Ciliates and this, obviously, leads to many interesting problems in relation to conjugation and endomixis.