Stomatogenesis in Paramecium tetraurelia: genetics and phenotypes of mutants affecting the development of the oral apparatus

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Abstract
Seven temperature-sensitive mutants of Paramecium tetraurelia, in which the development of the oral apparatus was affected, were recovered from mutagenesis with N-methyl-N′-nitro-N-nitrosoguanidine and ultraviolet irradiation and analysed. Five of them (short-1, buccalless-1, crook-1, monster-1, monster-2) were shown to possess single mutant recessive genes (designated sh1, bu1, cr1, mo1, and mo2, respectively). The other two, short-2 and short-3, were probably of the same mutagenic origin, which involved two linked recessive loci acting in concert. The short mutants are characterized by reduction in the length of the buccal cavity and oral membranelles and disruption of the organization of the membranelles. The crook mutant shows extra curvature and lengthening of the oral membranelles. The buccalless mutant exhibits loss of the buccal cavity, in addition to the absence, shortening, or disruption of the organization of oral membranelles. The monsters develop abnormal oral apparatuses and abnormalities in cell division giving rise to monstrous cells. All of these mutations are pleiotropic in expression. While the genes for short-1 and crook-1 affect stomatogenesis in the asexual and sexual cycles to similar extents, the other five mutants exhibit defects only in asexual stomatogenesis. The developmental interests of these mutants are discussed.