A mutant of Tetrahymena thermophila with a partial mirror-image duplication of cell surface pattern. I. Analysis of the phenotype.
Cells of a mutant clone, CU-127, of Tetrahymena thermophila (formerly T. pyriformis, syngen 1) manifest three anatomical abnormalities. First, the stable number of ciliary meridians is 21-25, above the usual number (17-21) in this species. Second, up to 30% of the cells have two oral apparatuses (OAs), one normal and the other abnormal. Third, more than one-half of the cells possess two distinct sets of contractile vacuole pores (CVPs). In some living cells two contractile vacuoles are seen. These abnormalities have persisted unchanged during more than 500 generations of vegetative propagation, and are similarly expressed in subclones. The normal and abnormal OAs are topographically segregated, with normal OAs developing along the "primary oral axis" and abnormal OAs developing along a "secondary oral axis" that is situated 170 degrees of the cell circumference to the cell's right of the primary oral axis. CVPs always appear within this 170 degree arc and never within the complementary 190 degrees arc to the left of the primary oral axis. A unique feature of the CU-127 clone is the commonly expressed mirror image reversal of the structural pattern of OAs that develop along the secondary oral axis. The primordia of such OAs initially appear (as usual) to the cell's left of a ciliary meridian, but as membranelles develop the frequently come to be oriented in a mirror image of the normal pattern, and an undulating membrane sometimes develops on the wrong (left) side of the oral primordium. When two sets of CVPs are formed, their average positions are roughly equidistant with respect to the two oral axes, with the two sets located 50-60 degrees to the right and left respectively of the primary and secondary oral axis. Such cells are thus bilaterally symmetrical about a plane defined by the central longitudinal axis and the halfway point between the two CVP sets (see Fig. 25). This plane bisects the cell into a normal and a "reversed" half-cell. However, only oral asymmetry and large-scale CVP positioning are subject to such reversal; all ciliary meridians remain of normal asymmetry and all CVPs are situated on the left side of CVP meridians. The fact that major aspects of large-scale cellular organization can be reversed while the "fine-positioning" associated with the ciliary meridians remains normal indicates that the two aspects of cell organization are distinct.