Discontinuities and overlaps in patterning within single cells.

Ciliates such as Tetrahymena manifest longitudinal vegetative growth and periodic equatorial subdivision. Evidence is presented suggesting that this subdivision involves the formation of discontinuities associated with the fission zone that closely resemble expressions of the segment border in multicellular organisms. Unlike latitudes, cellular longitudes can potentially maintain clonal continuity. Features of the system of longitudinal positioning of contractile vacuole pores (c.v.ps) in wild-type cells are suggestive of a circular positional system wrapped around the cell circumference, with a reference border coinciding with the axis of oral development. This border marks a discontinuity that, unlike the fission zone, can be clonally propagated. A recessive mutant, janus (jan), brings about alterations in c.v.p. positioning that suggest that a second longitudinal reference border is located about 45% of the cell circumference to the cell's right of the first. This second border, along which abnormal oral structures sporadically appear, seems to maintain a positional system oriented in a direction opposite to the primary system. When jan first comes to expression in cells previously jan+, the pattern of c.v.p. longitudes changes gradually from that characteristic of wild-type cells to the jan pattern; this change begins before abnormal oral structures first appear along the second reference border. We suggest that the two reference borders, and the positional systems that they control, might be present in wild-type as well as jan cells. The oppositely directed positional systems are likely to overlap. A simple model is proposed to illustrate how overlapping positional systems might cooperate to generate patterns such as those observed.