bcd: A mutation affecting the width of organelle domains in the cortex of Tetrahymena thermophila.

A single-gene recessive mutation, bcd (broadened cortical domains), of Tetrahymena thermophila is characterized by a variable broadening of the spatial domains within which cortical organelles, including both the contractile vacuole pores (CVP) and oral apparatus (OA), are formed. The phenotype is not temperature-sensitive. During the development of the organelles of the mutant prior to cell division, extra CVPs and extra oral primordia (OP) appear near ciliary rows adjacent to the rows at which these structures normally form. In the later stages of development, some, but not all, of these extra structures are resorbed, or in the case of the oral domain, multiple adjacent OPs may be completely or partially integrated into a single enlarged OA. When multiple OAs persist, one or more of these may display a reversed orientation reminiscent of those encountered in janus mutants. However, unlike janus, bcd cells do not express any sign of a mirror-image global organization.Our results can best be accounted for by postulating that the bcd mutation affects some common determinant of the widths of both CVP and OA domains. Studies are in progress which explore the relationship between this width-determining mechanism(s) and the mechanism(s) determining the location of cortical organelles around the cell circumference.