ABSTRACT
The CU-127 clone of Tetrahymena thermophila, which manifests an unusually high number of ciliary rows plus a second set of abnormal oral structures and of contractile vacuole pores with partial mirror-image reversal of asymmetry (Jerka-Dziadosz & Frankel, 1979), has been subjected to breeding analysis. The progeny ratios obtained in various crosses indicate that the abnormalities of cell-surface asymmetry are brought to expression as a result of the action of a recessive allele at a single gene locus, here named janus. When previously normal cells were made homozygous for the jan allele, the cortical pattern characteristic of the CU-127 clone came rapidly to expression, often without associated change in number of ciliary meridians. Conversely, when cells previously expressing jan re-acquired the wild-type (jan+) allele, they returned to the normal pattern of a single normal oral structure and a single nor-mally located set of contractile vacuole pores while still retaining the high ciliary meridian number characteristic of the original CU-127 clone. The capacity for manifestation of the unique asymmetry pattern depends solely on homozygous expression of the janus allele, whereas the stable number of ciliary meridians in janus clones and the degree of expression of secondary OAs is modulated by other factors, probably at least in part genic. These results, taken together with those of the preceding paper, indicate that the janus allele promotes the propagation and/or expression of a condition of reversed asymmetry in a precisely located cell region, and further indicates that the propagation and expression of this condition are largely independent of the number and asymmetry of ciliary meridians.
It is not, however, certain that all of the 58 cycloheximide resistant clones that manifested sensitivity to 6-methylpurine on the subsequent test necessarily lacked the Mpr allele. Mpr/Mpr+ heterozygotes are known to be unusually apt to undergo early phenotypic assort-ment to express only the allele conferring methylpurine sensitivity (Bleyman & Bruns, 1977).
Heterozygosity of three of these clones was verified by genomic exclusion crosses, the aggregated results of which are given by the bottom line of Table 2.
The only anomaly inconsistent with this explanation is the immaturity of clone Y. As progeny of the first round of typical genomic exclusion retain their old macronuclei, they should be sexually mature (cf. Allen, 1967). We are therefore forced into the ad hoc postulate that clone Y, for some unknown reason, did not conjugate when challenged with suitable testers. The fact that clone Y, unlike the two others, retained its old mating type is consistent with this postulate, though it is not compelling evidence for it as the mating type in question (IV) is the commonest encountered in this species.
Recent results obtained by Ares & Bruns (1978) indicate that progeny of short-circuit genomic exclusion may be heterozygous for alleles newly arisen as a result of mutagenesis of the parental clone. But if the CU-127 clone were heterozygous for the janus allele, it would have had to undergo phenotypic assortment to full expression of the janus allele. This is un-likely in view of our failure to achieve such assortment in known janus heterozygotes.
This high ciliary meridian number was not retained indefinitely; there was a drift back to the ‘stability center’ characteristic of the wild type. This will be described more extensively in a subsequent communication.
