TRANSITIONS BETWEEN HELICAL AND PROTOFIBRILLAR CONFIGURATIONS IN DOUBLET AND SINGLET MICROTUBULES IN SPERMATOZOA OF STYLOCHUS ZEBRA (TURBELLARIA, POLYCLADIDA)

¹ Department of Zoology, University of North Carolina, Chapel Hill, North Carolina 27514

1. In spermatozoa of Stylochus zebra negatively stained with phosphotungstic acid, both cortical singlet and axonemal doublet microtubules can exist in a protofibrillar or a helical configuration. In cortical singlets the helical configuration appears more frequently; transitions to the protofibrillar condition occur at random and appear to result from the treatment with PTA. In doublet microtubules, the protofibrillar arrangement is the usual one. The helical structure is encountered only at the distal end of the axial unit in the single subtubule that persists distal to the termination of the other member of the doublet.

2. Comparison of the dimensions of protofibrillar and helical configurations in both singlets and doublets reveals that the two types of microtubules are similar. The observation that each type of microtubule can assume the characteristic configuration of the other type suggests that the structural differences of the two classes of microtubules result from alternative associations of equivalent subunits.

3. The central partition between the two subtubules of a doublet appears to be more closely associated with one of the two subtubules, since it remains when one subtubule drops out, either along the length of the doublet or at its distal end.

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