In the ciliate Paramecium caudatum, only mating-reactive cells can attach to the polystyrene surface of Petri dishes. The attachment of mating-reactive cells occurs exclusively at the tips of ventrally located cilia (mating-reactive cilia) and appears to involve a hydrophobic interaction. An increase in the hydrophilic nature of the polystyrene surface following treatment with sulphuric acid results in the loss of attachment of reactive cells. The presence or absence of Ca ions and varying the ionic strength of the medium have no effect upon attachment. Although attachment reactivity is closely correlated with mating reactivity, the attachment sites appear distinct from the mating recognition sites, as suggested by the following experimental results: (I) cells that lose mating reactivity following treatment with trypsin are still able to attach to the polystyrene surface; (2) the attachment of cells appears temperature-dependent over a range where mating reactivity remains uneffected; (3) mating reactivity is quickly lost as a result of attachment to the surface of the dish. While the mechanism of attachment is not the same as sexual recognition, the attachment to the polystyrene surface initiates the early stages of the conjugation process. Cells that attach show micronuclear migration while unattached cells do not. The maximum rate of micronuclear migration occurs approximately 30 min after attachment. Meiosis does not occur following migration in the attached cells as in cells that are sexually mating. The relationship between the mating substance and the attachment substance is discussed, while the utilization of the attachment phenomenon for other experimental procedures is proposed.

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