Pronuclei of Heliophrya erhardi Matthes during conjugation and their differential association with coated and uncoated microtubules.

Microtubules surrounding the pronuclei during conjugation of the suctorian ciliate Heliophrya erhardi can be divided into 2 distinct classes by electron microscopy. Microtubules around the stationary nucleus have a conventional appearance and presumably serve as a skeleton, anchoring that nucleus in its cytoplasmic position. Microtubules surrounding the prospective migratory nucleus are coated with electron-dense material and are in some cases associated with 7-nm filaments. These coated microtubules supposedly function to transport the migratory nucleus into the conjugation partner.