Nuclear Roles in the Post‐Conjugant Development of the Ciliate Euplotes aediculatus II. Experimentally Induced Regeneration of Old Macronuclear Fragments 1

Following conjugation in ciliates, the usual fate of the old pre‐conjugant macronucleus is resorption. In some species, however, old macronuclei, or their fragments, have the ability to reform functional vegetative macronuclei when new macronuclear anlagen are defective. The present work on Euplotes shows that if anlagen are allowed to carry out their essential roles in early exconjugant development, including influence on cortical reorganization such that feeding can resume, they can then be permanently damaged by UV‐microbeam irradiation and regeneration of old macronuclear fragments can occur. E. aediculatus exconjugants were anlage‐irradiated at 40–60 hr of development and the irradiated cells cultured individually and fed. Squashes revealed enlargement and anteriorward migration of the persistent (posterior) macronuclear fragments. The first post‐conjugant fission of such cells was delayed (times ranged 6–43 days) and did not seem to involve the damaged anlagen, which remained rudimentary, did not divide along with the cells, and were subsequently resorbed. It appeared that cell fission was supported by the fragments of the old macronuclei, which either divided or partitioned themselves between the two daughter cells. Mating tests performed on early clones derived from irradiated exconjugants revealed ample conjugation competence; intraclonal conjugation in such clones was also apparent. The absence of the immature period seen in normal exconjugants provides further evidence that the clones arose from cells with regenerated macronuclei.