Nuclear processes in Euplotes octocarinatus during conjugation

Summary The pregamic and metagamic nuclear divisions during conjugation of Euplotes octocarinatus and the development of new micro- and macronuclei were investigated with the help of phase contrast microscopy and a staining of the cells by aceto carmine. The most important stages are illustrated with microphotographs and a time table of the events based on experiments carried out at 26°C is presented. It is shown that E. octocarinatus has n = 35 chromosomes and that all of them enter the macronuclear anlage and undergo polytenisation before they break down into the short DNA-pieces typical of the adult macronucleus of hypotrich ciliates. The nuclear events suggest that, similar to what has been reported for E. patella , frequently sister nuclei become the pronuclei which should result in the formation of a high proportion of isogenic co-conjugant lines. A comparison of the nuclear events in heterotypic pairs (co-conjugants of different mating types) with those in homotypic pairs (co-conjugants of the same mating type) revealed no differences apart from a tendency of homotypic pairs to become arrested at the premeiotic division and then to separate. Although hundreds of cells were investigated, regeneration of pieces of the old macronucleus or fusion of fragments with a newly formed anlage, as is reported for certain other Euplotes species, has not been observed.