Somatic function of the micronucleus of Stylonychia mytilus during asexual propagation

The somatic function of the micronucleus during vegetative propagation of the hypotrichous ciliate Stylonychia mytilus was investigated, by generating amicronucleate cell lines with microinjection or amputation. The amicronucleate cell lines exhibited reduction in fission rate and ingestion shortly after the loss of the micronucleus. There was partial recovery of growth rate in the following months of propagation. Amicronucleate cultures carried both normal-looking and abnormal cells. The normal-looking ones underwent binary fission, and also physiological reorganization, in the usual manner. After binary fission, normal-looking amicronucleates gave rise to normal-looking post-dividers, which might then undergo abnormal transformation in the interfission period by developing a bulge on the posterior-right side of the cell (Type A), or on the dorsal surface (Type B). Type A cells could enter binary fission directly, eventually recovering normal cell shape. The hump-back Type B cells went through atypical cortical reorganization(s) to generate spherical cells, which then recover to normal shape by normalizing reorganization(s). The emergence of these two types of abnormalities was correlated with the failure of macronuclear division during binary fission. These observations indicate that the micronucleus possesses morphogenetic function, and that this function is division-related. The micronucleus probably affects cytoskeletal/membranellar development during binary fission, and in its absence the cortical abnormalities exhibited represent a delayed expression of the infirmities incurred during binary fission. In view of the persistence of such abnormalities in amicronucleate cultures for up to a year of culture, the morphogenetic function of the micronucleus appeared not to be replaced; this resembles the situation in the hypotrich Pseudourostyla cristata, but differs from Paramecium where recovery to near-normal is the rule. Chromatin bodies resembling pseudomicronuclei were present in low frequency in the culture, and these appeared to arise in connection with macronuclear development in Type B cells.