Mitochondrial mutations conferring erythromycin resistance (ER) are available in Paramecium and it is possible to obtain (by conjugation and cytoplasmic exchange) exconjugant cells containing a majority of wild-type erythromycin-sensitive (ES) mitochondria and a minority of ER ones. In the presence of erythromycin, such ‘mixed’ cells progressively become resistant.

This process of acquisition of resistance has been studied cytologically (on thin sections of single cells) and genetically (by evaluating, on the basis of previous data, the proportion of ER/ES mitochondrial genomes at various times).

While at early stages of the process of transformation the whole mitochondrial population appears rather homogeneous, at later stages, (i.e. when the cell has resumed growth in the antibiotic-containing medium) one finds, side by side, both resistant-looking mitochondria (structurally normal) and sensitive-looking ones, showing the typical alterations induced in Es cells by erythromycin. Conversely, a progressive decrease in the number of ES genomes can be demonstrated.

The complete genetical and cytological transformation from erythromycin sensitivity to erythromycin resistance can occur after less than three fissions in erythromycin-containing medium.

The results indicate that intensive selective multiplication of ER mitochondria occurs, under the pressure of erythromycin, virtually in the absence of cellular division. The possibility of dissociating mitochondrial division from cell division emphasizes the extent of mitochondrial autonomy.

*Present address: Centre de Génétique Moleculaire, CNRS, 91190 Gif-sur-Yvette, France.