Reversal form of endosymbiotic theory?
Endosymbiosis leads to an irreversible loss of independence of the endosymbiont and the host.
For example, all members of the insect superfamily Aphidoidea (aphids) have an obligatory association with bacteria, commonly called primary endosymbionts (Munson et al., 1991; emphasis mine):

The primary endosymbionts are found in specialized organs, the
mycetomes, which consist of cells (mycetocytes) containing the
endosymbionts. The endosymbionts are enclosed within vesicles which
are derived from the cell membranes of the mycetocytes.
None of the [primary] endosymbionts has been successfully cultivated in
bacteriological media or in tissue culture. [These] organisms are essential
for the life of the aphids since the removal of the endosymbionts by
antibiotics leads to aphid sterility.
The endosymbionts are  maternally transmitted, and
there are complex mechanisms which assure that the progeny are
infected.

Munson et al. (1991) Buchnera gen. nov. and Buchnera aphidicola sp. nov., a taxon consisting of the mycetocyte-associated, primary endosymbionts of aphids. Int J Syst Bacteriol 41: 566--568.