II. IMPLICATIONS AND EXTENSIONS OF THE SERIAL ENDOSYMBIOSIS THEORY OF THE ORIGIN OF EUKARYOTES

<jats:title>Summary</jats:title><jats:p>In comparison to other theories put forward so far, the Serial Endosymbiosis Theory appears to be the most critically favoured as an explanation for the origin of mitochondria and chloroplasts. For this reason some of its implications are drawn attention to. In particular it is shown that present hierarchical concepts and terminology based on the classical cell theory are inadequate to cope with the S.E.T. (as the theory is abbreviated here). It is shown that the topological relations of mitochondria and chloroplasts to the cell suggest that they are topologically “outside” it, “embraced” rather than lying truly within it. This position is consistent with the S.E.T. On the other hand microtubule‐organising‐centres are truly within the cell. An autogenous origin for the latter seems to be more likely than the endosymbiotic proposal of Margulis.</jats:p><jats:p>In addition to the primary origin of organelles by the transformation of entire prokaryotic endosymbionts, the possibility of the maintenance of eukaryotic organelles produced by one cell, taken into another, is discussed. Examples of some recently discovered, unusual photosynthetic states in ciliates, brought about by temporary foreign chloroplast maintenance, and possibly degeneration of endosymbiotic photosynthetic flagellates, are discussed in this context. The existence of some contemporary examples of photosynthetic bodies which seem to be intermediate between endosymbiotic blue‐green algae and chloroplasts, suggests that some organelles may have had a more recent origin than the Precambrian Era, the process being a continuous one.</jats:p><jats:p>Some of the difficulties inherent in alternative theories for the origin of eukaryotes are discussed. It is stressed that alternative proposals have not yet received intensive exposition and comparisons with the S.E.T. are thus difficult. Attempts to use single criteria (particularly biochemical) as phylogenetic “markers” should be made with due consideration of the S.E.T.</jats:p><jats:p>Finally, it has bearing on the recognition of kingdoms, for it widens the already acknowledged chasm between prokaryotes and eukaryotes, intermediates between the two types of organisation being impossible (as stressed by Margulis).</jats:p>