PROGRESS IN PROTOZOOLOGY

PHYLOGENETIC RELATIONSHIPS AMONG PROTOZOA 211
maintain their normal activity when transferred into a prokaryotic cell.
The second problem pertains to the ways in which transition from
Pro- to Eukaryota was accomplished over two billion years ago.
Two main groups of hypotheses have been concerned with this
problem.
According to the first theory, the eukaryotic cells originated by symbiosis
with prokaryotes, the latter ultimately becoming organelles of
the former. The theory of symbiotic origin of the eukaryotes, first formulated
by Russian biologists Merezkhovsky (1905) and Famintsyn
[1907 (1915)], was subsequently totally forgotten. In recent years
it has been revived, mainly due to the publications of Lynn M a rg
u 1 i s and her colleagues. Her book, showing the great talent of its
author, which appeared in 1970, contains a brilliant and well documented
presentation of the symbiosis theory which is considered in terms
of modern biology. I shall not go into details of this theory for I am
certain that my audience is familiar with the book. I would like, however,
to discuss some of the consequences of the concept represented by
the symbiosis theory.
The theory allows that eukaryotic cells may have originated from
their prokaryotic ancestors on many separate occasions, thus providing
conditions for an almost unlimited polyphyly. According to Margulis
and her coworkers, the majority of photoautotrophic and heterotrophic
flagellates may have originated by symbiogenesis. She placed all the
organisms in five kingdoms: Monera (Prokaryota), Protists, Fungi, Plants,
and Animals. Among the protists, in the broader sense, she recognized
31 phyla that include both the protozoa and algae. MASTIGOPHORA
are included in nine of the phyla. Clearly, Margulis does not regard
PROTOZOA as a natural group. Indeed, her concept of PROTOZOA
appears valid only with regard to a certain level of organization of
unicellular eukaryotes belonging to a high but uncertain number of
independently evolved groups.
The symbiosis theory is not accepted by all, and its opponents raise
serious objections to it. I believe that the transition from Prokaryota
to Eukaryota occurred not by symbiogenesis, but by a progressive increase
in the complexity of organization of the prokaryotic cell. Polymerization,
as defined by Dogiel (1929, 1954) and his colleagues, i.e.,
an increase in number of cell components, appears to have played
a leading role in this complexity increase.
I shall discuss here only some of the possible objections to the symbiosis
theory of eukaryote evolution. As yet, no cases of symbiosis between
two prokaryotic cells have been reported. Still, such a relationship
is postulated by the proponents of this theory. Most illustrations
of transformation of prokaryotic to eukaryotic cells are unconvincing,
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