This bacterium would have multiplied inside its host
(Fig. 4.17g, h). This association would have become permanent and, both partners finding mutual benefit, would have
evolved into intracellular mutualistic symbiosis. The bacterium that consumes dioxygen found ‘board and lodging’
in its host, that is, protection and abundant food.
The exact nature of the immediate benefit to the host of
the mitochondrial symbiosis is still debated today. The ability to synthesise ATP with maximum efficiency by oxidative
phosphorylation could have been originally selected. Alternatively, this character appeared only secondarily and the
primary selective effect was that the symbiotic dioxygen
consumer was able to protect its host from damage caused
by dioxygen, thus allowing it to ‘conquer’ aerobic ecological
niches. In any event, a major consequence of this endosymbiosis for early eukaryotes would be the transition from an
anaerobic to an aerobic lifestyle. The transition from the
prokaryotic symbiont to the mitochondrion was accompanied by gene transfers from the prokaryotic genome to
the nuclear genome of its host (Fig. 4.17i).
Following the acquisition of mitochondria, major extant
eukaryotic groups diversified. Later, a new symbiotic association occurred in one lineage with the capture of a photosynthetic bacterium related to cyanobacteria (Fig. 4.17j).
Similarly to mitochondrial endosymbiosis, gene transfers
occurred from the cyanobacterium to the nucleus of the
host (Fig. 4.17k), and the symbiotic photosynthetic bacterium evolved into the chloroplast (Fig. 4.17l).
A comprehensive study of processes involved in these
endosymbioses is presented in Chap. 5.
4.3.1.3 Ample Evidence Supporting
the Endosymbiotic Theory
1. Phylogenetic studies based on the comparison of
sequences of rRNAs and protein-encoding genes from
the genomes of endosymbionts have clearly demonstrated
that mitochondria and chloroplasts are related to Bacteria,
specifically alphaproteobacteria for mitochondria and
cyanobacteria for chloroplasts. Mitochondrial genomes
contain between 10 and 100 genes, which is much lower
than the number of genes found in extant alphaproteobacteria. Two complementary explanations can be
advanced: (a) many genes initially present in the symbiont
were lost during evolution and (b) genes were transferred
from the symbiont to the nucleus of the host. Supporting
this point, molecular phylogenies have shown that some
nuclear genes coding for proteins that sometimes act
in the mitochondria are close to alphaproteobacteria.
HGTs to the host nuclear genome are also frequent in the
case of chloroplasts, whose genomes contain a higher
number (100–200) of genes. The large size of the chloroplast genome in comparison with that of mitochondria
has sometimes been interpreted as an indication that
mitochondrial endosymbiosis predates the chloroplastic
endosymbiosis. However, it may also simply indicate a
different mode of evolution. A second argument in favour
of the temporal priority of mitochondria is that the reception of a dioxygen producer is only conceivable for a host
capable of consuming dioxygen. But the most decisive
argument is that all current eukaryotic cells have or have
had mitochondrion-harbouring ancestors, while the chloroplast endosymbiosis is specific to some eukaryotic
lineages.
2. Mitochondria and chloroplasts contain their own
ribosomes which are undoubtedly of the bacterial type.
In addition, the same antibiotics as those acting on freeliving bacteria, for example, streptomycin, inhibit their
function.
3. Photosynthesis is virtually identical in chloroplasts and in
extant cyanobacteria. Moreover, the respiratory chain
process of prokaryotes (localised in the cytoplasmic
membrane) is comparable to that of eukaryotes (located
in the mitochondrial inner membrane).
4. The acquisition of photosynthesis continues today
(see Sect. 5.4.4).
4.3.2 Eukaryotes Result of an Association
Between a Bacterium and an Archaeon
Many other hypotheses apart from that proposed by De Duve
have been put forward to explain the origin of eukaryotes.
Two in particular suggest that the eukaryotic cell is the result
of the establishment of a syntrophic association between
bacteria and archaea. Such hypotheses have been advanced
to explain the fact that the eukaryotic genome contains both
genes that are close relatives of archaeal homologues and
genes that are relatives of bacterial genes.
The hydrogen hypothesis was proposed in 1998 by
Martin and Mu ¨ller. It is based on the establishment of
associations in an anoxic environment between H 2 - and
CO 2 -producing fermentative alphaproteobacteria and
methanogenic archaea that could use the dihydrogen and
CO 2 produced by the alphaproteobacteria. Such associations, based on interspecies hydrogen transfer, have been
observed among extant organisms. For these authors, an
increase of the contact surface between the partners by
expansion of the membranes of the methanogens around
alphaproteobacteria would have resulted in total engulfment
of the latter (Fig. 4.18a, b). In parallel, several alphaproteobacterial genes were transferred to the genome of the
methanogenic archaea and this is how the genome of
the primitive eukaryote (or proto-eukaryote) would have
evolved. The other part of the alphaproteobacterial genome
98
J.-C. Bertrand et al.
(Fig. 4.17g, h). This association would have become permanent and, both partners finding mutual benefit, would have
evolved into intracellular mutualistic symbiosis. The bacterium that consumes dioxygen found ‘board and lodging’
in its host, that is, protection and abundant food.
The exact nature of the immediate benefit to the host of
the mitochondrial symbiosis is still debated today. The ability to synthesise ATP with maximum efficiency by oxidative
phosphorylation could have been originally selected. Alternatively, this character appeared only secondarily and the
primary selective effect was that the symbiotic dioxygen
consumer was able to protect its host from damage caused
by dioxygen, thus allowing it to ‘conquer’ aerobic ecological
niches. In any event, a major consequence of this endosymbiosis for early eukaryotes would be the transition from an
anaerobic to an aerobic lifestyle. The transition from the
prokaryotic symbiont to the mitochondrion was accompanied by gene transfers from the prokaryotic genome to
the nuclear genome of its host (Fig. 4.17i).
Following the acquisition of mitochondria, major extant
eukaryotic groups diversified. Later, a new symbiotic association occurred in one lineage with the capture of a photosynthetic bacterium related to cyanobacteria (Fig. 4.17j).
Similarly to mitochondrial endosymbiosis, gene transfers
occurred from the cyanobacterium to the nucleus of the
host (Fig. 4.17k), and the symbiotic photosynthetic bacterium evolved into the chloroplast (Fig. 4.17l).
A comprehensive study of processes involved in these
endosymbioses is presented in Chap. 5.
4.3.1.3 Ample Evidence Supporting
the Endosymbiotic Theory
1. Phylogenetic studies based on the comparison of
sequences of rRNAs and protein-encoding genes from
the genomes of endosymbionts have clearly demonstrated
that mitochondria and chloroplasts are related to Bacteria,
specifically alphaproteobacteria for mitochondria and
cyanobacteria for chloroplasts. Mitochondrial genomes
contain between 10 and 100 genes, which is much lower
than the number of genes found in extant alphaproteobacteria. Two complementary explanations can be
advanced: (a) many genes initially present in the symbiont
were lost during evolution and (b) genes were transferred
from the symbiont to the nucleus of the host. Supporting
this point, molecular phylogenies have shown that some
nuclear genes coding for proteins that sometimes act
in the mitochondria are close to alphaproteobacteria.
HGTs to the host nuclear genome are also frequent in the
case of chloroplasts, whose genomes contain a higher
number (100–200) of genes. The large size of the chloroplast genome in comparison with that of mitochondria
has sometimes been interpreted as an indication that
mitochondrial endosymbiosis predates the chloroplastic
endosymbiosis. However, it may also simply indicate a
different mode of evolution. A second argument in favour
of the temporal priority of mitochondria is that the reception of a dioxygen producer is only conceivable for a host
capable of consuming dioxygen. But the most decisive
argument is that all current eukaryotic cells have or have
had mitochondrion-harbouring ancestors, while the chloroplast endosymbiosis is specific to some eukaryotic
lineages.
2. Mitochondria and chloroplasts contain their own
ribosomes which are undoubtedly of the bacterial type.
In addition, the same antibiotics as those acting on freeliving bacteria, for example, streptomycin, inhibit their
function.
3. Photosynthesis is virtually identical in chloroplasts and in
extant cyanobacteria. Moreover, the respiratory chain
process of prokaryotes (localised in the cytoplasmic
membrane) is comparable to that of eukaryotes (located
in the mitochondrial inner membrane).
4. The acquisition of photosynthesis continues today
(see Sect. 5.4.4).
4.3.2 Eukaryotes Result of an Association
Between a Bacterium and an Archaeon
Many other hypotheses apart from that proposed by De Duve
have been put forward to explain the origin of eukaryotes.
Two in particular suggest that the eukaryotic cell is the result
of the establishment of a syntrophic association between
bacteria and archaea. Such hypotheses have been advanced
to explain the fact that the eukaryotic genome contains both
genes that are close relatives of archaeal homologues and
genes that are relatives of bacterial genes.
The hydrogen hypothesis was proposed in 1998 by
Martin and Mu ¨ller. It is based on the establishment of
associations in an anoxic environment between H 2 - and
CO 2 -producing fermentative alphaproteobacteria and
methanogenic archaea that could use the dihydrogen and
CO 2 produced by the alphaproteobacteria. Such associations, based on interspecies hydrogen transfer, have been
observed among extant organisms. For these authors, an
increase of the contact surface between the partners by
expansion of the membranes of the methanogens around
alphaproteobacteria would have resulted in total engulfment
of the latter (Fig. 4.18a, b). In parallel, several alphaproteobacterial genes were transferred to the genome of the
methanogenic archaea and this is how the genome of
the primitive eukaryote (or proto-eukaryote) would have
evolved. The other part of the alphaproteobacterial genome
98
J.-C. Bertrand et al.