Encyclopedia of Evolution.pdf - Online Reading Center

comes from the unfertilized egg, rather than from the sperm;
the cytoplasmic factors showed a pattern of inheritance that
was passed on solely through the maternal line. This indicated
that there was DNA in the cytoplasm, not just in the
nucleus (see DNA [raw material of evolution]), and she
had an idea where that DNA was located.
All independently living cells (and most viruses as well)
have genetic information encoded in DNA. In contrast, organelles
(specialized structures within the cell) generally do not
have their own DNA; their production and operation are
dictated by the DNA in the cell’s nucleus. Margulis wrote a
paper in 1967 in which she proposed that chloroplasts and
mitochondria had and used their own DNA (see photosynthesis,
evolution of; respiration, evolution of). The
DNA was there because these organelles are the evolutionary
descendants of bacteria that had merged with ancestral
eukaryotic cells (see eukaryotes, evolution of). The bacteria
remained, without killing their host cells; and the host cell
lineages evolved processes that accommodated and made use
of the bacteria. The bacteria became symbionts. If chloroplasts
and mitochondria were merely cell structures, one would not
expect them to have their own DNA. The chloroplasts evolved
from photosynthetic bacteria (often called blue-green algae;
see bacteria, evolution of) while the mitochondria evolved
from aerobic bacteria that consumed organic materials.
These symbiotic events happened more than a billion
years ago. Frequently in cells, segments of DNA can
move from one location to another (see horizontal gene
transfer). Many of the genes of the mitochondria and
chloroplasts were transposed from the symbiotic bacteria to
the nuclei of the host cells. The simplified symbionts were
now incapable of surviving on their own, and they became
the chloroplasts and mitochondria that are today found
only in eukaryotic cells.
Though not accepted by many scientists at first, Margulis’s
proposal of symbiogenesis quickly advanced in the
scientific community. As better chemical and microscopic
techniques became available, convincing evidence of the bacterial
origin of chloroplasts and mitochondria emerged. Chloroplasts
and mitochondria:
• have their own DNA, which resembles bacterial DNA more
than it resembles the DNA of eukaryotic chromosomes.
• have ribosomes, which are structures that use genetic information
in DNA to produce proteins. As a result, mitochondria
and chloroplasts produce some of their own proteins.
In the case of an important photosynthetic enzyme called
rubisco, part of the enzyme (the large subunit, rbcL) is
produced by the chloroplasts while the small subunit is
produced by the ribosomes out in the chloroplast, using
information now found in the nucleus.
• reproduce themselves, rather than being constructed by
the cell.
In addition, some marine invertebrates consume algae
but keep the algal chloroplasts alive in some of their cells.
This strongly suggests that chloroplasts and mitochondria
may have arisen in a similar way, as symbiotic bacteria that
symbiogenesis
lost most of their genetic independence—but not quite all of
it, thus betraying their symbiogenetic origins.
As techniques became available to actually determine the
sequence of nucleotides within DNA and other nucleic acids,
it became possible to make comparisons between the DNA of
different species to determine their evolutionary relationships.
Following the initial lead of Carl R. Woese of the University
of Illinois (see Woese, Carl R.), biologists constructed a tree
of life showing the branching patterns of evolution (see DNA
[evidence for evolution]). The tree was based solely upon
living species, from which DNA and RNA could be obtained.
Mitochondrial genes turned out to be very close to aerobic
bacteria on the tree of life, and not at all similar to the genes
in the nuclei of the cells in which mitochondria live. Chloroplast
genes turned out to more closely resemble cyanobacteria
on the tree of life than the genes in the nuclei of the plant
cells in which they now live. This is precisely what would be
expected as confirmation of Margulis’s symbiogenetic theory
of their origins.
While nearly all biologists accept Margulis’s symbiogenetic
explanation of the origin of chloroplasts and mitochondria,
some of her subsequent proposals have not met with
widespread acceptance. Perhaps the most famous example
is her proposal that the motile structures of eukaryotic cells
were originally symbiotic bacteria as well.
Margulis uses the term undulipodia (“undulating feet”
in Greek) to denote structures in eukaryotic cells that are
usually called cilia and eukaryotic flagella. Cilia are hairlike
structures on the outside of many eukaryotic cells, for example
free-living protists, and the cells that line animal respiratory
passages, and that have the power of movement. Cilia
assist in the movement of protists such as Paramecium, and
some small aquatic animals. Flagella are whiplike structures
with which many eukaryotic cells can swim, for example protists
like Euglena, and sperm cells. Many bacteria also have
flagella with which they propel themselves, but the smaller
rotary structure of bacterial flagella is totally unlike the more
complex structure of eukaryotic flagella, which is why Margulis
does not use the same term for both of them.
Margulis has also pointed out similarity in composition
between undulipodia and the mitotic spindle apparatus in
eukaryotic cells. Before mitosis begins, chromosomes replicate;
in mitosis, small protein strands in the spindle move the
chromosomes into two groups, which then form the nuclei of
two cells. Margulis points out structural similarities between
undulipodia and the spindle apparatus; and that eukaryotic
cells do not have undulipodia and a spindle apparatus at the
same time. This implies that, even though they now look and
act very differently, undulipodia and the spindle apparatus
evolved from the same ancestral structure.
Margulis claims that the protein strands within undulipodia
and within the spindle apparatus are the remnants of
spirochete bacteria. How might this have occurred? Today,
there are examples of spirochete bacteria that burrow partway
into protist cells. The protruding parts of the bacteria
flap in a coordinated rhythm, propelling the protist cell.
These protists, when viewed under the microscope, look just