[Paleopsych] SW: On the Domains of Life
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Evolutionary Biology: On the Domains of Life
http://scienceweek.com/2005/sw050610-3.htm
The following points are made by D.A. Walsh and W.F. Doolittle
(Current Biology 2005 15:R237):
1) The key molecular player in microbial classification has been the
RNA component of the small subunit of ribosomes (SSU rRNA, or 16S/18S
rRNA), which Carl Woese picked in the early 1970s as a convenient and
reliable "universal molecular chronometer". His goal was nothing less
than a global Tree of Life, relating all living things, but most
immediately his purpose was to sort out the prokaryotes. In 1977, he
and his postdoc George Fox were ready to announce to the world that
these could be unequivocally divided into two very distinct groups, on
the basis of SSU rRNA sequence. The first group comprised mostly
well-studied organisms, such as E coli, cyanobacteria and anthrax,
which they called "eubacteria". The second was made up of less
well-known types, such as methanogens and (as they soon discovered)
extreme halophiles and some thermophilic acidophiles, which they named
collectively "archaebacteria".
2) That prokaryotes are diverse was no surprise, but that they could
be so neatly divided into two, and only two, groups certainly was a
surprise, and so the division was not widely accepted until other
characteristics that distinguished the domain Archaea from the domain
Bacteria (as they are now called) were described. By the early 1980s,
such traits were known to include: the possession of RNA polymerases
more like their eukaryotic than their bacterial counterparts in
subunit composition and sequence; some features of translation shared
specifically with eukaryotes; insensitivity to most antibacterial
antibiotics; and unique membrane glycerolipids composed of isoprenols
ether-linked to glycerol-1-phosphate, those of bacteria and eukaryotes
being fatty acids ester-linked to glycerol-3-phosphate. Ether-linked
lipids have, however, now been found in several thermophilic bacteria,
and fatty acids were recently detected in an archaeon, leaving only
the stereoisomeric form of the glycerol phosphate backbone as a
diagnostic tool to differentiate absolutely between archaeal and
bacterial membranes.
3) In the early 1970s, only partial sequence information (catalogs of
oligonucleotides generated by nucleases) could be obtained. Now, of
course near complete genes are easily PCR-amplified, cloned and
sequenced. The SSU rRNA database as of February 2005 included more
than 125,000 entries. These continue to support the division of
prokaryotes into two domains, each with subdivisions most commonly
called "phyla". Archaea show so far only two or three major
constituent groups (perhaps they should be "kingdoms"): the
Euryarchaeota, the Crenarchaeota and (possibly) the Korarchaeota.
4) Among Bacteria there are at least 52 phyla; some of these turn out
to correspond closely to divisions of bacteria recognized in
pre-molecular sequence days by molecular and cellular phenotype alone,
such as cyanobacteria and spirochaetes. Some unexpected groupings that
could not be easily unified by phenotypic similarities include the
Chloroflexi assemblage and the Proteobacteria subdivisions. Even for
previously recognized phyla, SSU rRNA sequencing provides the
advantage of quick identification and the ability to define
within-phylum phylogenetic relationships down to the level of
'species' in a uniform way.[1-5]
References (abridged):
1. Allers, T. and Mevarech, M. (2005). Archaeal genetics - the third
way. Nat. Rev. Genet. 6, 58-73
2. Charlebois, R.L. and Doolittle, W.F. (2004). Computing prokaryotic
gene ubiquity: rescuing the core from extinction. Genome Res. 14,
2469-2477
3. DeLong, E.F. and Pace, N.R. (2001). Environmental diversity of
bacteria and archaea. Syst. Biol. 50, 470-478
4. Esser, C., Ahmadinejad, N., Wiegand, C., Rotte, C., Sebastiani, F.,
et al. (2004). A genome phylogeny for mitochondria among
alpha-proteobacteria and a predominantly eubacterial ancestry of yeast
nuclear genes. Mol. Biol. Evol. 21, 1643-1660
5. Forterre, P., Brochier, C., and Philippe, H. (2002). Evolution of
the Archaea. Theor. Popul. Biol. 61, 409-422
Current Biology http://www.current-biology.com
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Related Material:
EVOLUTIONARY BIOLOGY: PHYLOGENETIC TREES AND MICROBES
The following points are made by W. Martin and T. M. Embley (Nature
2004 431:134):
1) Charles Darwin (1809-1882) described the evolutionary process in
terms of trees, with natural variation producing diversity among
progeny and natural selection shaping that diversity along a series of
branches over time. But in the microbial world things are different,
and various schemes have been devised to take both traditional and
molecular approaches to microbial evolution into account. For example,
Rivera and Lake(1), based on analysis of whole-genome sequences, call
for a radical departure from conventional thinking.
2) Unknown to Darwin, microbes use two mechanisms of natural variation
that disobey the rules of tree-like evolution: lateral gene transfer
and endosymbiosis. Lateral gene transfer involves the passage of genes
among distantly related groups, causing branches in the tree of life
to exchange bits of their fabric. Endosymbiosis -- one cell living
within another -- gave rise to the double-membrane-bounded organelles
of eukaryotic cells: mitochondria (the powerhouses of the cell) and
chloroplasts. At the endosymbiotic origin of mitochondria, a
free-living proteobacterium came to reside within an archaebacterially
related host. This event involved the genetic union of two highly
divergent cell lineages, causing two deep branches in the tree of life
to merge outright. To this day, biologists cannot agree on how often
lateral gene transfer and endosymbiosis have occurred in life's
history; how significant either is for genome evolution; or how to
deal with them mathematically in the process of reconstructing
evolutionary trees. The report by Rivera and Lake(1) bears on all
three issues: Instead of a tree linking life's three deepest branches
(eubacteria, archaebacteria and eukaryotes), they uncover a ring.
3) The ring comes to rest on evolution's sorest spot -- the origin of
eukaryotes. Biologists fiercely debate the relationships between
eukaryotes (complex cells that have a nucleus and organelles) and
prokaryotes (cells that lack both). For a decade, the dominant
approach has involved another intracellular structure called the
ribosome, which consists of complexes of RNA and protein, and is
present in all living organisms. The genes encoding an organism's
ribosomal RNA (rRNA) are sequenced, and the results compared with
those for rRNAs from other organisms. The ensuing tree(2) divides life
into three groups called "domains". The usefulness of rRNA in
exploring biodiversity within the three domains is unparalleled, but
the proposal for a natural system of all life based on rRNA alone has
come increasingly under fire.
4) Ernst Mayr(3), for example, argued forcefully that the rRNA tree
errs by showing eukaryotes as sisters to archaebacteria, thereby
obscuring the obvious natural division between eukaryotes and
prokaryotes at the level of cell organization. A central concept here
is that of a tree's "root", which defines its most ancient branch and
hence the relationships among the deepest-diverging lineages. The
eukaryote-archaebacteria sister-grouping in the rRNA tree hinges on
the position of the root. The root was placed on the eubacterial
branch of the rRNA tree based on phylogenetic studies of genes that
were duplicated in the common ancestor of all life(2). But the studies
that advocated this placement of the root on the rRNA tree used, by
today's standards, overly simple mathematical models and lacked
rigorous tests for alternative positions(4).
5) One discrepancy is already apparent in analyses of a key data set
used to place the root, an ancient pair of related proteins, called
elongation factors, that are essential for protein synthesis(5).
Although this data set places the root on the eubacterial branch, it
also places eukaryotes within the archaebacteria, not as their
sisters(5). Given the uncertainties of deep phylogenetic trees based
on single genes(4), a more realistic view is that we still don't know
where the root on the rRNA tree lies and how its deeper branches
should be connected.
References (abridged):
1. Rivera, M. C. & Lake, J. A. Nature 431, 152-155 (2004)
2. Woese, C., Kandler, O. & Wheelis, M. L. Proc. Natl Acad. Sci. USA
87, 4576-4579 (1990)
3. Mayr, E. Proc. Natl Acad. Sci. USA 95, 9720-9723 (1998)
4. Penny, D., Hendy, M. D. & Steel, M. A. in Phylogenetic Analysis of
DNA Sequences (eds Miyamoto, M. M. & Cracraft, J.) 155-183 (Oxford
Univ. Press, 1991)
5. Baldauf, S., Palmer, J. D. & Doolittle, W. F. Proc. Natl Acad. Sci.
USA 93, 7749-7754 (1996)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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Related Material:
EVOLUTIONARY BIOLOGY: ON THE PRIMEVAL KINGDOMS
Notes by ScienceWeek:
During most of the past 100 years, the consensus view among biologists
was that all life on Earth evolved from a universal common ancestor, a
primitive cellular form that lived approximately 3.5 to 3.8 billion
years ago. This view capped centuries of detailed classifications of
living systems, with relationships between organisms deduced and
revised and revised again as new discoveries were made. Detailed
analysis of many traits indicated, for example, that primates in the
human family (hominids) shared a common ancestor with apes, that this
common ancestor shared an earlier common ancestor with monkeys, and
that that common ancestor, in turn, shared an even earlier common
ancestor with primitive primates (prosimians; e.g., lemurs), and so
on.
The view was thus of a "tree of life", with discrete branches rising
ever higher, but with all branches deriving from a single primeval
trunk. The known organisms that might have comprised the primeval
trunk and its lowest branches, however, did not provide enough
organismic information to define detailed relationships, so that
biologists were left with apparent mysteries concerning radical
evolutionary innovations between primitive cells and more complex
cells, between the first biological cells and the appearance of
multicellular fungi, plants, and animals.
The following points are made by W. Ford Doolittle (Scientific
American February 2000):
1) In the mid-1960s, Zuckerkandl and Pauling proposed a revolutionary
strategy that might supply the missing information concerning
evolutionary branching. The essential idea was that instead of
investigating anatomy and physiology, family trees of living organisms
should be based on differences in the monomer sequences in selected
genes or proteins. This approach became known as "molecular
phylogeny", and its essential basis was that as a result of changes in
genes caused by mutations, as two species diverge from an ancestor,
the gene sequences they share will also diverge, and as time passes,
the genetic divergence will increase. Researchers could thus
reconstruct the evolutionary past of living species by assessing the
apparent history of divergence of genes or proteins isolated from
those species. Protein studies completed in the 1960s and 1970s
demonstrated the general utility of molecular phylogeny by confirming
and then extending the already established family trees of
well-studied groups such as the vertebrates.
2) A new research development occurred in the late 1970s, when Carl
Woese proposed that the two-domain view of life that divided living
organisms into a) bacteria and b) cells with internal membrane-bound
organelles (eukaryotes) was no longer tenable on the basis of
molecular analysis. Woese suggested that certain so-called "bacteria"
formed a distinct third primary group -- the archaea -- and that
members of this group were as different from other bacteria as
bacteria were different from eukaryotes. Woese suggested that although
certain cells without internal membrane-bound organelles (prokaryotes)
classified as bacteria might look like bacteria, they were genetically
much different, and their *ribosomal RNA (rRNA) supported an early
evolutionary divergence.
3) Once the idea of three rather than two primeval domains was
accepted by researchers, an important question was which of the two
structurally primitive groups -- bacteria or archaea --gave rise to
the first eukaryotes? Because of evidence indicating an apparent
kinship between the gene expression/protein synthesis machinery of
archaea and eukaryotes, the consensus was that eukaryotes diverged
from the archaea.
4) One important result of research in molecular phylogeny during the
past 15 years has been the production of strong evidence supporting
the "endosymbiont hypothesis". In biology, the term "symbiosis" refers
in general to an intimate and protracted association of individuals of
different species, and "endosymbiosis" refers to a symbiotic
association between cells of two or more different species in which a
smaller cell inhabits a larger host cell. The endosymbiont hypothesis
in evolutionary biology, now a consensus view, proposes that the
mitochondria components of eukaryotes, so essential for eukaryote
metabolism, formed when an early eukaryote engulfed and then retained
one or more primitive bacteria of a certain type
(alpha-proteobacteria). Eventually, these bacteria relinquished their
ability to live on their own and transferred some of their genes to
the nucleus of the host cell, and these bacteria then evolved into the
extant mitochondria. In addition, and similarly, the hypothesis
proposes that some mitochondria-bearing eukaryotes ingested bacteria
capable of producing oxygen during photosynthesis (cyanobacteria), and
these resident symbiotic bacteria subsequently evolved into the
chloroplasts, the present internal structures that drive
photosynthesis in certain eukaryotes (e.g., in plant cells).
5) Until very recently, therefore, the consensus view in biology could
be summarized as follows: The early descendants of the last universal
common ancestor -- a small prokaryote cell --divided into two
prokaryotic groups: the bacteria and the archaea. Later, the archaea
gave rise to the eukaryotes. Subsequently, the eukaryotes gained
valuable energy-generating organelles --mitochondria and (in the case
of plants, for example) chloroplasts -- by taking up and retaining
certain symbiotic bacteria.
6) Several years ago, however, the consensus view stated above became
complicated by a large amount of evidence concerning the phenomenon of
"lateral gene transfer" (horizontal gene transfer). Biologists
recognize two types of gene transfer from one organism to another:
vertical and horizontal. Vertical gene transfer occurs between parents
and offspring, and horizontal gene transfer is the transfer that may
occur between organisms otherwise. It is in bacteria that horizontal
gene transfer has been studied most extensively, particularly in the
last decade. Three types of horizontal gene transfer are known:
conjugation, transduction, and transformation. Conjugation is a type
of sexual reproduction exhibited by some bacteria, the process
involving the exchange of genetic material by means of a tube or
bridge, the transfer of DNA occurring either in one direction or in
both directions.
7) Transduction involves the transfer of genetic material from one
bacterium to another with the intermediation of a virus. Essentially,
when the virus infects one bacterium, it often carries away pieces of
that bacterium's genome, and those pieces, upon the infection of a new
bacterium, become incorporated into the second bacterial genome.
Finally, transformation is the process involving the uptake or
incorporation of DNA fragments (plasmids) by a bacterium, first
observed in 1944 by Oswald Avery. In this context, the important
aspect of horizontal gene transfer is that in primitive cells such as
prokaryotes it is now apparent that horizontal gene transfer readily
occurs across species. As a consequence of the new evidence, the
consensus view of the interrelations between the primeval three
kingdoms has now been seriously destabilized.
8) In general, the current situation concerning the evolutionary "tree
of life" is as follows: The conceptual tree-like structure with
discrete branches is retained at the top of the eukaryote domain, and
also retained is the idea that eukaryotes obtained mitochondria and
chloroplasts from bacteria. But the lower parts of the tree are now
seen to involve an extensive anastomosis of branches -- branches
joining other branches in a complex network of intersecting links --
resulting from extensive horizontal gene transfer of single or
multiple genes, the horizontal gene transfer known to be common in
unicellular organisms. Thus, the author (Doolittle) suggests that the
"tree of life" lacks a single organism at its base, and that "the
three major domains of life probably arose from a population of
primitive cells that differed in their genes."
Scientific American http://www.sciam.com
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