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Lecture 7

BIOC 4403 Lecture Notes - Lecture 7: Cytosine, Experimental Evolution, Wild Type


Department
Biochem & Molecular Biology
Course Code
BIOC 4403
Professor
Archibald John
Lecture
7

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1. Genome
reduction
common in intracellular bacteria, such as
obligate intracellular pathogens and
endosymbionts.
contributes to the evolution of strictly host-
dependent bacterial variants -- as bacteria
rely on the host cell to compensate for the
gene functions that are lost.
2. What is the
'minimal
genome'?
The minimal set of genes that can support
the basic cellular processes of a free-living
organism in a 'rich' growth medium and in
the absence of any selective pressure.
3. why do people
care about the
'minimal
genome'?
This concept is interesting but problematic:
-Which organism are we talking about?
What exactly do we mean by 'free-living'?
-Yes, many genes are 'non-essential' in the
lab (including ~100 in M. genitalium), but
what about in nature?
Perhaps the minimal genome is the set of
genes essential for life in all growth
conditions? (but how will we go about
determining that?!?).
Synthetic biologists are particularly
interested in the minimal genome concept
4. Insect
nutritional
symbioses
Mutual interdependence of the
metabolism of a host insect and one or
more bacterial symbionts.
10-15% of insect species are dependent on
bacteria for viability and reproduction.
NOTE—insect nutritional symbioses have
evolved many times. In each case, the
(endo)symbiont genomes have become
much smaller than those of their closest-
free living bacterial relatives.
5. Mutual
interdependence
in Insect
nutritional
symbioses -
Example
The best known examples are aphids,
plant pests, capable of causing major
agricultural damage.
They feed on plants—they suck the sap
out of the phloem. This diet is rich in
carbohydrates and deficient in amino
acids. Some of these amino acids cannot be
synthesized by the insect but are supplied
by the intracellular symbionts Buchnera
aphidicola.
The interaction between the two partners
dates back 150-250 million years and both
have become so dependent on each other
that under natural conditions they cannot
exist without each other (the insects and
bacteria are said to be 'co-evolving').
6. bacteriocyte Endosymbionts of insects are characterized
by their occurrence in specialized host cells,
which are located at various positions in
the insect body depending on the host
group.
7. bacteriomes Bacteriocytes are sometimes grouped into
organ-like structures called bacteriomes (or
mycetomes) that occur in the body cavity of
aphids and in the anterior gut region of
tsetse flies.
8. Important
message about
Insect
nutritional
symbioses
Percentage
Genes involved in amino acid biosynthesis,
co-factors (e.g., vitamins), etc. represent a
significant fraction of the genomes of
Buchnera, Wigglesworthia and other
nutritional symbionts: they are making
these compounds for their insect hosts!
(they are also receiving compounds from
their hosts; some more than others).
9. complementary
in Insect
nutritional
symbioses
Sometimes there are two distinct
symbionts, both 'deficient' in their own way
but nevertheless complementary:
they help the insect and each other.
EXAMPLE—sharpshooter insects.
10. EXAMPLE of
complementary
in Insect
nutritional
symbioses
Some compounds produced by the
bacterial symbionts are needed by the
host. Some of those are hypothesized to be
shared between symbionts.
Compounds or genes are shared between
the two bacterial symbionts.
11. genome
reduction in
the extreme
Many insects rely on bacterial symbionts
with tiny genomes specialized for
provisioning nutrients lacking in host diets.
Together, these symbionts retain the
capability to synthesize the 10 essential
amino acids. Both symbionts have lost
genes underlying ATP synthesis through
oxidative phosphorylation, possibly as a
consequence of the enriched sugar
content of phloem.
12. why do
genomes get
smaller?
QUESTION—Is genome shrinkage due to
selection for efficiency? Is it somehow
'better' to have a smaller genome, e.g.,
because you can replicate your DNA
faster?
ANSWER—If so, there is no evidence for it.
For example, E. coli genomes can vary by as
much as 25%, but growth rates do not
correlate with genome size, under rich or
poor nutrient conditions.
4403 - 7 Symbiosis & Genome Reduction
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