Lecture 9
Pt 1. Another look at organism-organism interactions.
Pt 2. The lessons of the SH for biochemical evolution
Organism – organism interactions
What lessons might the Screening Hypothesis contain for those
studying such interactions?
One can assume no function for any one NP
Biological activity that is found cannot be assumed to be
important or relevant
Inducibility – what is it? Why has it evolved?
One can assume no function for any NP
Gas chromatograph
trace shows the many
volatile NPs from a corn
plant
The response of one
insect olfaction cell to
those chemicals
Time
Biological activity that is found cannot be assumed
to be important or relevant
Which of these chemicals is made because the plants gains fitness by making
that NP?
• The anti-cancer drug vincristin?
• The anti-cancer drug taxol?
• The stimulant caffeine?
• Every one of the furanocoumarin family?
• All chemicals with antifungal properties?
More likely that the organisms making these NPs gain fitness from the
overall ability to make many diverse chemicals, a very few of which
enhance the fitness – quite possibly chemicals not yet found,
characterised or known? Mammals gain fitness by having an immune
system not by having a particular antibody?
Inducibility – what is it? Why has it evolved?
Inducible defenses – where the rate of development of damage is slow,
there are resource benefits to be had if the plant only makes defensive
chemicals after the attack has started.
Phytoalexin model illustrates the paradigm.
Examples are known of the systemic induction of chemicals – where
the part of the plant initially attacked sends a “signal” to adjacent areas
to induce defenses there before those areas are attacked.
Thousands of papers have been published on this topic. It is a subject
of great commercial interest.
But the story has become less simple …..
Inducibility – what is it? Why has it evolved?
Cost saving – only make a defence when needed?
But Agrawal & Karban give some alternative theories
• Making the plant harder to find
• Reducing the rate of development of resistance to
defensive chemical
• Reduce autotoxicity
• Reduces chances of negative side effects
• and many more ……..
Inducibility – what is it? Why has it evolved?
“Cross talk”
As people began to investigate the
stimuli (“elicitors”) evoking defense
reactions in plants, and in particular
after the developed methods to
analyse changes in gene
expression, it became clear that the
various pathways interacted. This is
termed “cross talk”.
What can the Screening Hypothesis add to this debate
about cross talk?
A -> B
A -> B -> C -> Insecticide?
A -> B -> C -> Fungicide?
A -> B -> C -> Attractive scent?
A -> B -> C -> Colour?
How are appropriate regulatory systems evolved when
pathways can serve multiple roles?
How are appropriate regulatory systems evolved
when pathways can serve multiple roles?
Gene 1
A
Enzyme 1
B Antifungal
w hich cells
w hen
w hich factors (light, cold, insect attack, fungal attack, etc.)
A lot of other proteins and machinery needed for this, hence a big investment
How are appropriate regulatory systems evolved
when pathways can serve multiple roles?
Gene 1
Gene 1'
A
Enzyme 1
Is it necessary to duplicate
the control processes at
every step?
B Antifungal
Enzyme 1'
C
Repels a herbivore
How are appropriate regulatory systems evolved
when pathways can serve multiple roles?
Gene 1
Gene 1'
A
Could it be that most NP pathways
have several common control points
near the start which allow many
stimuli to upregulate them?
Enzyme 1
B Antifungal
Enzyme 1'
C
Repels a herbivore
Thus insect or fungal attack might
turn a pathway but which product, if
any, is important could be unclear.
If cross-talk is inevitable, inducibility becomes a
poor predictor of function
Cross-talk is the term used to describe the fact that certain control
systems seem to interact. A particular gene turned on by fungal
attack might also turned on by insect attack. Cross talk is very
fashionable but was without explanation. Maybe we now have an
explanation.
Inducibility has often been an important part of the evidence
produced in favour of any one NP playing a specific role in a
particular circumstance.
Organism – organism interactions
Summary
Nearly all such studies of the role of NPs in the organisms that
make them have been conducted using the chemical co-evolution
model to guide the design and interpreation of the experiments.
The SH suggests that scrutiny will now be needed.
Whether greater progress is made in future …..
Vermouth is a fortified wine
flavored with NPs in
recipes that are closelyguarded trade secrets. The
inventor of vermouth,
Antonio Benedetto
Carpano from Turin, Italy,
chose this name in 1786
because he was inspired
by a German wine fortified
with wormwood, an herb
most famously used in
distilling absinthe.
(Wikipedia)
Pt 2 – Bichemical evolution.
The bigger picture
The Screening Hypothesis is now part of a
bigger model
The Screening Hypothesis is now part of a bigger
model
Lipids
The Screening Hypothesis is now part of a bigger
model
Colours – carotenoids, anthocyanins
Matrix pathways with low substrate specificity enzymes found
making chemicals with useful physicochemical properties.
Reservoir for more chemical diversity?
The Screening Hypothesis is now part of a bigger
model
The Screening Hypothesis is now part of a bigger
model
Terms such as primary metabolism,
secondary metabolism and even NP are
not really meaningful classifications.
 How science works. One simple
idea - potent biological activity is rare
- led to renewed debate. The debate
did not need:
 amazing new instruments
 expensive new techniques
 a huge input of human
resources
Just a little knowledge, some
imagination, simple logic and thought.
The knowledge was not even new - it
had been widely available for
decades.
Teaching you to think creatively is
Thanks
Clive Jones
http://www.scielo.br/scielo.php?pid
=S010040422000000100019&script=sci_
arttext