Transcript Bicoid-nanos - Studentportalen

Reaction norms and genetic
assimilation
Two individuals of a single clone of the Asian and African water flea, Daphnia
lumholtzi. The individual on the left was exposed to chemical cues from
predaceous fish (induced); the individual on the right was not (control).
Sundew: green & red morphs
H1: Morphs have different genotypes.
H2: Morphs have same genotype but developed in different environments.
If H2, why should the plant have the capability of producing different forms?
3.3 Two morphs of Araschnia levana, the European map butterfly
What are the questions?
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Can flexibility be an adaptation?
What is phenotypic plasticity?
How does p.-p. contrast with canalization?
What aspects of phenotypic plasticity are
heritable?
How can we test the hypothesis of
adaptive plasticity?
Phenotypic plasticity defined
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environment-dependent
phenotypic expression
identical genotypes are
exposed to different
environments
traits of interest measured
Antonym: canalization
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a developmental process that produces the
same phenotype in spite of environmental
variability (= homeostasis)
reduces the effect of environmental “noise”
variability in a trait in a population represents a
balance between plasticity and canalization
Measuring phenotypic plasticity
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Norm of Reaction
Different phenotypes produced by the
same genotype in different environments
“X- axis” represents range of
environmental conditions
“Y-axis” represents the resulting
phenotype
Forms of Norms of Reaction
Red lines indicate optimal
reactions
Arrows indicate optimal
phenotypes for each
environment
Norm of Reaction curves for biomass
allocation in Polygonum
From Sultan, S. E. 2003. Phenotypic plasticity in plants: a case study in
ecological development. Evolution & Development 5:25-33.
Norms of reaction for leaf area
(morphological plasticity)
Norms of reaction for reproductive plasticity
(lifetime output)
Is phenotypic plasticity an adaptation?
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It depends
Are patterns heritable?
Do “plastic” genotypes have more success
than “canalized” genotypes?
Nijhout, H. F. 2003. Development and
evolution of adaptive polyphenisms.
Evolution & Development 5: 9-18.
What is a “polyphenism”?
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Two or more discrete alternative
phenotypes, without intermediate forms
Why, in nature, are there just 2 forms?
3.4 Two morphs of Nemoria arizonaria
Hormonal & developmental programs influence
polyphenism: a heritable mechanism
Correlated characters in beetles:
size & mating tactics (variation in success)
Other examples of polyphenism
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castes of social insects (Nature 26 Oct 2006)
alternative seasonal form of insects
alternative leaf forms in plants
predator-induced polymorphism in
cladocerans polymorphism in migratory
behavior of locusts
sedentary vs. dispersal morphs of aphids &
other animals
Is Polyphenism adaptive? - tadpoles
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Relyea, R. A. 2005. The heritability of inducible
defenses in tadpoles. J. Evol. Biol. 18: 856-866.
Tadpole norms of reaction
Heritability of Plasticity
What is different about phenotypic
plasticity as an adaptation?
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Compared to selection on a continuous
trait influenced by additive genetic
effects?
Consider rate of response
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in context of rate of environmental change
in context of shifting gene frequencies
As a response to spatial heterogeneity?
Spea hammondii
Pelodytes punctatus
Scaphiopus couchii
Pelobates syriacus
Genetic assimilation: the work of
Schmalhausen and Waddington and
an introduction to systems biology
Seen as vague competitors…
Schmalhausen (1884-1963)
Waddington (1905-1975)
The role of selection in evolution
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Schmalhausen noted that there are two
opposing tendencies in evolution:
selection, which tends to lead to
phenotypic change; and heredity, which
tends to lead to phenotypic conservation.
He tried to come up with a synthetic
theory that reconciled these two forces.
Mutations and environment
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For example, it is wrong to think about
mutations producing a definite phenotypic
effect:
E.g. The case of vestigial
Female
Male
Male
Female
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Shows that the effect of mutations is both
variable and often dependent on the
environment
Basis for this thermosensitivity has been further
examined.
In the wildtype, the amount of vg transcribed
stays constant with temperature. So this
suggests that low levels of normal vg are what
trigger the temperature dependency of the
system.
But mutations not needed!
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Introduces the concept of “phenocopies”:
environmentally induced morphologies
that mimic mutations.
Temperature shocking Aglais urticae produces phenocopies of geographic
variants. (A) Usual central European variant; (B) heat-shock phenocopy
resembling Sardinian form; (C) a Sardinian form of the species. (After
Goldschmidt 1938.)
The Baldwin Effect
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“A New Factor in Evolution” - Baldwin, 1896
Argued that behaviour could alter selective
regime. In time, selection would reinforce that
behaviour until it became instinctual
Note that this effect converts phenotypic
plasticity into genetically determined behaviour!
Has always been controversial
The classic example of ostriches
How did the ostrich get its
calluses?
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Calluses form where the ostrich’s skin is
under pressure by contact with the ground
when it sits.
But ostriches actually hatch with such
calluses!
Baldwin’s explanation
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When ostriches sit on the ground, the
pressure on the skin causes calluses to
form.
However, this reaction will be variable,
and under genetic control.
Hence, those ostriches that best form
calluses will be selected for.
Baldwin’s explanation part II
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In time, assuming a constant environment,
the population as a whole will form
calluses more reliably and with less
stimulus.
And in time, why need any stimulus at all?
Similar arguments are ongoing about the
origin of language in humans.
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Baldwin’s work was theoretical, but
Waddington provided some experimental
evidence for just such an effect, which he
called “genetic assimilation”, and which
Schmalhausen called “stabilising selection”.
Also controversial.
Waddington’s Fruitfly experiments
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Waddington took a population of
Drosophila and dumped their eggs in
ether.
Rather surprisingly, a proportion of the
subsequent adult flies produced a
bithoracic phenotype
This is of course a phenocopy of a wellknown set of homeotic mutations
Actually a set of homeotic mutations!
Waddingtonian fruit fly.
Note he has removed
anterior wings for
clarity!
Selection on Waddington’s flies
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Waddington selected the flies that
produced the best attempts at wings and
bred from them. Of course, their offspring
produced normal wings.
But he also dumped the eggs in ether, and
again the population produced some
bithorax flies.
Continued selection
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He carried on doing this, and noted two changes
in the population:
I) On average, the flies produced better wings.
II) More of them did so.
With some complications, on the 29th
generation, flies started producing transformed
halteres even without treatment!
Even further selection…
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By breeding from the bithoracic flies in the 29th
population, Waddington managed to eventually
fix the phenotype: all the flies produced it in the
population without ether treatment.
Conversely, the down selection experiments
produced the opposite effect: they produced
flies that did not respond to ether treatment.
Another experiment: cross-veins in
wings
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Waddington also showed that when fly
eggs were heat shocked, a certain wing
character appeared, that could also be
selected for and would eventually appear
without any heat shocking.
Reactions to G.A.
‘...
[genetic assimilation] is an interesting but, I would judge,
relatively minor outcome of the [synthetic] theory’
(Simpson,1953)
‘It represents merely a degeneration of a part of an
original adaptation’ (Williams, 1966)
‘A baroque hypothesis’ (Orr, 1999).
The genetics of genetic assimilation!
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Hsp90 (heat shock protein): the chaperone
molecule
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Plays a role in stabilising several unstable
developmental pathways, for example, steroid
hormone receptors.
Its function can be knocked down or out using the
drug geldanamycin, or by mutations
Rutherford and Lindquist found that when this was
done, a variety of abnormalities appeared in various
systems. These could be selected for and would
eventually appear no matter what the Hsp90 levels
were.
C after selection…
Can one have “internal assimiliation”?
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Budd (1999, 2006) suggested that a
process analogous to genetic assimilation
could take place within the genome too.
May help explain how deep developmental
shifts can take place without
developmental disasters!
Example: bicoid, nanos
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In Drosophila, maternal mRNA is
deposited in the egg from the nurse cells
at the anterior. Bicoid stays put; nanos is
transported to the posterior.
The mRNA is then translated, and a
double gradient is set up. This acts as the
framework to establish the A-P axis along
which future differentiation
(segmentation, hox genes etc) takes
place.
Bicoid and nanos regulate the translation
of two other maternal genes; hunchback
and caudal (nanos inhibits hunchback).
Hunchback seems to be a mistake
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As all nanos does is repress it. If you
knock out both, flies develop normally!
What about bicoid?
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Its sequence is similar to Hox3!
It is only found in the higher dipterans!
Bicoid-nanos is derived.
Higher dipterans are very “long germ band”. Ancestral AP patterning very different
Example two: “homeotic takeover”
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How do Hox gene expression patterns
evolve?
Gellon and McGinnis - hopeful monsters!
Selective pressure?
Budd 1999
But there is another way…
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…of looking at this, which is more
subversive.
Development can be seen as a
cooperation between genes and
environment, of variable influence of both.
I will discuss extreme “environment”
issues tomorrow.
Environmental stress…
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…leads to altered developmental outcomes.
Through time, and through selection, there is
true genetic alteration of development too (not
necessarily involving mutation, of course!).
Plan A (environmental stress plus development
1) is replaced by Plan B (development 2, without
environmental stress).
Thus, the same morphologies in Plan A and Plan
B have different genetic backgrounds - Roth’s
“genetic piracy”; Weiss and Fullerton’s
“phenogenetic drift”.
Gene pirates and selfish genes
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All this implies a systems approach to
evolution. Selection truly acts on the
phenotype; any genotype that generates a
favoured phenotype under particular
conditions will be selected for; and one
can trace, via genetic assimilation
experiments, precisely this process.
Gene pirates and selfish genes (ii)
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And if within a population, several equivalent
genotypes persist, they can shift via drift.
Implication is that selection acts at the whole
organism level, not at the genetic level. In this
way of thinking about things, genetic networks
are the slaves of the phenotype, not the other
way round.
Strict genetic encoding may not be the only
repository of information that is stable enough
to enable selection to take place.
Remember heterochrony
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May be environmentally induced (e.g.
temperature affecting speed of
development) - could become genetically
assimilated?
Cf Island dwarfism
23.20 Co-option of signal transduction
pathways in the formation of butterfly
eyespots