Transcript The Major Transitions in Evolution

Mi öröklődik a géneken
kívül?
Szathmáry Eörs
Collegium Budapest
Eötvös University
Units of evolution
1. multiplication
2. heredity
3. variation
hereditary traits affecting
survival and/or
reproduction
The formose ‘reaction’
formaldehyd
e
autocatalysi
s
glycolaldehyde
Butlerow, 1861
The reductive citric acid cycle
Von Kiedrowski’s replicator
Peptide replicator networks
Classification of replicators
Limited
heredity
Holistic
formose
Modular
Von
Kiedrowski
Unlimited
heredity
genes
Limited
(# of individuals)  (# of types)
Unlimited
(# of individuals) << (# of types)
King (1980): evolution of the
coenzymes
• He looked at the metabolic maps then
• Coenzymes looked auto- and cross-catalytic
• BUT the situation is slightly more
complicated
• The idea nicely links to the assumed
primitive ancestry of coenzymes (related to
the idea of the RNA world)
An autocatalytic cycle in the given
environment
Although A is autocatalytic, it is not
strictly needed
Dependent on the environment!
Autocatalysis of the pair (A, B) is
more complicated, but easy to see
If this is big, you may not realize the
autocatalysts
The basic question
• Could one kick-start metabolism just with
external molecules and macromolecules
(genes an enzymes)?
• Influx  buildup of metabolism?
Metabolic networks
Membrane heredity
Principle of membrane heredity
Prions
Strain-specific prion propagation
Yeast and fungal amyloid prions
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•
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The soluble forms of the yeast proteins Ure2p
and Sup35p function in nitrogen regulation
and transcription termination, respectively.
Their amyloid forms are non-functional.
Soluble Rnq1p has no known cellular
function and the amyloid form can
sporadically prime polymerization of Sup35p
or Ure2p resulting in the generation of the
[PSI+] and [URE3] prions.
The soluble form of the HETs protein has no
known function, but its amyloid form is
necessary for heterokaryon incompatibility,
which is a limitation on the fusion of
neighbouringcolonies.
Red domains are apparently unstructured in
the native form and become amyloid in the
prion form. Green shapes are natively
structured domains.
Epigenetic inheritance
1. Structural inheritance (e.g. cortical
inheritance in ciliates)
2. Autocatalytic gene activity
3. Chromatin marking (e.g. methylation)
Genetic and epigenetics
Regulation of gene expression by
constitutive expression of a
protein
• After division the state is inherited because
enough protein is around
Stable and unstable epigenetic
markings
Inheritance of DNA methylation
patterns
Linaria flower inheritance
Linaria (gyújtoványfű)
• A naturally occurring mutant of Linaria vulgaris, originally described
more than 250 years ago by Linnaeus, in which the fundamental
symmetry of the flower is changed from bilateral to radial.
• The mutant carries a defect in Lcyc, a homologue of the cycloidea
gene which controls dorsoventral asymmetry in Antirrhinum.
• The Lcyc gene is extensively methylated and transcriptionally silent in
the mutant.
• This modification is heritable and co-segregates with the mutant
phenotype.
• Occasionally the mutant reverts phenotypically during somatic
development, correlating with demethylation of Lcyc and restoration
of gene expression.
• It is surprising that the first natural morphological mutant to be
characterized should trace to methylation, given the rarity of this
mutational mechanism
• in the laboratory.
• This indicates that epigenetic mutations may play a more signi®cant
role in evolution than has hitherto been suspected.
Somatic instability of peloric plants
Types of transmitted variation
Hypermutation in derepressed
operons
• Starvation for leucine in an Escherichia coli auxotroph
triggers metabolic activities that specifically target the leu
operon for derepression, increased rates of transcription,
and mutation.
• Derepression of the leu operon was a prerequisite for its
activation by the signal nucleotide, guanosine
tetraphosphate, which accumulates in response to
nutritional stress (the stringent response).
• A quantitative correlation was established between leuB
mRNA abundance and leuB2 reversion rates.
• These investigations suggest that guanosine tetraphosphate
may contribute as much as attenuation in regulating leu
operon expression and that higher rates of mutation are
specifically associated with the derepressed leu operon.
Types of mutation
Language is not Weismannian
soma
germ
soma
germ
protein
DNA
germ
sentence
Neural
germ
representation
protein
DNA
sentence
Neural
representation
RNA-directed DNA methylation
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•
•
•
Target loci (in this case tandemly repeated
sequences; coloured arrows) recruit an RNA
polymerase IV complex consisting of NRPD1A
and NRPD2 through an unknown mechanism, and
this results in the generation of a single-stranded
RNA (ssRNA) species.
This ssRNA is converted to double-stranded RNA
(dsRNA) by the RNA-dependent RNA polymerase
RDR2. The dsRNA is then processed into 24nucleotide siRNAs by DCL3. The siRNAs are
subsequently loaded into the PAZ- and PIWIdomain-containing protein AGO4, which
associates with another form of the RNA
polymerase IV complex, NRPD1B–NRPD2.
AGO4 that is ‘programmed’ with siRNAs can
then locate homologous genomic sequences and
guide the protein DRM2, which has de novo
cytosine methyltransferase activity.
Targeting of DRM2 to DNA sequences also
involves the SWI–SNF-family chromatin
remodelling protein DRD1. The NRPD1B–NRPD2
complex might generate a target transcript
(ssRNA) to which the AGO4-associated siRNAs
can hybridize.
Given that siRNAs homologous to some loci are
absent in drm2 mutants and ago4 mutants, it is
possible that DNA methylation (blue circles) also
stimulates siRNA generation and reinforces
silencing.
PcG-protein-mediated silencing
throughout the A. thaliana life cycle
•
•
•
•
•
•
•
•
a, FLC is transcriptionally active in seeds and
seedlings, preventing the plant from flowering and
prolonging vegetative development.
b, Exposure to a long period of cold (that is,
vernalization) results in the expression of VIN3
(red), which initiates repression of FLC
transcription, and the binding of the PcG protein
VRN2, as well as VRN1 and LHP1 (blue). In this
process, chromatin at FLC is epigenetically modified
by the trimethylation of H3K27.
c, After warmer temperatures return, FLC
repression is maintained, allowing flowering to be
induced by other cues.
d, During flower development, the anthers
and ovaries are sites of meiotic differentiation,
giving rise to haploid cells known as microspores
and megaspores, respectively.
e, These meiotic products undergo mitotic
proliferation to form the multicellular embryo sac
and pollen gametophytes.
f, PcG-protein-mediated repression at FLC is
removed during an undefined resetting process.
g, Then, the pollen contributes sperm nuclei to the
embryo sac, and these fertilize the haploid egg cell
and diploid central cell (not shown), forming the
embryo and endosperm (respectively) in a new seed,
in which FLC is re-expressed.
Chimpanzee culture
• Each chimpanzee community has its own unique array of traditions
that together constitute the local ‘culture’.
• ‘Customary’ acts are those typical in the community, ‘habitual’ ones
are less common but consistent with social transmission, and ‘absent’
acts are those missing with no apparent straightforward environmental
explanation.
• Traditions are defined as behaviour patterns that are customary or
habitual in at least one site but absent elsewhere.
• Transmission is attributed to social learning on the basis of a complex
of circumstantial evidence, ranging from intense observation by
juveniles to distributions inconsistent with alternative explanations.
The cultures of wild chimpanzees
The different social conventions
of neighbours: the grooming
hand-clasp
Tool-set for harvesting termites
Selective copying
Why is language so interesting?
• Because everybody knows that only we talk
• …although other animals may understand a
number of words
• Language makes long-term cumulative
cultural evolution possible
• A novel type of inheritance system with
showing “unlimited hereditary” potential
Design features of language
• Compositionality (meaning dependent on how
parts are combined)
• Recursion (phrases within phrases)
• Symbolicism (versus icons and indices)
• Cultural transmission (rather than genetic)
• SYMBOLIC REFERENCE and SYNTAX
Three interwoven processes
• Note the different time-scales involved
• Cultural transmission: language transmits itself as
well as other things, has its own dynamics