Download The Major Transitions in Evolution

A biológia néhány
megválaszolatlan nagy kérdése
Szathmáry Eörs
ELTE Biológiai Intézet
Theodosius Dobzhansky (1900-75)
• „A biológiában
minden csak az
evolúció fényében nyer
értelmet”
• Mi?
• Hogyan?
• Miért?
Az evolúció egységei
1. sokszorozódás
2. öröklődés
3. változékonyság
Vannak olyan öröklődő
bélyegek, melyek a
termékenységre és/vagy
a túlélőképességre
hatnak
A replikáció eredete?
• Kémiai szempontból mindig autokatalízist
igényel
• A legegyszerűbb forma:
A+X2A+Y
• A biológia számára kiemelt jelentőségű
• Sokkal általánosabb, mint a DNS-replikáció
A formóz ‘reakció’
formaldehid
autokatalízis
glikolaldehid
Butlerow, 1861
Replikáció a formóz reakcióban
•
•
•
•
•
Nem-információs replikáció
Autokatalízis – IGEN
Öröklődés – NEM
Jó az anyagcseréhez
Nem jó genetikai célokra
A fordított citrát-kör
korai eredete?
• Günter
Wächtershäuser
(1990)
• CO2-fixálás piritfelszíneken, a
mélytengeri
hévforrásoknál
• Nincs rá bizonyíték
Az élet keletkezésének a fő
problémája a metabolitok kellő
kanalizásása a lehetséges reakciók
terében
• Az enzimek a fontos reakciókat a kellemetlen
reakciókhoz képest gyorsítják
• A spontán mellékreakciók száma rendkívül nagy
• A fenntartás, nemcsak a szaporodás, feltétele az
autokatalízis
dx/ dt = k x – d x = 0
Minden, a mellékreakciókat
elhanyagoló „hálózati” modell
bűnösen csökött
• Pl. a Kauffman által javasolt fehérjehálózatok
• A modellekben általában azt feltételezik,
hogy a lehetséges reakciók vagy előnyösek,
vagy semlegesek
• Egy kémiai „kanyonból” jött ki az élet?
A kémiai evolúció az élet kialakulása
és a kátrány képződése közötti
verseny volt
Kémiai hálózatok
Élet
Kátrány
A bolygók hanyad részén lenne
végül csak kátrány?
1986: az első mesterséges replikátor
És mi van, ha hosszabb?
• Kísérlet és elmélet nemes versenye
Esetleg hőmérsékleti oszcillációval?
A spontán hosszabbodás gátolja a
replikációt!
The extended genetic alphabet
New base pair in the code for new
amino acid
• Implementation with
isoC:isoG pair
• UAG nonsense codon for
iodotyrosine
• Or the (iso-C)AG codon
• Challenge: coupling of
non-standard amino acids
to nonstandard tRNAs by
nonstandard synthetases
Fisher’s (1930) question
"No practical biologist
interested in sexual
reproduction would be led to
work out the detailed
consequences experienced by
organisms having three or
more sexes; yet what else
should he do if he wishes to
understand why the sexes
are, in fact, always two?"
The fitness of ribo-organisms
W(N) = A(N) Q(N)
(vö. Eigen, 1971)
W
fitness
A
reproduction rate
Q
fidelity of replication
N
size of the alphabet
W(N) = A(N) Q(N)
There is somewhere an optimal N
Mi hajtotta a kambriumi
robbanást?
A „robbanás”
Before the Cambrian…
Cnidaria were some of the
earliest fossils recognized as
animals. This death
assemblage of Vendian
cnidarians (medusoids)
predate the Cambrian
Explosion by tens of millions of
years. These jellyfish-like
examples of Nemaia simplex
are relatively attributable to
jellyfish descendants alive
today. The incredible softbodied preservation is
believed to be the result of
impressions made in a
microbial mat contained within
the sand, a phenomenon that
is exceedingly rare in the fossil
record.
Temporal distribution (bars) and stratigraphic occurrences (black dots) of representative Ediacara genera, plotted against timescale
of Ediacaran Period and fossil localities or stratigraphic units. The three Ediacara assemblages (Box 1) are indicated by different
shades of gray. The Marinoan and Gaskiers glaciations, as well as the age range of the Doushantuo biota, are also marked.
Explanations
Environmental Explanations
The accumulation of enough oxygen to sustain large
animals, large-scale and environmental change that
occurred at about the time of the initiation of the
radiation.
(a) the geological evidence that points to the Snowball
Earths
(b) the large negative carbon isotopic anomaly at the
precambrian/Cambrian boundary, followed by a period
of unusual volatility,
(c) paleomagnetic evidence for true polar wander
(whereby, on geological timescales, the continents
rapidly shifted their positions).
Explanations II.
Developmental Explanations
1.
2.
3.
4.
A small kit of key developmental genes was used in a new flexible way
and generated the Cambrian diversity.
Dynamical patterning modules (units that mobilize physical processes
characteristic of chemically and mechanically excitable meso- to
macroscopic systems such as cell aggregates: cohesion, viscoelasticity,
diffusion, spatiotemporal heterogeneity based on lateral inhibition and
multistable and oscillatory dynamics) generated all the body plans that
emerged in the Cambrian.
Stress due to climatic and chemical changes (snow ball earths) could
have activated stress-managing genes (such as the gene coding for
Hsp90) leading to great increase in variation.
miRNA may be involved: it is present in eumetazonas and is absent in
basal groups that lack organs including sponges, cnidarians, and acoel
flatworms
Several workers point to the origin of the bilaterian developmental system,
including the origin of the Hox genes, etc., as the primary cause of the origin
of the bilaterian developmental “explosion”
‘Dynamical
patterning modules (DPMs) generated the multicellular forms
during the explosive radiation of animal body plans in the middle Cambrian,
approximately 530 million years ago; multicellular organisms could have explored an
extensive morphospace without concomitant genotypic change or selection for
adaptation.
A role for miRNA??? (Peterson)
A core set of microRNAs is conserved in
animal with organs (protostomes and
deuterostomes), but is absent in basal
groups that lack organs including sponges,
cnidarians, and acoel flatworms. In
addition, the continuous acquisition and
fixation of miRNAs in various animal groups
strongly correlates both with the hierarchy of
metazoan relationships (A) and with the
non-random origination of metazoan
morphological innovations through geologic
time. This suggests that microRNAs could
have played a pivotal role in evolution of
both animal body plans and morphological
complexity (B), with organs being the
ultimate manifestation of this
innovation. And because microRNAs
regulate tens to hundreds of protein-coding
genes, they may also constrain the future
evolution of animal taxa (C bottom).
Animal miRNAs and morphological
complexity
Trichoplax
• Oikopleura:
urchordate
Tissue-specific miRNA expression
in the bilaterian ancestor
miRNA expression onset is
coupled to differentiation
miRNA and siRNA systems
Animal miRNA biogenesis
Plant miRNA biogenesis
Earliest possible stages of evolution
of the eukaryotic RNAi protein
machinery
Prokaryotic connections
Origin of eukaryotic miRNA
machinery
• Comparative genomic analysis shows that the protein machinery of
RNAi is conserved in all major eukaryotic lineages, independent loss
in many unicellular forms notwithstanding.
• It appears most likely that LECA possessed relatively complex RNAi
machinery. At a minimum, this primordial RNAi machinery consisted
of an Argonaute-like protein, a Piwi-like protein, a Dicer, and an
RNA-dependent RNA polymerase.
• Comparison of these proteins with their prokaryotic homologs suggests
that, during eukaryogenesis, the protein machinery of RNAi was
pieced together from archaeal, bacterial and phage proteins that
performed diverse functions in DNA repair and RNA processing,
unrelated to RNA interference.
Origin of miRNAs from genomic
repeats or transposable elements
in animals
Explanations III
Ecological explanations – arms races
Evolution of predation
Hard body parts
The Eye
The nervous system
Associative learning
Andrew Parker : In the Blink of an Eye 2003. Perseus
Books
Parker suggests that the Cambrian explosion was caused by the development of the
eye, an organ that not only senses light, but can see an image. He states that the
trilobite evolved its eye about 543 million years ago, which would have been in
time to help ignite the Cambrian explosion. Once eyes developed, there would
have been a revolution in predation. According to Parker, the period before the
Cambrian ''was rather an experimental stage for predation, occupied mainly by
peace-loving vegetarians that were willing enough to accept any occasional animal
matter they stumbled upon.'' But once the ''light switch'' was turned on, the
randomness of the hunt was gone. Since every animal could be seen -- predators
included -- the rules of survival changed. ''So to adapt to vision,'' Parker writes, ''an
animal must evolve a response in terms of adapting its visual appearance, whether
it is warning shapes and colours, camouflage or hiding behind physical barriers.''
The Eye?
Illustration of a possible evolutionary path for the development of the basic eye types Andre
Metazoa
Bilateria
Triploblastica
LBCA
Nephrozoa
LCTA
Deuterostomia
Placozoa [-]
Porifera [-]
Ctenophora [-]
Cnidaria [-]
Acoelomorpha [??]
*
protostomia
Ecdysozoa
Lophotrochozoa
Chordata [+]
Nematoda [+]
Platyhelminthes [+]
Echinodermata [?]
Arthropoda [+]
Mollusca [+]
Annelida [+]
Classical (Pavlovian) conditioning: a neutral stimulus is paired with a
primary stimulus which elicits a specific response (the response to this
stimulus in either inborn, or was learnt earlier). The neutral stimulus is
neutral with respect to this specific response, and precedes the primary
stimulus. An animal that exposed to the paired stimuli will first respond to
the primary stimulus, but later will respond to the neutral stimulus alone.
Pavlov presented dogs with food, and measured their salivary response
(how much they drooled). Then he began ringing a bell just before
presenting the food. At first, the dogs did not begin salivating until the food
was presented. After a while, however, the dogs began to salivate when
the sound of the bell was presented. They learned to associate the sound
of the bell with the presentation of the food.
Cubozoa
Increased size required better motor and sensory
coordination and neural integration, and was
associated with bigger, metabolically-expensive
brains and ganglia. Active life-span was probably
extended, so learning the associations among cues
became worthwhile. The initial stages of associative
learning evolution may have required only very
simple modifications in the threshold concentrations
of molecules that strengthened synaptic connections
between newly-linked, co-activated neurons.