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

Vannak olyan öröklődő bélyegek, melyek a termékenységre és/vagy a túlélőképességre hatnak

1. sokszorozódás

2. öröklődés

3. változékonyság

A replikáció eredete?

• Kémiai szempontból mindig autokatalízist igényel

• A legegyszerűbb forma:

A + X 2 A + Y

• A biológia számára kiemelt jelentőségű

• Sokkal általánosabb, mint a DNS-replikáció

A formóz ‘reakció’

formaldehid

glikolaldehid

autokatalízis

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 pirit-felszíneken, 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 fehérje-

hálózatok

• 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”

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 soft-bodied 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.

Before the Cambrian…

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 ExplanationsThe 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 Explanations1. A small kit of key developmental genes was used in a new flexible way

and generated the Cambrian diversity.2. 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.

3. 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.

4. 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.

Animal miRNAs and morphological complexity

• Oikopleura: urchordate

Trichoplax

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.

Explanations III

Ecological explanations – arms racesEvolution of predationHard body partsThe 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

Platyhelminthes [+]

Mollusca [+]

Annelida [+]

Nematoda [+]

Arthropoda [+]

Chordata [+]

Echinodermata [?]

Acoelomorpha [??]Cnidaria [-]Ctenophora [-]Porifera [-]Placozoa [-]

Metazoa

LBCA

LophotrochozoaEcdysozoa

protostomiaDeuterostomia

Nephrozoa

Bilateria

Triploblastica

LCTA

*

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.