Symbiotic Sympatric Speciation:
Compliance with Interaction-driven
Phenotype Differentiation from aSingle Genotype
Kunihiko Kaneko (Univ of Tokyo, Komaba)
withTetsuya Yomo (Osaka Univ.)
experiment Akiko Kashiwagi, T. Yomo
（KK, T. Yomo; Proc. Roy. Soc. B, 267 (2000) 2367-2373；
KK；
Population Ecology, 44 (2002) 71-85）

Sympatric Speciation: Difficult?
If slight genetic change leads to slight phenotype changethen distinct organisms compete each other for the same niche
Coexistence : difficult ( unless neutral )Most theory for (sympatric) speciation so far:Search for a scheme that two groups are `effectively' isolated* `minimize‘ interaction --- separation in space, inmating etc.
??? origin of mating preference???
?? robust speciation?? (two species; not necessary)

* and/or almost neutral in some characteristics?? character essential to survival ??
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Recall underlying assumption in population genetics:
Selection by fitness=Function(Phenotype, environment)
but
assumption; Genotype ----> Phenotype: Single-Valued
determined uniquelythen Fitness=function(Genotype,environment)
individuals with little change in geneshould compete under the same niche

Reconsider Genotype-Phenotype mapping
Consider seriously G -------[Development] Æ P;** G-P relationship can be one to many
(1) Low penetrance (often in mutants?)(---observation)

non-unique phenotypes (often in mutants)(2) Interaction-induced differentiation in experiments
bacteria (E.coli) Shapiro, Yomo, …..
(3) Isologous diversification(---theory)

Measurement by fluoresecent
proteins
Character of bacteria differentiate in a crowded condition

Isologous Diversification:(adopted from cell differentiation model)internal dynamics and interaction : development
instability
distinct phenotypes
interaction-induceddx m= f m ( x1 , x 2 ,.., x k )dt

Example: chemical reaction network
specialize in the use of some path

phenotypeÆWith the increase of the number

Distinct types are formed through instability in ‘developmental
dynamics’ and interaction (both types are necessary)

Model with Evolution :Each unit Phenotype :: Variable X = ( X 1 , X 2 ,.., X k )Gene :: Parameter in the modele.g., reaction rate

( g1 , g 2 ,..., g k )Parameter

Variable (dynamical systems)
X(t=0)ÆX(t)Reproduction when maturity threshold condition(given by X) is satisfied
Mutation ---- small change in parameter in reproduction
Competition for survival:( remove some units (either randomly or under some condition))

Example of numerical simulation
Phenoptype(variable)

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Sympatric Speciation observed(1) First interaction-induced phenotype differentiation;
homogeneous state is destabilized by the interaction
e.g., by the increase of population, decrease of resources(2) Amplification of the difference through geno-pheno relation
Two groups form symbiotic relationship, and coevolve(3) Genetic Fixation and Isolation of Differentiated Groups
consolidated to genotypes(Example) chemical secreted out by one group are used as resources
for the other, and vice versa
create a niche each other and specialized in this created nicheCharacteristics of the Symbiotic Sympatric Speciation*Valid (possible) in the presence of strong interaction*Robust speciation; two groups coevolve(fig)*Genetic separation always follows if there appears interaction-induced
phenotypic differentiation(deterministic)*Fast and deterministic in nature*Relevance of the phenotypic differentiation,rather than genetic change, to genetic diversification

Stable under sexual reproduction?

i.e., stable against mixing of genes

Extension of the Model:* two individuals satisfying maturity cond. mate randomly to haveoffspring* offspring: mixed in genes (parameters) (and in loci)i,j ÆmSpeciation observed

gm between g(i) and g(j)Post-mating isolation

stable under mixing by sexual reproduction
because symbiotic speciation is robust…?still hybrid is formed…??

StageIÆIIÆIIIÆIVÆV

Parameter

Completion of speciation
Although hybrid is formed, but they cannot leave offspringinteraction-induced phenotype differentiation
genetic change
hybrid sterility (cf. definition of species)Æ Basis for mating preferenceEvolution of mating preference
extend the model to include loci for mating preference parameters:( ρ1 (i ), ρ 2 (i ),..., ρ k (i )) ⇔ ( X 1 (i ), X 2 (i ),..., X k (i ))if the other partner has

X m (i ) < ρ m ( j )i denies the mating with j and vice versa===> ρ m ( j ) < 0for all m, then no mating preference ;start from ρ m ( j ) < 0 later ρ m ( j ) > 0 for some m ==> evolution of mating
preference(postmating isolation first, premating isolation later)* Coexistence of the two species is further stabilized by thisTwo-allele case; correlation between two alleles is established??
Model with two alleles and random shuffling by mating

Speciation proceeds in the sameway;
Later, correlation between twoalleles are formed

Significance* fast speciation process once the condition is satisfiedÆpunctuated equilibrium; adaptive radiation* relevance of competitive interaction to speciationÆniche is created by each other(cf. Tilman)• relevance of developmental plasticity (ÅÆ the so-calledphenotypic plasticity)Ædifference of tempo in evolution by species•Degree of penetrance ( why low penetrance is frequent in mutants)* speciation in asexual and sexual reproduction: unified

Sympatric speciation can generally occur under strong interaction,
if the condition (for interaction-induced phenotype differentiation) issatisfied
Reversing the order:
phenotypic differentiation Æ genetic (cf Baldwin effect)(Æ postmating isolation Æ mating preference)verifiable in the speciation in Cichlid??
Doubting the conventional ordering???
Observed is `Correlation' A-B; but guess causal relationship A Æ B.
e.g., Allopatric speciation; spatial variation really cause?
(cf. the residence separation in city between rich/poor)
Sympatric speciation could be later consolidated spatially?

Two types, (blue and green) are speciated;
Later they start to be separated in space

Model with spatial location,Slowly moving, mating within some range

Plasticity in phenotype from loose dynamics Æ interaction-induced
phenotypic differentiation
Consolidated to Genes Æ Mating Æ Allele-correlation, Space..
Prove the above scenario?? From observation-- often remains a guess…
Real experiment wanted:
E Coli ; interaction-induced phenotypic differentiation observed
Evolution (Yomo’s group)
genetic fixation ---not yet; but

coexistence of diverse types by ‘crowded’ condition is confirmed

DNA Sequencing

Mutagenesis

genomeHost E. coliΔglnA strain

Mutant gene poolPlasmid DNA extraction

continuous culture

1st chemostat

1stW1mutant library
W1270 hA1W1W2A1W2

2nd chemostat

2nd mutant library

270 h

A2 A1 W3 W1 B1 C1W1 W2W3W4W5A1A2
B1C1

W2 W4 W5

W1A1

W1C1C1

3rd mutant library

3rd chemostat

552 h

A1

G1
E1F1
D1C3A1C2552552 hh

A1 A3

C1 C2 C3 D1 E1 F1 G1C1A3

Coexistence of several types of E coli under identical enviroment

W1

1st270 h

A1W1

2nd
2nd chemostat

mutant libraryThe fitness of W2 strain changes in accord to thechanges of its surrounding members.
NOTE: second chemostat population and thirdchemostat one.

W2mutant library

270 h A2A1 W3W1B1C1 W2W4W5

3rd chemostat

552 h

W1

3rd(2 / 0)

552 h

A1

A1A3

C1

mutant library

C1C2 C3D1E1F1 G1Frequency of W2 strain

1st chemostat

Does each mutant cell change its fitness through the cellular interaction?

110 7 cells /75ml

0.8
0.6

10 10 cells /75ml

0.4
0.200200400600

Time (h)Crowded condition rendering sufficient interaction among the cells can cause fitnesschange and lead to the coexistence of closely-related mutants.

Crowded Condition

Diluted Condition

L-glutamate + glutaminase

L-glutamate

L-glutamine
A1

L-glutamate

glutaminase

L-glutamate

Question:L-glutamine
W2

Is the coexistence
temporal orreproducible?
Frequency of W2 strain

L-glutamine

stochastic mechanisms
does not explain thecoexistenceContinuous Culture
10.8

Without glutaminase

0.6

With glutaminase* *0.4*0.8μM 1.9 μM

0.2*2.2 μM 1.3 μM

Leaked glutamine

00200400600Time (h)The cellular interaction through the glutamine leaked into themedium is necessary for the state of coexistence.

How do cellular interactions affect molecular evolution toallow genetic diversity in a population?
NH3 ATP ADP + Pi
L-glutamate

L-glutamine

Glutamine Synthetase
L-glutamate

L-glutamate

L-glutamine
Cell A

L-glutamate
L-glutamate

L-glutamine
Cell B

Continuous Culture