T. WenseleersUniversity of Sheffield2002

Wolbachia: microbial manipulator of insect reproduction

Wolbachia: microbial manipulator of insect reproduction

T. WenseleersUniversity of Sheffield2002

Selfishness & Altruism course

Conflicts in societies

Intragenomic conflict

Similar conflicts occur among genes within individual organisms

“Intragenomic conflict”

E.g. conflict– between genes on homologous

chromosomes over transmission to gametes (meiotic drive)

– between nucleus and cytoplasm over optimal sex-ratio (cytoplasmic sex-ratio distorters)

– between cells over who ends up in the germ-line

Meiotic drive

normal Mendeliansituationeach homologue transmittedto half of the gametes

meiotic drive gene transmitted to all gametes (but only half as much sperm produced)

section through sperm bundle

Cytoplasmic sex-ratio distorters

Cytoplasmic symbionts that manipulate their host into producing a female-biased brood

Benefits their transmission to future generations because of their exclusively maternal inheritance

Mechanisms– Selective killing of male offspring / function

– Feminisation of genetic males

– Induction of parthenogenesis

– Increasing the fertilisation frequency in male-haploids

W.D. Hamilton (1936-2000)

“Extraordinary sex-ratios”, Science 1967

Wolbachia

Example of a Cytoplasmic Sex-Ratio Distorter

Alpha-proteobacterium

Occurs in 15-75% of all insects + in crustacea, spiders and nematodes

Biases sex-ratio via– Male killing– Feminisation– Parthenogenesis induction

May cause mating incompatibilities

High temperature or tetracycline cure the host

MK

F

PI

First observed by Hertig & Wolbach (1924)

Intracellular rickettsial bacterium in ovaries of mosquito Culex pipiens

“Wolbachia pipientis”

Amplification of Wolbachia DNA up to detectable levels has become possible using PCR-techniques

Cloning and sequencing of various genes (16S rRNA, ftsZ, wsp) allows detailed analysis

The DNA revolution

0.1 changes per nt

EUBACTERIAARCHAEBACTERIA

EUCARYA

HomoCoprinus

Paramecium

Naegleria

pSL 22

pSL 50Thermofilum

Methanospirillum

Methanobacterium

Thermococcus

Thermotoga

ThermusSynechococcus

Bacilllus

CytophagaChlorobium

WolbachiaE. coliRiftia

macroscopic

organismsZea

Porphyra

Dictyostelium

EntamoebaEuglena

Trypanosoma

Physarum

Encephalitozoon

Vairimorpha

GiardiaHexamita

Tritrichomonas

pJP 78

pJP 27marinegroup1

pSL 12

pSL 4

ThermoproteusSulfolobus

Haloferax

Methanosarcina

Methanococcus

Methanopyrus

EM 17Aquifex

Thermomicrobium

chloroplastEpulopiscium

mitochondriaChromatium

origin

C. WoeseC. Woese

Scott O’NeillUniversity of Queensland, AU

Richard StouthamerUniversity of California,

Riverside Ary Hoffmann,La Trobe, AU

Jack Werren,University of

Rochester

Gregory Hurst,UCL, London

Phylogeny

Oth

er a

lpha

pro

teobacte

ria

Ehrlichieae

Neorickettsia

Gamm

a

prote

obac

teria

0.1

Wolbachia

CaedibacterMtK

MitochondriaCMS

Orientia MK

Rickettsia MK

No match with host phylogeny

pratensis

lemani

fusca

rufa

O

100

99

polyctena

truncorum84100

0.02(10 MY)

...and their symbionts

rufa

polyctena

pratensis

truncorum

lemani

fusca

O

Formica hosts...

Gyllenstrand, unpublished

Male killing

Selective killing of males

In Tribolium and ladybird beetles, Drosophila and Acraea butterflies

Increases the survival of sisters in the same brood, who carry copies of the maternal element

Kin selected benefit

Male killing in ladybird beetle

Male killing

Causes cost at population/species level(dearth of males, decreased female mating success)

E.g. in Acraea encedana : 96% of all wild-caught butterflies are female

Still male killing remains selected for since even at high frequency a sex-ratio distorter transmits more of its genes to future generations than a symbiont not distorting the sex-ratio

Feminisation

In woodlice

Causes genetic males (ZZ) to develop as ZW females

Works by suppressing the androgenic gland

Also causes cost at population/species level(dearth of males, decreased female mating success)

Induction of parthenogenesis

Induction of asexual reproduction, resulting in an all-female brood

Sex-ratio benefit + avoids cost of sex

Occurs in various parasitoid waspse.g. Trichogramma, Muscidifurax, Aphytis, Diplolepis

Restoration of diploidy via gamete duplication

Other Examples

Feminization– microsporidia in Amphipods

Male killing

– Spiroplasma and Rickettsia in Drosophila and ladybird beetle

– Arsenophonus nasoniae in Nasonia vitripennis (parasitoid jewel wasp)

– microsporidia in mosquitoes

Other Sex Ratio Distorter Types

Cytoplasmic male sterility– cf. male killing, but in plants– male function inhibited

Increased fertilization frequency– in haplo-diploids:

fertilized eggs females– “maternal sex-ratio”

Cytoplasmic male sterility

in approx. 4% of all hermophrodite plants

determined by mitochondrial gene

mitochondria kill themselves when they find themselves in tissue of male function

nuclear genes may suppress CMS

“Maternal sex-ratio”

manipulates her host (Nasonia) to fertilise more eggs than she is selected to

Nasonia is haplodiploid, so fertilised eggs develop as females.

exact nature unknown

NormalOffspring

Production

Reduces fitness of Uninfected Female x Infected Male Crosses

Gives an advantage to infected females

Sterility in diploids, but production

of males only in haplo-diploids

Cytoplasmic incompatibility

Inviable

++--

----

--

++++

++

Mechanism

Condensation of paternal genome in infected male

Rescue by Wolbachia in egg upon fertilisation of infected oocyte

CI may drive speciation Unidirectional incompatibility

• UNINFECTED FEMALE x INFECTED MALEincompatible

• other crosses unaffected

Bidirectional incompatibility

• A STRAIN INFECTED FEMALE x B strain INFECTED MALEincompatible and vice versa

• may drive sympatric speciation

Wolbachia in nematodes

Mutualistic– Wolbachia required for nematode

reproduction– Nematodes die when treated with tetracycline

Strict host-parasite coevolution(concordant phylogenies)

Offers new avenues for treatment of filarial diseases

Concluding remarks Two ways for Wolbachia to increase its fitness

– Increase host fecundity (cf. nematodes)– Manipulation

How are conflicting genetic interests resolved?

Parliament of the Genes? (E. Leigh)

Majority Interests Prevail

small Wolbachia genome powerless against a large autosome ?

Take-home questions

1. Meiotic drive genes often do not go to fixation because drive/drive homozygotes tend to be near-sterile. If k is the fraction of drive gametes produced by a drive/wild type heterozygote and H is the fitness of a drive/drive homozygote, what is the equilibrium frequency of the drive allele in the population?

2. Are male killing elements selected to kill just as many males in large as in small populations? In the limit where on would have a population of only one female, what should the male-killing symbiont do?

Game theory(hawk-dove game)

0 -B

B -C

DOVE HAWK

DO

VE

HA

WK

Maynard Smith & Price 1973

Solving for an ESS

Fitness player 1 wFitness player 1 w11 =B.z =B.z11-B.z-B.z22-C.z-C.z11.z.z22

zz1 1 en zen z22= phenotypes of players 1 & 2= phenotypes of players 1 & 2

(hawk=1, dove=0)(hawk=1, dove=0)

Personal benefit of playing hawk = influence of own Personal benefit of playing hawk = influence of own

behaviour on own fitness = behaviour on own fitness = ww1//zz1

= B-C.z= B-C.z2 2 (depends on what other player does)(depends on what other player does)

At equilibrium benefit = B-C.zAt equilibrium benefit = B-C.z2 2 = 0, and the ESS is to play = 0, and the ESS is to play

hawk with a probability of z*=B/Chawk with a probability of z*=B/C

- SYNERGY- SYNERGY

COOPERATE DRIVE

CO

OP

ER

ATE

DR

IVE

(1-k)1/2

k H.(1/2)

Meiotic drive

Solution problem 1

If z1 and z2 are the probabilities that genes on chromosomal homologue 1 and 2 play as drive alleles, we can write the fitness of the gene on homologue 1 as

w1 = (1-z1).(1-z2).(1/2) (neither of them drive, hom. 1 gets half of the gametes)

+ z1.(1-z2).k (homologue 1 shows drive, the other hom. does not,

hom. 1 then gets a share k > 1/2 of the gametes)

+ (1-z1).z2.(1-k) (homologue 1 is mendelian, the other hom. shows drive)

+ z1.z2.(H/2) (both drive, each get half of the gametes, but drive/drive type only produces a fraction H of the gametes of the normal wild type)

The benefit for homologue 1 to drive with a probability z1 is

D[w1,z1] (where D[] stands for a partial derivative - the idea is to calculate how your behaviour influences your reproduction)

= -(1-z2).(1/2)+k.(1-z2)-(1-k).z2+z2.H/2

Solution problem 1

As one can see, in a population where all chromosomes play the fair Mendelian strategy (z2=0), drive confers a benefit (when z2=0 the benefit=k-1/2). This benefit reduces as drive becomes more common, however, because of the cost that arises in drive/drive pairs.

The selective pressure to drive with higher probability stops when the D[w1,z1] drops to zero.

At that point the ESS (evolutionary stable state) is reached.

So at the ESS we have

-(1-z2).(1/2)+k.(1-z2)-(1-k).z2+z2.H/2 = 0

from which we can solve for the ESS strategy, z*=(2k-1)/(1-H),

i.e. if a gene drives with this probability it cannot be invaded by any other gene that drives with a another (higher) probability.

Solution problem 1Technically we have now derived what is known as a mixed strategy ESS, that is the situation whereby the players adopt a probabilistic strategy, e.g. drive or play hawk with a particular probability.

Alternatively, we could also have derived what is known as a pure strategy ESS where players play fixed strategies. The ESS will then be reached when these different types (genotypes) of players occur in some equilibrium mix in the population. Fortunately, it has been shown that for 2-player games the mixed and pure ESS always coincide so that the above result also describes the equilibrium frequency of a drive allele in a population (assuming that driving and non-driving alleles have a different genetic constitution, which is indeed the case).

A more orthodox way to calculate this result would be to construct a genetic model and write down a recurrence equation that describes how a drive allele increases in frequency from one generation to the next. The equilibrium frequency is then reached when the frequency of the drive allele does not change between generations. Hartl & Clark (1997) has a simple derivation.

Hartl, D. L. & Clark, A. G. 1997. Principles of population genetics. Sunderland, MA: Sinauer Ass.

The result is the same though, and which approach to use is a matter of taste.For a general overview of game theory as applied to problems in biology see

Maynard Smith, J. 1982. Evolution and the Theory of Games. New York: Cambridge University Press.

Solution problem 2

In a large population the best strategy for a cytoplasmic male killer is to kill all males in a brood so as to bias the brood sex ratio to a maximum extent.

In a very small population, however, biasing the sex ratio will also have costs to the females in the same brood, because they will be unable to find a male to mate with, and the Wolbachia will die with them. In a large population this wouldn’t matter since this cost would be carried by females in the population at large, which contain Wolbachia unrelated to actual male killing Wolbachia in the focal brood. In a small population, however, this is no longer true because it will be females of the very same brood that will have a reduced mating success as a result of the male killing.

In the limit where we would have a population of only one female, that produces offspring that mate among themselves, the best strategy for the Wolbachia would be to kill no males at all, and have the host produce an equal sex ratio (this assumes that one male is needed to fertilise one female).