Presentation on theme: "T. Wenseleers University of Sheffield 2002 Wolbachia: microbial manipulator of insect reproduction T. Wenseleers University of Sheffield 2002 Selfishness."— Presentation transcript:
T. Wenseleers University of Sheffield 2002 Wolbachia: microbial manipulator of insect reproduction T. Wenseleers University of Sheffield 2002 Selfishness & Altruism course
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 Mendelian situation each homologue transmitted to 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 E UBACTERIA A RCHAEBACTERIA E UCARYA Homo Coprinus Paramecium Naegleria pSL 22 pSL 50 Thermofilum Methanospirillum Methanobacterium Thermococcus Thermotoga Thermus Synechococcus Bacilllus Cytophaga Chlorobium Wolbachia E. coli Riftia macroscopic organisms Zea Porphyra Dictyostelium Entamoeba Euglena Trypanosoma Physarum Encephalitozoon Vairimorpha Giardia Hexamita Tritrichomonas pJP 78 pJP 27 marine group1 pSL 12 pSL 4 Thermoproteus Sulfolobus Haloferax Methanosarcina Methanococcus Methanopyrus EM 17 Aquifex Thermomicrobium chloroplast Epulopiscium mitochondria Chromatium origin C. Woese
Scott ONeill University of Queensland, AU Richard Stouthamer University of California, Riverside Ary Hoffmann, La Trobe, AU Jack Werren, University of Rochester Gregory Hurst, UCL, London
No match with host phylogeny pratensis lemani fusca rufa O 100 99 polyctena truncorum 84 100 0.02 (10 MY)...and their symbionts rufa polyctena pratensis truncorum lemani fusca O Formica hosts... Gyllenstrand, unpublished
Male killing ò ò Selective killing of males ò ò In Tribolium and ladybird beetles, Drosophila and Acraea butterflies ò ò Increases the survival of sisters in the same brood, who carry copies of the maternal element ò ò Kin selected benefit
Male killing ò ò Causes cost at population/ species level (dearth of males, decreased female mating success) ò ò E.g. in Acraea encedana : 96% of all wild-caught butterflies are female ò ò 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 ò ò In woodlice ò ò Causes genetic males (ZZ) to develop as ZW females ò ò Works by suppressing the androgenic gland ò ò Also causes cost at population/species level (dearth of males, decreased female mating success)
Induction of parthenogenesis ò ò Induction of asexual reproduction, resulting in an all-female brood ò ò Sex-ratio benefit + avoids cost of sex ò ò Occurs in various parasitoid wasps e.g. Trichogramma, Muscidifurax, Aphytis, Diplolepis ò ò 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 ò ò in approx. 4% of all hermophrodite plants ò ò determined by mitochondrial gene ò ò mitochondria kill themselves when they find themselves in tissue of male function ò ò nuclear genes may suppress CMS
Maternal sex-ratio ò ò manipulates her host (Nasonia) to fertilise more eggs than she is selected to ò ò Nasonia is haplodiploid, so fertilised eggs develop as females. ò ò exact nature unknown
Normal Offspring 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 MALE incompatible other crosses unaffected Bidirectional incompatibility A STRAIN INFECTED FEMALE x B strain INFECTED MALE incompatible 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 DOVEHAWK DOVE HAWK Maynard Smith & Price 1973
Solving for an ESS Fitness player 1 w 1 =B.z 1 -B.z 2 -C.z 1.z 2 z 1 en z 2 = phenotypes of players 1 & 2 (hawk=1, dove=0) Personal benefit of playing hawk = influence of own behaviour on own fitness = w/ z = B-C.z 2 (depends on what other player does) Personal benefit of playing hawk = influence of own behaviour on own fitness = w 1 / z 1 = B-C.z 2 (depends on what other player does) At equilibrium benefit = B-C.z 2 = 0, and the ESS is to play hawk with a probability of z*=B/C - SYNERGY
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 1 Technically 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 wouldnt 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).