Wolbachia pipientis, an agent of speciation? "So, naturalists observe, a flea / Hath smaller fleas...
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Wolbachia pipientis , an agent of speciation?
"So, naturalists observe, a flea / Hath smaller fleas that on him prey / And these have smaller fleas to bite 'em / And so proceed ad infinitum. Jonathan Swift, of Gulliver's Travels Made by Robin Groch
What is Wolbachia?Obligate endosymbiont bacteriaa genus of the eubacteria domain
Wolbachia is an -proteobacteria descended from gram-negative bacteria
Related to mitochondria also a gram-negative, fused to ancestor eukaryote 2 billion years ago
Symbiont of Ecdyzoa (arthropods) and nematodes
Cytoplasmically inherited rickettsiae that are found in reproductive tissues (ovaries and testes)
Tree of Life
Alpha-ProteobacteriaWolbachiaincludes nitrogen-fixing bacteria, chemoautotrophs, and chemoheterotrophs. Members of this sub-phylum grow at low-nutrient levels (oligotrophic) and some have stalks(prosthecae). Wolbachia lost the ability to cause disease in vertebrates
Distribution of WolbachiaNo vertebrates (humans included) are known to carry Wolbachia
infection is rampant in the invertebrate world (est. ~ 20 million species)Estimates range from 15-20% of all insect species, 154 species so far17 isopods and freshwater shrimpArachnids (mites and spiders)Parasitic nematodes
Closest relative Wolbachia are a group of rickettsiae Ehrlichia equii, Ehrlichia canis, Cowdria ruminata, and Anaplasma marginale. These are blood parasites of mammals that are vectored by arthropods
Bacteria in the genus Rickettsia are still more distantly relatedgenus includes several arthropod-vectored disease agents, including the causative agents of Rocky Mountain spotted fever, murine typhus, and scrub typhus, as well as a cytoplasmically inherited male-killing bacterium found in ladybird beetles
Some of the Hosts of WolbachiaAchaera encedonDrosophiliaNasoniaParasitic nematodeArmadillidium vulgareAdalia bipunctataFormica exsecta
Wolbachia Strains and their hostsNematodesArthropods600 mya?? Mya 100 Mya, proposedtime of endosymbiosis of Wolbachia58-67 Mya100 MyaD*C*B (isopods and insects)A (insects)Proposed horizontal transmission from arthropods to nematodes 100 mya*Strains C & D are found in filarial nematodes
What is Wolbachia? Diversity of BehaviorsParasitismManipulation of host reproduction in which one organism (the parasite) lives in or on the body of another (the host) and obtains nutrition and other benefit from it. A parasitic relationship is usually to the detriment of the host but some host species have evolved remarkable tolerance to high levels of parasitic infection MutualismWolbachia Hypothesized to help host metabolism, host benefits Wolbachia has a home and a method of infectious transmissionCommensalismloose association in which Wolbachia benefits with no apparent advantage or disadvantageNote: The boundaries between these three kinds of symbiosis are not always clear because it can be difficult to establish advantage or disadvantage in the relationship.
Filarial Worms: Mutualism ExampleWolbachia are parasites in most invertebrates, researchers suspect that they live mutualistically with nematodes. Perhaps the clearest sign that the worms derive some benefit from an infection is the fact that they suffer if their Wolbachia are wiped out by antibiotics. Onchocerca ochengi, a filarial nematode in cattle, for example, die when their bacteria are destroyed. In other species, the females simply become sterile Filarial worms (nematodes are associated withElephantiasis and River-blindness diseases.
Wolbachia and skewing the hosts sex-ratioAlteration of timing of hosts reproductive cycle results in a variety of reproductive phenotypes :
Parthenogenesis (Infected virgins produce only daughters)Hymenoptera
Male Killing (male embryos die, female embryos develop into infected females)Acraea encedon (Ugandan butterfly)Adelia bipunctata (two-spot ladybird)
Feminization (Infected genetic males reproduce as females)Armadillidium vulagare (wood louse)
Cytoplasmic Incompatibility (infected male and uninfected female yield no viable embryos)Most common mechanism
Infected virgins produce only daughters, score one for the symbiont Appears to be restricted to Hymenoptera (wasps) evidence of over 40 species affectedHymenoptera have particular sex determination arrhenotoky (males are haploid, females diploid)Reversed with antibioticsBest case scenario for bacteria No superfluous malesMechanismTakes place in infected females unfertilized eggs1st mitotic division aborted in anaphaseYields 2N in unfertilized egg, therefore femaleCosts and Benefits for the hostInfected females produce less offspring If bacteria completely successful, asexual reproduction results (bad for both)Older females harbor less Wolbachia (have laid more eggs)Older females produce more males.
Male Killing or Dead Man Walking
Maternal inherited factors- kill male progeny during embryogenesisHigh sex-ratio of femalesIn Acraea encedon, 80-90 % population femaleWolbachia very adaptableMale killing in male heterogametic AdaliaMale killing in female heterogametic AcraeaCan reverse with antibiotics
Acraea encendana and leksSexist microbe. Wolbachia favor females, like this Ugandan butterfly, because they will carry on the lineage. CREDITS: COURTESY OF BACCHI AND BANDI; FRANCIS JIGGINS/CAMBRIDGE UNIVERSITY
Lets LekSex-ratio shift towards females, bacteria kills off malesNow the males are choosy, a valuable commodityFemale role reversed, form dense swarms (leks) (350 butterflies/200m)Males prefer uninfected females, but mistakes occur (which if fine with Wolbachia)
FeminizationIn Armadillidium individuals become female unless androgenic gland present Gland produces androgenic hormones to promote male sex determination
When Wolbachia is inherited from female a suppression factor prevents the formation of the gland converting males into reproductively competent females (although intersexes can also produced).
Symbiont wins, all female offspringYum, Adalia eats unhatched malesCurable with antibiotics
Wolbachia-induced CI is a reproductive incompatibility between sperm and egg, which typically results in zygotic death in diploid species or male production in haplodiploid species The bacteria are transmitted in eggs but are not transmitted through sperm
CI takes two forms, unidirectional and bidirectional.Unidirectional incompatibility typically occurs when the sperm from a Wolbachia-infected male fertilizes an uninfectedThe reciprocal cross (uninfected male and infected female) is compatible. Bidirectional incompatibility typically occurs when a male and a female harbor different strains of Wolbachia that are mutually incompatible egg.
CI OutcomesPaternal Chromosome eliminated embryo become haploidEmbryo diesCI mortalityIf a wasp, Normal male Male-biased Sex-ratioDiploid organismHaplodiploid organismSome mites, embryoDies, CI mortality
Whats Happening?Uninfected organisms
Sperm enters eggSperm chromatin decondenses, forms paternal pronucleusPaternal histones removed and replaced by female histonesReplication commences, chromosomes condense for mitosisPaternal and maternal pronuclei fuse to make diploid nucleus of zygoteInfected organismsOnly female pronucleus forms and undergoes first cleavagePaternal pronucleus does not condense into individual chromosomesPaternal chromosomes appear as diffuse tangle of chromatin and gets fragmented during 1st mitotic divisionIf 2N organism, then embryo is haploid, & either fails to develop or female sterile or has reduced fecundity Post-zygotic barrier (aneuploidy or failure of syngamy)
Modification-Rescue Modelof CIIncompatibility apparently involves a two component systembacterial "modification" of sperm and a bacterial "rescue" in the fertilized egg. bacteria present in the testes modify the developing sperm (possibly via chromatin binding proteins). The same bacterial strain must then be present in the egg to rescue this modification. If rescue does not occur, then incompatibility between the egg and sperm results. consistent with unidirectional incompatibility (modified sperm from infected males are not rescued by uninfected eggs) and bidirectional incompatibility (different bacterial strains use somewhat different modification-rescue systems). Two general biochemical models have been proposed, either (a) Wolbachia in the male produce a product that disrupts sperm processing in the egg (unless rescued) or (b) bacteria in the male act as a "sink" to bind away a product necessary for normal processing of the sperm in the egg consistent with the "sink" hypothesis, a number of host chromatin-binding proteins (such as H1 histone-likeprotein) have been found to bind to Wolbachia within host cells
Nasonia Timing Is Everything for Wolbachia HostsCarl Zimmer (Science 296, 999-1000 (2002)
Cytoplasmic Incompatibility in Nasonia A case of bad timingLatest model:Asynchrony between maternal and paternal pronuclei chromosomes during 1st mitosisCause: CdK1/cyclin B and disruption of cell cycle checkpointsWolbachia inhibiting cell cycle timing in unidirectional CIInfected sperm modified, no rescue in eggsRescue = female pronucleus delayed also
A case of bad timing continuedWhy do reciprocal matings work?Infected female have live Wolbachia which can change the timing of the uninfected spermWhy dont bidirectional matings work?Males and females infected with different strains that cant align the timing of mitosis, asynchrony still occurs
Figure 1 Wolbachia and cytoplasmic incompatibility. Cytoplasmic