Endosymbiosis - Wolbachia

Endosymbionts
1. An organism that lives within the body or cells of another organism
  1. Obligate endosymbiont: cannot survive without the host
2. Often endosymbiont is heritable - passed on by host to progeny, with very infrequent lateral transfer
3. Endosymbiont can have major influence on host biology
Transmission
1. Via egg
2. Many reproductive parastism relationships promote maintenance of infection
3. Often can trace close endosymbiont/mitochondrial genotype relationship
  1. Consequence of maternal transmission
4. It is clear lateral transfer to new hosts can occur
  1. Evidence for survival outside of host
Categories
1. Obligate mutualists (primary symbionts)
  1. Required for host development
  2. Restricted for host development
  3. Cannot invade naive hosts and dependent on host-based mechanisms for transmision
  4. 'Domesticated' by host
2. Facultative mutualists
  1. Confer fitness benefit to host
    1. Heat
    2. Protection against natural enemies
    3. A myriad of metabolic advantages
3. Reproductive manipulators (maternal transmission)
  1. Parasites spread by increasing host reproduction through daughters at the expense of reproduction through sons
    1. Incompatabilities (infected vs uninfected)
    2. Feminisation of genetic males
    3. 'Son killing' which increases investment in daughters
Hosts
1. Wide range
  1. Free living amoeba
  2. Arthropods
  3. Nematodes
  4. Plants
2. Often host specific endosymbionts
  1. Reflects ancient origins?
Origins
1. 'Domestication' of intracellular bacterium
2. Evolution of mutualistic interaction into dependence
3. Many endosymbionts have lost many functions - genome reduction
4. Mitochondria and chloroplasts are thought to have evolved from endosymbionts
Wolbachia
1. Members of order Rickettsiales
2. Very wide distribution among insects and other arthropods
  1. 65% of insect species harbour Wolbachia
  2. Infect >10^6 insect species
3. Wide range of striking host manipulations
  1. Many affect reproduction
4. Wolbacha-host interactions
  1. Strict maternal inheritance via the egg
  2. Variety of means to induce productive distortions to increase the reproductive success of infected females, thus enhancing their maternal transmission
  3. Evidence for direct fitness benefits to the host by affection nutrition of development
  4. Controversy surrounds the question whether Wolbachia drive evolution through the creation of reproduction barriers and genetic divergence
  5. Recent evidence shows gene transfer to invertebrate hosts
5. Manipulation of reproduction
  1. Feminisation
    1. Genetic males develop as females
    2. Probably several mechanisms, not clear
  2. Parthenogenesis
    1. Species with arrhenotokous development (males develop from unfertilised eggs)
    2. Infected females produce daughters from unfertilised eggs (normally only from fertilised eggs)
    3. Cell cycle is affected by Wolbachia early in embryo development
  3. Male killing
    1. 'Lethal feminisation'
    2. e.g. genetic become female and die during development
  4. Cytoplasmic incompatibility
    1. Most common phenotype
    2. Wolbachia induced modification of spermatogensis
    3. Rescue of the modification in embryos infected by the same Wolbachia strain
    4. Asynchronous development of female and male pronuclei
6. Gentics/genomics
  1. Generally, endosymbionts have small genomes compared to free living relatives
  2. Once thought the small genomes of endosymbionts represent the ancestor (primitive) state
    1. Large genomes of free living bacteria evolved by gene acquisition
  3. Now clear the endosymbionts have undergone large scale gene loss
7. Mutualism - host benefits
  1. Probably Wolbachia-host dependent
  2. Asobara tabida (wasp) - regulation of apoptosis in nurse cells allows oocyte development
    1. Removal of Wolbachia causes higher levels of apoptosis of nurse cells
  3. Drosophila - Wolbachia protect against a range of RNA viruses, interfere with proliferation?
  4. Nutritional mutualism - iron homeostasis, protection from oxidative stress?
  5. Theraputic window?
    1. Wolbachia essential to survival of some hosts
      1. Target Wolbachia as a drug target to remove harmful pathogens from body?
8. Antibiotic treatments of filarial diseases
  1. Filariasis - disease caused by nematodes of the superfamily Filarioidea
    1. Transmission by arthropods
  2. Lymphatic filariases - e.g. elephantiasis
  3. Subcutaneous - e.g. river blindness
  4. Serious common disease
  5. Resistance to broad spectrum of antihelmintics is becoming more common
  6. Now appreciated clearance of Wolbachia from nematodes can sterilise the worms
  7. Doxycycline shown as effective combination therapy
9. Endosymbionts as biological control agents?
  1. Most endosymbionts should theoretically be evovevd to beneficially co-exist with their hosts, but there are some well studies exceptions
  2. Some Wolbachia strains are unable to synchronise their replication with their host cells, resulting in significant life-span reductions
  3. Best studied example is wMelPop (popcorn) which over-replicate and fill the brain of infected flies leading to 50% reduction in life expectancy
  4. Major interest in using Wolbachia strains as a tool to control mosquito-born diseases
  5. The transmission of mosquito-born pathogens (malaria and dengue fever) is influenced by the age of the mosquito to replicate within their host until they reach the salivary glands

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Endosymbiosis - Wolbachia

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