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