Lecture 27: The Insect Microbiome Flashcards
What is “Wolbachia”?
“An inherited intracellular bacteria that live within the cells of the reproductive tracts of insects”
- In alpha-proteobacteria class
- Gram negative
- Diverse - numerous species of
Wolbachia - Though to be member of rhizobium
- Cannot survive outside the host cell
- Primarily; arthropods (~66% of species &
insects comprise ~85% of all animal
species) - Therefore may be most common
endosymbiont in the world
How is Wolbachia so successful?
- Primary mechanism for propagation is vertical transmission
- (Can propagate horizontally)
Vertical transmission:
“passed parent to offspring with high efficiency”
Horizontal transmission:
“dispersal of bacteria between organisms living in same environment”
What doe Wolbachia do inside the cell?
- Can manipulate cellular components so
that it can move across spindles - Therefore when cell divides, it is split
into both cells equally - can pass itself into germ cells, which
ultimately lead to eggs and sperm
What difficulties may Wolbachia phase in transmission? What does it do about this?
- Paternal transmission is very difficult
(almost never) due to sperm being so
small
a. it can get into sperm heads, but not
often at high enough dosages to infect
offspring
2. It gets round this by various mechanisms
tailored to different organisms
of reproductive manipulation
a. Feminization
b. Parthenogenesis
c. Male killing
d. Cytoplasmic incompatibility
Why did the various reproductive manipulation mechanisms evolve?
Despite the different techniques, they involve the end goal of producing more females than males.
This means more propagation.
How does cytoplasmic incompatibility work?
- A “toxin antidote system”?
a. Bacteria inserts a toxin into
sperm, despite not being able to fit
itself, able to interrupt reproduction
b. If bacteria is present in the oocyte
when fertilised, there must be some
form of antidote to counteract the toxin
How does the toxin/antidote system in Wolbachia work genetically?
- Identification is difficult:
a. cannot be cultured outside insects
hosts - Progress was made with whole
genome sequencing and comparative
genomics with “non-parasitic strains”
a. allowed narrowing down of potential
target genes - Analysis of proteins in sperm
a. looking at proteins in infected &
non-infected sperm
LED TO IDENTIFICATION OF “cidA cidB Complex”
What is the cidA cidB complex?
- A Wolbachia gene operon
- Expression of cidA OR cidB on their own
produces no effect - Males expressing cidA/cidB = sterile
- Females expressing cidA/cidB = fertile
- Female expression in cidA in ovaries can
“rescue” sterile males
Therefore
a. production of cidA & cidB together
= toxic sperm
b. presence of cidA alone = antidote
What do cidA and cidB do?
- cidB causes “post-fertilisation embryonic
defects” - When sperm fertilises oocyte and tries to
form embryo, it fails at early stages of
nuclear division
PROCESS
1. Paternal nuclear envelope breakdown
delayed
2. Male pronuclear DNA replication is
incomplete
3. Nuclear division fails
What is bi-directional cytoplasmic incompatibility?
- Once sufficiently diverged from each other, Wolbachia lineages can start to fail to rescue each other
I.E., If sperm and oocyte are too evolutionary divergent, the females rescue operon may not match up to the toxic operon
HOW CAN SOMETHING DIVERGE IF ITS DIVERGENCE IS A SELECTIVE PRESSURE
DESCRIPTION AND ADAPTIVE RATIONALE OF REPRODUCTIVE MANIPULATION ON POWERPOINT/COLLABORATE
What is feminization
Conversion of hosts that would develop as males to female development
What is parthenogenesis induction?
Induction of asexual daughter development
What is male killing?
Death of male host during embryogenesis
What is cytoplasmic incompatibility
Sterility of crosses between infected males and female that are either uninfected or infected with a different strain of symbiont
