title: Effect of temperature on mortality, sterility and Wolbachia titre that may cause behavioural changes
Creators: name: Rashid, Amirah
Publisher: name: University of Liverpool Repository
Year: 2029

description: Temperature is one of the important factors used to predict the distribution range of insects
and observing pathogen-insect relationships. It also affects the fitness of ectotherms such as
their adult lifespan, fecundity and biting rate. Taken together, all these factors are crucial for
prediction of disease transmission risk and subsequent vector control programmes.
Previous study has proven that observing the thermal fertility limit (TFL) of Drosophila flies is
better at predicting their distributions as compared to critical thermal limit (CTL) alone. In
Chapter 2, I looked at the mortality limit and sterility limit of Aedes aegypti and Culex
molestus. Unmated adult mosquitoes were heat and cold shocked for 4 hours and 6 hours
respectively to represent day/night extremes. Then, they were allowed to mate for four days
before given an egg dish to lay eggs. Sterility was observed based on blood feeding, eggs laying
and eggs hatching. To further support the findings, morphology of both the male and female
reproductive organs were dissected and observed under microscope to compare between the
stressed and normal. We found that Aedes are sterile at 38 °C. We also found that Aedes males
are much more sensitive to high temperatures, while female mosquitoes have higher
tolerance. On the other hand, both males and females are sterile at -2 °C which is confirmed
from the shrunk reproductive organs. Apart from looking at adult mortality and sterility, eggs
of Aedes can stand desiccation, hence we also looked at maximum temperature that can cause
a detrimental effect on the eggs. Results from this study were then analysed and used for
temperature modelling in predicting distributions of Aedes in the current climate and future
climate.
Wolbachia is a bacterium commonly found in insects. They do not naturally occur in Ae.
aegypti. However, Ae. aegypti have been artificially infected with Wolbachia (wAlb) since this
bacterium can confer protection against RNA viruses (i.e. dengue and Zika viruses) to their
host. Moreover, Wolbachia is also known for their manipulative reproductive phenotypes in
mosquitoes by causing cytoplasmic incompatibility. Due to these abilities, Wolbachia are used
for vector and arbovirus control. More recently studies have shown that Wolbachia can also
change its host behaviour, such as feeding behaviour in older wMelPop-infected female Ae.
aegypti. In Chapter 3, I studied the thermal preference of Wolbachia-infected female Ae.
aegypti and compared them to the uninfected controls using a thermal gradient setup.
Knowing that developmental temperature is crucial for the Wolbachia-conferred antiviral
protection, we have compared temperatures where infected and uninfected mosquitoes
choose to lay eggs. We observed that Wolbachia alters the mosquito oviposition preference.
We have also observed interaction between the presence of Wolbachia, mosquito
developmental time, and the temperature where they choose to oviposit. Our results will help
us to understand the relationship between Wolbachia, Ae aegypti, and temperature. Such
interactions are useful to evaluate the influence of environment on releasing Wolbachia infected mosquitoes and may imposed in future vector control programmes.
In Chapter 4, I looked at another endosymbiont, Spiroplasma in infected Drosophila flies. Even
though insects choose temperatures optimal for them, presence of parasites force them to
change their thermal preference in order to benefit the microbe or minimize the parasite induced harm. Spiroplasma is a bacterium which protects D. melanogaster hosts against
parasitoid wasps. It has been shown before that endosymbiont (i.e. Wolbachia) can modulate
thermal preference of Drosophila – flies infected with the symbiont seek colder temperatures
to minimize Wolbachia replication. In this chapter, I compared the temperature preference of
Spiroplasma-infected Drosophila to their non-infected counterparts in the presence of
parasitoid wasps. We confirmed that parasitoid reduce the number of eggs laid by D.
melanogaster. And that both Spiroplasma and wasps presence increases the D. melanogaster
egg laying temperature. Our result will help understand the ecology of these symbiotic
interactions.
Together, these analyses provide valuable updates to our current understanding on
temperature, relationship between host and endosymbiont which will be beneficial for
improving preparedness and strategies in vector control programs.