CASE STUDY: The Wolbachia Files
Part I: Mosquitos: Ecosystem Service or Pest Control!
As the sun went down, Marcus and Annie were relaxing by the pool as they lamented the fact that
summer was coming to an end and college would be starting in just a few short weeks.
“Argh, these mosquitoes are so irritating!” exclaimed Marcus as he sprayed enough bug repellant to kill
an elephant. Trying to sooth his mosquito bites, he said “A world without mosquitoes wouldn’t bother
me a bit.”
“Relax. For every pest there’s a predator.” replied Annie.
“Well, I’m wondering where the nearest predator is then, as Marcus swatted away one buzzing
mosquito after another. I just read an article in the newspaper that said because of the early warm
spring and all the wet weather we’ve been having, we’re probably going to see more mosquitoes. They
need stagnant or slow moving water to lay eggs.” Marcus looked around the pool area and realized
there might be places where mosquitoes could easily make a comfy home for egg laying.
Later that night Annie thought about what would happen if there was a world without mosquitoes, but
also what if there was a world without mosquito control. Sure they’re annoying but she also knew they
spread diseases. Do they do any good she wondered? Annie looked at a flyer reminding folks to practice
the “3D’s”:
Public Health Announcement
Take a Bite out of Disease
Mosquito season is upon us again. Dr. Public Health suggests the
public practice the 3D’s to protect themselves from mosquito borne
illnesses caused by the pathogens they carry like West Nile Virus
(WNV).
Drain: Keep stagnant water from forming by cleaning out drains and
containers on a weekly basis.
Dress: Wear long sleeves and pants to prevent mosquito bites spreading
pathogens in humans such as West Nile virus.
Defend: Wear insect repellent.
“Mosquito control isn’t just about human health”, says Dr. Health.
“Mosquitos are also responsible for transmitting pathogens to other
animals. They carry the pathogen that causes heart worm in dogs, and the
virus that causes Eastern Equine Encephalitis in horses.” This year dog and
cat owners need to be even more vigilant about protecting their animals.
“The mild winter and early spring provide the perfect environmental
conditions for mosquitos breeding”, said Dr. Kind Heart from the American
Heart Worm Society.
A healthy community is a community working to prevent the spread of
disease to people and pets alike.
For more information contact the American Mosquito Control Association.
Student Questions:
1. What if mosquitoes were eliminated altogether? Identify at least two consequences to an
ecosystem if mosquitoes were eliminated?
2. What evolutionary adaptations do mosquitos have that make them successful as insects?
3. In what way is adaptive evolution a function of natural selection?
Optional Debate Question:
4. Does the public human health issue outweigh the impact of elimination of mosquitoes entirely
from an ecosystem?
Part II: Two weeks later….
Annie impatiently sat in front of Marcus’ house texting away at lightning speed: Marcus come-on!
We’ve got a class in an hour. We need to get to campus.” She just couldn’t believe the semester had
begun and fall was around the corner.
Rushing across campus to get to their biology class, Annie noticed a flyer looking for an undergraduate
student to take on a research role in the lab of Professor Skeeter. She grabbed it off the wall and
headed towards class.
Undergraduate Research Assistant Wanted!
Dr. Skeeter’s Research Lab
School of Molecular Biology
Dr. Skeeter’s research lab is currently exploring how
WOphage, a virus, contributes to genetic diversity in
mosquitoes and Wolbachia. Investigation into the
impact transposons have on genetic diversity will be
a major focus of the research project.
Other ongoing research projects explore the
relationship between mosquitoes, Wolbachia and
heartworm in dogs.
All inquiring minds, stop by Dr. Skeeter’r office in
Room 315, School of Molecular Biology.
“Wow” said Annie, Marcus, isn’t Professor Skeeter the one who’s done work for the World Health
Organization? Annie showed Marcus the flyer. “He’s been working on how mosquitos transmit disease.
This is a great opportunity to get experience with research”. Annie already knew a little bit about
mosquitoes, but she decided she’d better do some research on Wolbachia pipientis, WOphage, and
transposons though before she asked for an interview.
Student questions:
1. What is a virus? What is a bacteriophage? What is WOphage?
2. What is a transposon?
3. Transposons are sometimes called ‘jumping genes’ but these segments don’t actually detach
from the genome but instead are moved in one of two ways: a) transposon movement or b)
retrotransposon movement.
Briefly describe the differences between these two types of transposon movements.
4. Characterize Wolbachia pipientis:
a. What domain of life does it belong?
b. What are special characteristics of this organism?
Part III
With some ‘gutterflies’ twirling around inside her, Anni confidently knocked on Dr. Skeeter’s office door.
Annie: Hello, Dr. Skeeter I’m Annie.
Professor Skeeter: Come in Annie. Nice to meet you.
Anni: I’m interested in doing research in your lab. I’ve done some background research on bacteria and
viruses, but I’d like to know more.
Professor Skeeter: We’ve got two major lines of research going on in the lab. We’re interested in
studying how disease is transmitted through the mosquito as a vector. Arthropods, such as mosquitoes
serve as vectors and harbor parasites and bacteria, as well as, viruses. Perhaps we should bring you in
on the mosquito-heartworm project to get you started. You should probably look into the life cycle of
the nematode D. immitis and think about the how the other components of our research are all tied
together.
Annie: Thank you Dr. Skeeter. I’ll see you next week. I can’t wait to get started.”
Wow! I’m in, Annie exclaimed.
Student Questions:
1.
Characterize D. immitis. What domain does it belong to, what phylum, how does it
reproduce?
2. What is the relationship between mosquitoes and D. immitis?
3. Professor Skeeter’s work centers on mosquito adaptations, Wolbachia and WOphage. How
might transposable elements contribute to the genetic diversity of organisms such as
mosquitoes and bacteria?
Answer Key
Part I
Answers to Questions:
Q1: Mosquitoes are part of ecosystems and serve as food for others in a food chain. Larvae are food
for fish and other aquatic animals; adult mosquitoes are food for birds, bats, dragonflies, and spiders.
Mosquitoes get their nourishment from plant nectar and serve as pollinators. Mosquito larvae even
help plants like the pitcher plant grow, by producing nitrogen as a by-product of the waste from other
insects. Small fish feed on the larvae too.
Q2: Structural and Functional Components: Mosquitoes have wings, are light bodied, and have a
proboscis that enables them to feed. Male and female mosquitoes get their nutrients in the form of
nectar. Both male and females eat flower nectar for nourishment but only females need other protein
nourishment to promote the development of eggs. The mosquito uses its sensory palps to find a meal!
Male and female stylets are morphologically different as a result. Females have a piercing stylet that is
slender and needlelike with ridges while the male stylet is less neddlelike and does not have tiny ridges
to pierce skin. Females have chemicals in their saliva that help them draw blood out of the animal
without it clotting.
Habitat Adaptations: Legs to walk on water and used for weight balance; aquatic life cycle adaptations.
Q3: As favorable traits increase (due to changes in allelic frequencies that enhance survival and
reproduction), so too does the match between a species and its environment.
Q4: Debate Question. Does the public human health issue outweigh the impact of elimination of
mosquitoes entirely from an ecosystem? Students may bring into the conversation a variety of
perspectives including economics, health issues and health care access, and ecosystem balance.
PART II
Answers to Questions
Q1: a) W. pipientis is a genus of bacteria, specifically belonging to alpha-proteobacterium class and
belongs in Domain Bacteria. b) W. pipientis is an obligate endosymbiotic organism.
Q2: A virus is a non-biological pathogen. Viruses are not considered living because they lack a selfderived cell membrane and either contain DNA or RNA but not both. Fundamentally, living things are
membrane bound, can produce their own metabolites including a cell membrane and have both RNA
and DNA for cellular function.
Q3: A transposon is a stretch of DNA that can move from one position in a genome to another or move
from one genome to another on rare occasions. While it is generally thought that TE’s only move within
a genome, TE’s can move horizontally between species and are believed to be a contributing factor in
genomic evolution of other species.
Q4:
A. Wolbachia belong to domain Bacteria.
B. Wolbachia are a genus of gram-negative bacteria that are obligate endosymbionts which infect
arthropods and some nematodes. It can alter sex ratios in the affected host, male killing, and
induce parthenogenesis.
PART III:
Answers to Questions
Q1: Dirofiliaria immmitis is also known as heartworm. It belongs to domain Eukarya, phylum Nematoda.
It is a parasitic roundworm that uses the mosquito as its vector and the definitive host is the dog
although other animals can be infected (cats, foxes, coyotes and rarely humans). See teacher notes Part
I.
Q2: The mosquito is required to transfer L3 stage microfilaria to the host dog. The mosquito serves as a
transmission vessel for the development of L1 larva (microfilaria) in the mosquito. The L1 stage of
developing nematodes are called microfilaria. Once the female mosquito is infected, the microfilaria
incubate and develop to the L3 stage within the Malpighian tubules of the mosquito. The L3 microfilaria
migrate to the labium of the mosquito. In the process of the bite, an infected female mosquito will
deposit microfilaria near the bite opening. The dog is the primary host for the nematode. The
nematode is parasitic to both the mosquito and dog. The mosquito and nematode have a host-parasite
relationship, respectively.
Q3: Wolbachia and WOphage could contribute to change at the genomic level. Mutation underlies
much of evolution including duplication of chromosomes, alterations of chromosome structure.
Transposable elements contribute to genome evolution by contributing to genome recombination or
carry genes, individual exons, or movement of various other genomic elements. Oftentimes, genetic
movement of this sort is detrimental, but some may provide an evolutionary advantage over
evolutionary time.
References:
Tu, Z., & Coates, C. (2004). Mosquito transposable elements. Insect Biochemistry and Molecular Biology,
34(7), 631-644.
Teacher Notes and Background Resources
PART I:
Concepts for Focus:
Food chains, trophic levels, adaptive evolution, natural selection.
Student Learning Objectives:
Students will apply their understanding of organismal relationships in food chains and trophic levels.
Students will apply their understanding of adaptive evolution to the characteristics of an organism that
define structure and function.
Students will explain the relationship between natural selection and adaptive evolution.
Some background information on food chains, trophic levels, and ecosystem basics would be useful.
Knowledge of the mosquito life cycle would be helpful, but not necessary.
Mosquito as Vector and Nematode life cycle
Two critical environmental factors are necessary to promote a mosquito population and the spread of D.
immitis: temperature and water availability.
The mosquito is the vector for the transmission of heartworm. Mosquitoes become infected with the
first stage larva (L1) of the nematode Dirofilaria immitis while taking a blood meal from an infected
animal. Female mosquitoes bite. Males do not. Female mosquitoes of the Culix pipiens species require
a blood meal to promote egg development. The L1 stage of developing nematodes are called
microfilaria. Once the female mosquito is infected, the microfilaria incubate and develop to the L3 stage
within the Malpighian tubules of the mosquito. The L3 microfilaria migrate to the labium of the
mosquito. In the process of the bite, an infected female mosquito will deposit microfilaria near the bite
opening. The dog is the primary host for the nematode. The microfilaria wriggle their way into the
channel created by the mosquito bite and incubate under the skin where the L3 stage microfilaria
develop into L4 stage larvae. The L4 stage larvae migrate to other tissues including adipose and muscle
and complete its L5 stage. L5 then pass into the bloodstream where they migrate to the pulmonary
arteries.
The nematode could technically be called a vascular pathogen, with worm infections reaching into the
chambers of the heart by virtue of their length and overcrowding in the pulmonary vessels. Worms can
be found in the pulmonary arteries and right ventricle as a result.
Seventy to ninety days later they will develop into adults. Mating occurs approximately 6-9 months
after the adults become sexually mature and females release microfilaria into the bloodstream.
Part II
Concepts for Focus
Prokaryotic and eukaryotic cells, viruses, transposable elements
Student Learning Objectives
Students will apply their understanding of prokaryotic and eukaryotic organism characteristics to
Wolbachia and D. immitis.
Students will characterize viruses and WOphage.
Students will identify the difference between transposons and retrotransposons.
Students will distinguish the difference between prokaryotic and eukaryotic cells and non-biological viral
pathogens.
General summary of Bacteriophage structure and lytic and lysogenic modes.
Phage WO, is considered a temperate phage, that is, the phage has a lysogenic mode that enables the
viral DNA to be replicated and duplicated within the host genome and then transmitted during bacterial
replication. The phage genome is replicated and passed on to other bacteria species.
Transposon general summary:
Transposons are segments of DNA, viral in origin, that can duplicate themselves and/or move within a
host genome. Transposons can be duplicated throughout a genome and are often silent segments of
DNA that serve no purpose in the host genome.
PART III
Concepts for Focus
Evolution, nematode life cycle, transposable elements, bacteriophage
Student Learning Objectives
Students will describe the nematode life cycle.
Students will explore evolutionary mechanisms.
Students will describe bacteriophage structure and modes of replication.
Background Resources
Mosquito Identification
http://www.utep.edu/leb/mosquito/adslide33.htm
American Mosquito Control Association
http://www.mosquito.org/
Wolbachia Web site
https://www.wolbachiawebsite.org/about.cfm
Wolbachia Project
http://discover.mbl.edu/index.html
Heartworm: Dirofilaria immitis
http://microbewiki.kenyon.edu/index.php/Dirofilaria_immitis
Heartworm More Information
http://www.metapathogen.com/heartworm/
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/11300.htm
Video of HW infection
http://www.heartwormsociety.org/pet-owner-resources/canine.html
Heartworm map
http://www.veterinarypracticenews.com/vet-breaking-news/2011/05/13/heartworm-diagnosed-inevery-state-in-10-survey-finds.aspx
Fang, J. (2010). A world without mosquitoes [News Feature]. Nature, 466.