“Nobody has studied this before,”
said Neha Thakre, a postdoctoral researcher at the University of California,
San Diego, who studies Crispr as a mosquito control tool. Thakre, who was not
involved with the research, said she saw the study as a “great start” to
understanding what controls mosquito vision.
The female
mosquito quests for blood, as they require to lay eggs and thereby pass on
the viruses, which are capable of infecting tens of millions of people each year
with flaviviruses that lead to dengue, yellow fever and Zika. These pests hunt
in the morning, when the sun’s out, at dusk or dawn, unlike the Anopheles mosquito
that infects its host during the night.
“The better we understand how they
sense the human, the better we can control the mosquito in an eco-friendly
manner,” said Yinpeng Zhan, a postdoctoral researcher at the University of
California, Santa Barbara, and the lead author on the paper.
These insects
are attuned via various senses to detect blood, a minuscule gust of carbon
dioxide, gives an indication of someone or something has just respired in the
vicinity, making the mosquito hyperactive.
“They can also detect some of the
organic cues from our skin,” such as heat, humidity and stench, said Craig
Montell, a neurobiologist at the University of California, Santa Barbara, and
an author on the study. But if there is no suitable host, the mosquito will fly
straight to the closest-seeming target: a dark spot.
Quite a few
experiment, to detect mosquito vision take place in wind tunnels, large chambers
that cost tens of thousands of dollars. In previous experiments too, the
species where placed in the wind tunnel and given a whiff of carbon dioxide;
chose to fly to a dark spot over a white one.
However,
Montell’s lab lacks a wind
tunnel, so Zhan designed an inexpensive setup — a cage with a black circle and
a white circle inside, costing less than100$ and presented the same results. In
the spring of 2019, Zhan conducted spot tests in the cage. Jeff Riffell, a
biologist at the University of Washington, along with Claire Rusch, a graduate
student, and Diego Alonso San Alberto, a postdoctoral fellow, ran the same
experiment in the fall using a wind tunnel to double-check the original
results.
How was it done?
Montell and Zhan fostered the thought that,
out of the five light-sensing proteins expressed in the mosquito’s eye one held
the key to eradicate its ability to visually seek out human hosts by sensing
dark colors.
First, they
decided to knock out the rhodopsin protein Op1. Op1, the most widely expressed
vision protein in the mosquito’s compound eyes, seemed the best candidate for
interfering with the mosquito’s vision. Zhan injected the mutation into
thousands of tiny mosquito eggs using a tool with a special needle with a very
tiny tip.
After his
wee mutants had grown into adults, Zhan sucked 10 or so females into a tube
using a mouth-controlled aspirator. With each group, he held his breath, walked
over to the cage and released the females with one big exhale.
The Op1
mutants behaved exactly like the wild-type Aedes
aegypti: After huffing carbon dioxide, they flew directly to the black dot
in the cage. Montell and Zhan tried again, this time knocking out Op2, a
closely related rhodopsin. Still, the Op2 mutants showed no meaningful decline
in their vision.
But when the
researchers knocked out both proteins, the mosquitoes whizzed around aimlessly,
showing no preference between the white circle and black circles. They had lost
their ability to seek dark-colored hosts.
This raised
the next set of questions: whether the mosquitoes were complete blind or just
blind to people.
For the
answers, Montell and Zhan ran a series of tests to see how the double mutants
responded to light.
First, they
tested whether the double mutants would move toward light. Next, they connected
electrodes to the double mutants’ eyes to measure if the eyes displayed voltage
changes in response to light. Finally, they placed the double mutants in
rotating cylinders with vertical black and white stripes to see if the insects
would walk in the direction of the moving stripes. The double mutants passed
all three tests, although they had a weaker response than the wild types in the
last two tests.
The mosquitoes were not completely blind, after
all. “My first transgenic mosquito,” Zhan said proudly. “We had a happy
ending.”
The new
paper could inform future strategies to control mosquito populations. If female
mosquitoes were unable to see hosts, they would have a harder time finding the
blood required for their eggs to develop. “The
population would crash,” Montell said.
