Biology Now, 2e

348 ■ CHAPTER 19 Growth of Populations

ECOLOGY

no access to antibiotics—so the offspring die off

before becoming adults and therefore do not

reproduce. In this way the size of the population

is reduced (Figure 19.7).

Populations—mosquito, human, or otherwise—

can change in a density-dependent or density-

independent manner. Density-dependent popu-

lation change occurs when birth and death rates

change as the population density changes. The

number of offspring produced and the death rate

are often density-dependent. Food shortages,

lack of space, and habitat deterioration—all these

factors influence a population more strongly as it

increases in density (Figure 19.8).

In addition, when a population has many indi-

viduals, disease spreads more rapidly (because

individuals tend to encounter one another more

tests of GM mosquitoes in Brazil suggest that we

may have a new way to stop that spread.

Starting in 2002, British company Oxitec

began producing a GM line of the Aedes aegypti

mosquito. The company inserts a single gene

into a line of lab-bred mosquitoes. This gene,

designed to work only in insect cells, produces a

protein that prevents the mosquito from transi-

tioning between two of its life stages: larva and

adult. In the lab, the mosquitoes are exposed to

an antibiotic (tetracycline) that neutralizes the

protein so that the mosquitoes can grow to be

adults. Then, only males are released into the

wild, because the males don’t bite and so will not

contribute to the spread of disease. Once free,

the male GM mosquitoes mate with wild females

and produce offspring with the lethal gene—and

Normal mosquito

breeding cycle

with Zika

GM mosquito

breeding cycle

with Zika

Adult, infected

female transmits

virus while feeding

from human or

other species,

and ...

Female lays

eggs that

carry Zika virus.

Pupae develop

into infected

adults.

Eggs hatch

into infected

larvae.

... infected female

mosquito mates

with normal

male.

Larvae develop

into infected

pupae.

No infected,

GM adults

survive.

Female lays

eggs that carry

Zika virus

and are GM.

No infected,

GM pupae

survive.

Eggs hatch

into infected

GM larvae.

Infected

female

mosquito

mates with

GM male.

Larvae die

before

developing into

infected,

GM pupae.

Figure 19.7

How mosquitoes are genetically modified to stop the Zika virus

Genetically modified (GM) male mosquitoes have been created and released in Brazil and Florida, where they mate with wild

females and produce offspring that cannot grow to adulthood.

Q1: List all the stages of the mosquito life cycle.

Q2: Which life cycle stage is vulnerable to the GM treatment?

Q3: Why are only male GM mosquitoes released into the wild, rather than both males and females?