GENOMES OF MALARIA CARRYING MOSQUITOES SEQUENCED
Nora Besansky, O'Hara
Professor of Biological Sciences at the University of Notre Dame and a member
of the University's Eck Institute for Global Health, has led an international
team of scientists in sequencing the genomes of 16 Anopheles mosquito species from around the
world.
Anopheles mosquitoes are responsible for
transmitting human malaria parasites that cause an estimated 200 million cases
and more than 600 thousand deaths each year. However, of the almost 500
different Anophelesspecies, only a few dozen can carry the parasite
and only a handful of species are responsible for the vast majority of
transmissions. Besansky and her fellow researchers investigated the genetic
differences between the deadly parasite-transmitting species and their harmless
(but still annoying) cousins.
Two papers published
in today's (Nov. 27) editions ofScience Express, an electronic
publication of the journal Science in advance of print,
describe detailed genomic comparisons of these mosquitoes and the deadliest of
them all, Anopheles gambiae. These results offer new insights into
how these species are related to each other and how the dynamic evolution of
their genomes may contribute to their flexibility to adapt to new environments
and to seek out human blood. These newly sequenced genomes represent a
substantial contribution to the scientific resources that will advance our
understanding of the diverse biological characteristics of mosquitoes, and help
to eliminate diseases that have a major impact on global public health.
Malaria parasites are
transmitted to humans by only a few dozen of the many hundreds of species of Anopheles mosquitoes,
and of these, only a handful are highly efficient disease-vectors. Thus,
although about half the world's human population is at risk of malaria, most
fatalities occur in sub-Saharan Africa, home of the major vector species, Anopheles
gambiae. Variation in the ability of different Anopheles species
to transmit malaria -- known as "vectorial capacity" -- are
determined by many factors, including feeding and breeding preferences, as well
as their immune responses to infections. Much of our understanding of many such
processes derives from the sequencing of the Anopheles gambiae genome
in 2002, which was led by Notre Dame researchers and which has since
facilitated many large-scale functional studies that have offered numerous
insights into how this mosquito became highly specialized in order to live
amongst and feed upon humans.
Until now, the lack of
such genomic resources for other Anopheles limited comparisons
to small-scale studies of individual genes with no genome-wide data to
investigate key attributes that impact the mosquito's ability to transmit
parasites. To address these questions, researchers sequenced the genomes of 16 Anophelesspecies.
"We selected
species from Africa, Asia, Europe, and Latin America that represent a range of
evolutionary distances from Anopheles gambiae, a variety of
ecological conditions, and varying degrees of vectorial capacity,"
Besansky said.
DNA sequencing and
assembly efforts at the Broad Institute were funded by NHGRI and led by Daniel
Neafsey, with samples obtained from mosquito colonies maintained through BEI
Resources at the United States Centres for Disease Control and Prevention, and
wild-caught or laboratory-reared mosquitoes from scientists in Africa, India,
Iran, Melanesia and Southeast Asia.
"Getting enough
high-quality DNA samples for all species was a challenging process and we had
to design and apply novel strategies to overcome the difficulties associated
with high levels of DNA sequence variations, especially from the wild-caught
sample," Neafsey said.
With genome sequencing
complete, scientists from around the world contributed their expertise to
examine genes involved in different aspects of mosquito biology including
reproductive processes, immune responses, insecticide resistance, and
chemosensory mechanisms. These detailed studies involving so many species were
facilitated by large-scale computational evolutionary genomic analyses led by
Robert Waterhouse from the University of Geneva Medical School and the Swiss
Institute of Bioinformatics.
The researchers
carried out interspecies gene comparisons with the Anopheles and
other insects, to identify equivalent genes in each species and highlight
potentially important differences.
"We used
similarities to genes from Anopheles gambiae and other
well-studied organisms such as the fruit fly to learn about the possible
functions of the thousands of new genes found in each of the Anopheles genomes,"
Waterhouse said.
Examining gene
evolution across the Anopheles revealed high rates of gene
gain and loss, about five times higher than in fruit flies. Some genes, such as
those involved in reproduction or those that encode proteins secreted into the
mosquito saliva, have very high rates of sequence evolution and are only found
in subsets of the most closely-related species.
"These dynamic
changes," Neafsey said, "may offer clues to understanding the
diversification of Anopheles mosquitoes; why some breed in
salty water while others need temporary or permanent pools of fresh water, or
why some are attracted to livestock while others will only feed on humans."
The newly available
genome sequences also provided conclusive evidence of the true relations
amongst several species that are very closely related to Anopheles
gambiaebut nevertheless show quite different traits that affect their
vectorial capacity.
"The question of
the true species phylogeny has been a highly contentious issue in the
field," Besansky said. "Our results show that the most efficient
vectors are not necessarily the most closely-related species, and that traits
enhancing vectorial capacity may be gained by gene flow between species."
This study
substantially improves our understanding of the process of gene flow between
closely related species -- a process believed to have occurred from
Neanderthals to the ancestors of modern humans -- and how it may affect the
evolution of common and distinct biological characteristics of mosquitoes such
as ecological flexibility and vectorial capacity.
These two very
different evolutionary timescales -- spanning all the Anopheles or
focusing on the subset of very closely-related species -- offer distinct
insights into the processes that have moulded these mosquito genomes into their
present-day forms. Their dynamic evolutionary profiles may represent the
genomic signatures of an inherent evolvability that has allowed Anopheles mosquitoes
to quickly exploit new human-generated habitats and become the greatest scourge
of humankind.
Besansky's research
focuses primarily on African vectors of human malaria: the anopheline
mosquitoes known as Anopheles gambiae and Anopheles funestus.
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