FUNDAMENTAL BUILDING BLOCKS 319
had found the first evidence
that DNA could pass horizontally
between cells of the same
generation, as well as vertically
between generations.
In 1946, American biologists
Joshua Lederberg and Edward
Tatum demonstrated that bacteria
exchange genetic material as part
of their natural behavior. In 1959,
a team of Japanese microbiologists
led by Tomoichiro Akiba and
Kunitaro Ochia showed that this
kind of DNA transfer explains how
resistance to antibiotics can spread
through bacteria so quickly.
Transforming microbes
Bacteria have small, mobile rings
of DNA called plasmids that pass
from cell to cell when they come
into direct contact—taking their
genes with them. Some bacteria
contain genes that make them
resist the action of certain types of
antibiotics. The genes are copied
whenever the DNA replicates, and
can spread through a population of
bacteria as the DNA is transferred.
This sort of horizontal gene transfer
can also happen via viruses, as
Lederberg’s student Norton Zinder
discovered. Viruses are even
smaller than bacteria and can
invade living cells—including
bacteria. They may interfere with
the host genes, and when they
move from host to host, they may
take host genes with them.Genes for development
From the mid-1980s, Syvanen set
HGT in a wider context. He noted
similarities in how the development
of embryos is genetically controlled
at a cellular level—even between
distantly related species—and
attributed this to genes moving
between different organisms in
evolutionary history. He argued
that the genetic control of animal
development had evolved to be
similar in different groups because
this maximized the chances that
gene-swapping would work.
As genome sequences are
completed for more species, and
as the fossil record is reexamined,
evidence suggests that HGT mayThe flow of genes
between different species
represents a form of
genetic variation whose
implications have not been
fully appreciated.
Michael SyvanenDNA plasmids, colored blue in
this micrograph, are independent of
a cell’s chromosomes, yet they can
replicate genes and be used to insert
new genes into organisms.See also: Charles Darwin 142–49 ■ Thomas Hunt Morgan 224–25 ■ James Watson and Francis Crick 276–83 ■
William French Anderson 322–23
Michael Syvanen
Michael Syvanen trained in
chemistry and biochemistry at
the University of Washington
and the University of California
at Berkeley before going on
to specialize in the field of
microbiology. He was appointed
professor of microbiology and
molecular genetics at Harvard
Medical School in 1975, where
he conducted research in the
development of antibiotic
resistance in bacteria, and
insecticide resistance in flies.
His findings led him to publishhis theory of horizontal gene
transfer (HGT) and its role in
adaptation and evolution.
Since 1987, Syvanen has
been professor of medical
microbiology and immunology
at the School of Medicine in the
University of California at Davis.Key works1985 Cross-species Gene
Transfer: Implications for a
New Theory of Evolution
1994 Horizontal Gene Transfer:
Evidence and Possible
Consequencesoccur in not only microbes but also
more complex organisms, in both
plants and animals. Darwin’s tree
of life may look more like a net,
with multiple ancestors rather than
a last universal common ancestor.
With potential implications for
taxonomy, disease and pest control,
and genetic engineering, HGT’s full
significance is still unfolding. ■