To thrive in extreme environments like thermal springs, unicellular red algae (green) have taken in bacteria genes.
Debashish Bhattacharya
Plants and animals sometimes take genes from bacteria, study of algae suggests
Plantas e animais às vezes pegam genes de bactérias, sugere estudo de algas
Algae found in thermal springs and other extreme environments
have heated up a long-standing debate: Do eukaryotes—organisms with a
cell nucleus—sometimes get an evolutionary boost in the form of genes
transferred from bacteria? The genomes of some red algae, single-celled
eukaryotes, suggest the answer is yes. About 1% of their genes have
foreign origins, and the borrowed genes may help the algae adapt to
their hostile environment.
The new research, posted last week as a preprint on bioRxiv, has not
persuaded the most vocal critic of the idea that eukaryotes regularly
receive beneficial bacterial DNA. But other scientists have been won
over. The group provides a “fairly nice, rock-solid case for horizontal
gene transfer” into eukaryotes, says Andrew Roger, a protist genomicist
at Dalhousie University in Halifax, Canada.
Many genome studies have shown that prokaryotes—bacteria and
archaea—liberally swap genes among species, which influences their
evolution. The initial sequencing of the human genome suggested our
species, too, has picked up microbial genes. But further work
demonstrated that such genes found in vertebrate genomes were often
contaminants introduced during sequencing.
In 2015, after analyses of millions of protein sequences across many
species, William Martin, a biologist at the University of Dusseldorf
(UD) in Germany, and colleagues concluded in Nature that there
is no significant ongoing transfer of prokaryotic genes into eukaryotes.
Martin believes any such transfers only occurred episodically early in
the evolution of eukaryotes, as they internalized the bacteria that
eventually became organelles such mitochondria or chloroplasts. If
bacterial genes were continually moving into eukaryotes and being put to
use, Martin says, a pattern of such gene accumulation should be
discernible within the eukaryotic family tree, but there is none.
Debashish Bhattacharya, an evolutionary genomicist at Rutgers
University in New Brunswick, New Jersey, and UD plant biochemist Andreas
Weber took a closer look at a possible case of bacteria-to-eukaryote
gene transfer that Martin has challenged. The initial sequencing of
genomes from two species of red algae called Cyanidiophyceae had
indicated that up to 6% of their DNA had a prokaryotic origin. These
so-called extremophiles, which live in acidic hot springs and even
inside rock, can’t afford to maintain superfluous DNA. They appear to
contain only genes needed for survival. “When we find a bacterial gene,
we know it has an important function or it wouldn’t last” in the genome,
Bhattacharya says.
He and Weber turned to a newer technology that deciphers long pieces
of DNA. The 13 red algal genomes they studied contain 96 foreign genes,
nearly all of them sandwiched between typical algal genes in the DNA
sequenced, which makes it unlikely they were accidentally introduced in
the lab. “At the very least, this argument that [putative transferred
genes are] all contamination should finally be obsolete,” says Gerald
Schoenknecht, a plant physiologist at Oklahoma State University in
Stillwater.
The transferred genes seem to transport or detoxify heavy metals, or
they help the algae extract nourishment from the environment or cope
with high temperature and other stressful conditions. “By acquiring
genes from extremophile prokaryotes, these red algae have adapted to
more and more extreme environments,” Schoenknecht says.
Martin says the new evidence doesn’t persuade him. “They go to great
lengths to find exactly what I say they should find if [horizontal gene
transfer to eukaryotes] is real, but they do not find it,” he asserts.
Others argue that gene transfer to eukaryotes is so rare, and the
pressure to get rid of any but the most important borrowed genes is so
strong, that transferred genes might not accumulate over time as Martin
expects.
Of course, Roger says, “What’s happening in red algae might not be
happening in animals like us.” Humans and all other multicellular
eukaryotes, including plants, have specialized reproductive cells, such
as sperm or eggs or their stem cells, and only bacterial genes picked up
by those cells could be passed on.
Despite this obstacle, several insect researchers say they see
evidence of such gene transfer. John McCutcheon, a biologist at Montana
State University in Missoula who studies mealy bugs, is one. “I’ve moved
beyond asking ‘if [the bacterial genes] are there,’ to how they work,”
he says. The red algae, he adds, “is a very clear case.”
doi:10.1126/science.aaw8411
