|Duck-Billed Platypus Genome Sequence Published
Animal’s Reptilian-Mammalian Mix Reflected in Its DNA
The first analysis of the genome sequence of the duck-billed platypus
was published today by an international team of scientists, revealing
clues about how genomes were organized during the early evolution
of mammals. The research was supported in part by the National
Human Genome Research Institute (NHGRI), part of the National Institutes
of Health (NIH).
Fans of TV nature shows will remember that the duck-billed platypus,
native to Australia, is one of the few mammals that lays eggs.
However, platypus peculiarity does not end there. For example,
these odd animals boast what looks like a duck's bill, which houses
an electrosensory system used when foraging for food underwater,
along with thick fur coats adapted for the icy waters in which
it resides. Males also possess hind leg spurs that can deliver
venom powerful enough to wound territorial competitors during mating
season or cause excruciating pain in other mammals, including humans.
"At first glance, the platypus appears as if it was the result
of an evolutionary accident. But as weird as this animal looks,
its genome sequence is priceless for understanding how fundamental
mammalian biological processes have evolved," said Francis
S. Collins, M.D., Ph.D., director of NHGRI. "Comparisons of
the platypus genome to those of other mammals will provide new
insights into the history, structure and function of our own genome."
In a paper published in today's issue of the journal Nature, researchers
analyzed a high-quality draft genome sequence of Glennie, a female
platypus from Australia. The consortium included scientists from
the United States, Australia, England, Germany, Israel, Japan,
New Zealand and Spain. Sequencing of the platypus genome was led
by the Genome Sequencing Center at Washington University School
of Medicine in St. Louis, a part of NHGRI’s Large-Scale Sequencing
Once the sequence was produced, researchers began comparing the
genome of the platypus, whose ancestors split from the rest of
mammalian lineage some 166 million years ago, with the well-characterized
genomes of the human, mouse, dog, opossum and chicken, as well
as the draft genome sequence of the green anole lizard. The chicken
genome was chosen because it is descended from the ancestral group
of egg-laying animals, including extinct reptiles, which passed
on much of their DNA to animals like the platypus. Scientists were
particularly interested in finding features within the platypus
genome that could explain the odd mix of characteristics seen in
the platypus, those that were more like reptile and birds and which
were like mammals.
The team found that the platypus genome contains about the same
number of protein-coding genes as other mammals — approximately
18,500. The platypus also shares more than 80 percent of its genes
with other mammals whose genomes have been sequenced. Next, researchers
combed the platypus genome looking for genetic evidence of sequences
unique to platypuses, which have been lost from mammalian genomes.
Scientists were also eager to find out what characteristics of
the platypus were linked at the DNA level to reptiles or mammals.
"The mix of reptilian, mammalian and unique characteristics
of the platypus genome provides many clues to the function and
evolution of all mammalian genomes," said Richard K. Wilson,
Ph.D., director of Washington University School of Medicine’s Genome
Sequencing Center and the paper’s senior author. "Now, we’ll
be able to pinpoint genes that have been conserved throughout evolution,
as well as those that have been lost or gained."
The female platypus lays eggs, a reptilian characteristic, yet
also produces milk to nourish its young, which is a mammalian characteristic.
Interestingly, the platypus genome harbors both reptilian and mammalian
genes associated with fertilization of eggs. However, researchers
discovered that, like other mammals, the platypus genome contains
a tightly clustered set of genes that produce the casein proteins
that make up milk.
A good sense of smell is something mammals have in common. Compared
to most mammals, chicken and lizard have a relatively poor complement
of odorant receptor genes responsible for their ability to detect
smells. Interestingly, platypus has about half as many odor receptors
as do the mouse and other mammals. However, the researchers also
found that the platypus genome has experienced an expansion in
the genes that code for a particular type of odor receptor, called
a vomeronasal receptor. Researchers think the expansion of this
particular set of genes may be involved in the ability of the platypus
to detect odors during underwater foraging.
Researchers also found that genes related to the immune system
of the platypus are very similar to those of other mammals, with
a few key differences. For example, the platypus genome contains
expansions in the gene family that codes for a microbe-fighting
peptide called cathelicidin. Primates and rodents have only a single
copy of the cathelicidin gene. Scientists think that the expansion
of this gene family in platypus may be involved in boosting the
immune system of their very immature offspring, which hatch very
developmentally immature. Scientists postulate that the microbe-fighting
peptide may have been less critical for other mammals, whose offspring
spend relatively long periods of time developing within the womb
and are born at a more advanced stage of development.
Scientists were also eager to find out how venom production became
a part of the platypus genome. When researchers began analyzing
the genetic sequences responsible for venom production in the male
platypus, they made a surprising finding. They discovered that
venom produced by the male platypus arose from duplications in
certain genes over the course of evolutionary time which had been
passed on from ancestral reptile genomes. The reptilian lineage
displays a similar duplication of venom genes, but that duplication
appears to have occurred independently during the evolution of
reptiles, giving them similar powers to produce venom.
"This genome provides a unique perspective on what the genomes
of our earliest mammalian ancestors may have looked like. It is
fascinating that what we think of as being reptile-like and mammal-like
features can co-exist in the same genome." said Adam Felsenfeld,
Ph.D., who heads the Comparative and Sequencing Analysis Program
in NHGRI’s Division of Extramural Research.
The platypus genome sequence is publicly available from NIH's
National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov).
NCBI distributes the sequence data to the European Molecular Biology
laboratory’s Nucleotide Sequence Database, EMBL-Bank (http://www.ebi.ac.uk/embl/index.html),
and the DNA Data Bank of Japan, DDBJ (www.ddbj.nig.ac.jp).
To download high-resolution photographs of a platypus, go to http://www.genome.gov/17516871.
For more information on comparative genomic analysis, go to: http://www.genome.gov/11509542.
NHGRI is one of 27 institutes and centers at the NIH, an agency of
the Department of Health and Human Services. The NHGRI Division of
Extramural Research supports grants for research and for training
and career development at sites nationwide. Additional information
about NHGRI can be found at its Web site, www.genome.gov
The National Institutes of Health (NIH) — The Nation's
Medical Research Agency — includes 27 Institutes and
Centers and is a component of the U.S. Department of Health and
Human Services. It is the primary federal agency for conducting
and supporting basic, clinical and translational medical research,
and it investigates the causes, treatments, and cures for both
common and rare diseases. For more information about NIH and
its programs, visit www.nih.gov.