Their Genes Are Part Bird, Reptile, And Mammal
By Carly
Cassella
January 8,
2021 -- The first complete map of a platypus genome has just been released, and
it's every bit as strange as you'd expect from a creature with 10 sex
chromosomes, a pair of venomous spurs, a coat of fluorescent fur, and skin
that 'sweats' milk.
The
duck-billed platypus is truly one of the oddest creatures on Earth. Along with
the spiky echidna, these two Australian animals belong to a highly-specialised
group of mammals, known as monotremes, which both lay eggs but also nurse their
young with milk.
The genes of
both are relatively primitive and unchanged, revealing a bizarre blend of
several vertebrate animal classes, including birds, reptiles, and mammals.
As different
as the platypus might seem at first, it's those very differences that reveal
our similarities and our shared ancestry with Earth's other vertebrates.
Scientists
think its genome could tell us secrets about our own evolution and how our
distant mammalian ancestors went from laying eggs to giving birth.
"The
complete genome has provided us with the answers to how a few of the platypus'
bizarre features emerged," explains evolutionary biologist Guojie Zhang
from the University of Copenhagen.
"At the
same time, decoding the genome for platypus is important for improving our
understanding of how other mammals evolved - including us humans."
In previous
years, a female platypus had some of its genome sequenced, but without any Y
chromosome sequences, a lot of information was missing.
Using a male
platypus, researchers have now created a physical map with a highly accurate
platypus genome.
Today, living
mammals are split into three groups, including monotremes, marsupials, and
eutherians, or 'placentals'. We humans belong to that last group.
Together, the
latter two make up a subclass known as therian mammals. Therian mammals all
give birth to live young, but monotremes are simply too different to be lumped
in with that group as well.
It's still
unclear when all three of these distinct groups first began to diverge from one
another. Some think the monotremes split off first, with marsupial and
eutherians following suit. Others think all three groups diverged at roughly
the same time.
The genome of
the platypus has now helped clear up some of the dates. The data collected from
echidna and platypus lineages suggests their last common ancestor lived up to
57 million years ago.
Meanwhile,
monotremes as a whole appear to have diverged from marsupials and eutherian
mammals about 187 million years ago.
Even after all
that time, the semi-aquatic platypus has remained remarkably unchanged, fitting
a niche in the Australian bush that many marsupials and mammals simply
can't.
The authors
were particularly interested in the animal's sex chromosomes, which appear to
have originated independently from other therian mammals, all of which contain
a simple XY pair.
The platypus,
however, is the only known animal with 10 sex chromosomes (echidnas have nine).
Platypus have 5X and 5Y chromosomes organised in a ring that appears to have
broken apart into pieces over the course of mammalian evolution.
Comparing this
chromosome information to humans, opossums, Tasmanian devils, chickens, and
lizard genomes, the authors found the platypus's sex chromosomes have more in
common with birds like chickens than mammals such as humans.
But while
platypus lay eggs like chickens, they feed their young milk like therian
mammals.
It's not too
much of a surprise, therefore, that monotreme genomes contain most of the milk
genes that other therian mammals possess.
Casein genes
help encode certain proteins in mammalian milk, but monotremes appear to have
extra caseins with unknown functions. That said, their milk is not unlike what
comes from a cow, or even a lactating human.
As such, the
platypus is probably not as dependent on egg proteins as other bird and reptile
species because it can later feed their young through the lactation glands on
its skin.
Its genome
supports this. While birds and reptiles rely on three genes that encode for
major egg proteins, the platypus appears to have lost the majority of these
genes roughly 130 million years ago. Chickens today have all three egg protein
genes, humans have none, and the platypus has only one fully functional copy
left.
The platypus
is a weird in-between, and its genome is a sort of bridge to our own
evolutionary past.
"It
informs us that milk production in all extant mammal species has been developed
through the same set of genes derived from a common ancestor which lived more
than 170 million years ago – alongside the early dinosaurs in the Jurassic
period," Zhang says.
The full
genome has also revealed the loss of four genes associated with tooth
development, which probably disappeared roughly 120 million years ago. To eat,
the platypus now uses a pair of horn-like plates to grind up its food.
The venomous
spurs on its hind legs can possibly be explained by the creature's defensin
genes, which are associated with the immune system in other mammals, and appear
to give rise to unique proteins in their venom. Echidnas, which also had their
full genomes sequenced, appear to have lost this key venom gene.
The authors say
their results represent "some of the most fascinating biology of platypus
and echidna" alike.
"The new
genomes of both species will enable further insights into therian innovations
and the biology and evolution of these extraordinary egg-laying mammals,"
they conclude.
The study was
published in Nature.
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