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Male
or female? First sex-determining genes appeared in mammals some 180 million
years ago
Male
or female? First sex-determining genes appeared in mammals some 180 million
years ago
Date:
April 23,
2014
Source:
Swiss Institute of Bioinformatics
Summary:
The Y chromosome, which distinguishes males from
females at the genetic level, appeared some 180 million years ago. It
originated twice independently in all mammals. Scientists have managed to date
these events that are crucial for both mammalian evolution and our lives,
because the Y chromosome determines whether we are born as a boy or girl.
.......................
Man or woman? Male or female? In humans and other mammals,
the difference between sexes depends on one single element of the genome: the Y
chromosome. It is present only in males, where the two sexual chromosomes are X
and Y, whereas women have two X chromosomes. Thus, the Y is ultimately
responsible for all the morphological and physiological differences between
males and females.
But this has
not always been the case. A very long time ago, the X and Y were identical,
until the Y started to differentiate from the X in males. It then progressively
shrank to such an extent that, nowadays, it only contains about 20 genes (the X
carries more than one thousand genes). When did the Y originate and which genes
have been kept? The answer has just been brought to light by the team of Henrik
Kaessmann, Associate Professor at the CIG (UNIL) and group leader at the SIB
Swiss Institute of Bioinformatics, and their collaborators in Australia. They
have established that the first " sex genes " appeared concomitantly
in mammals around 180 million years ago.
4,3 billion
genetic sequences
By studying
samples from several male tissues -- in particular testicles -- from different
species, the researchers recovered the Y chromosome genes from the three major mammalian
lineages: placentals (which include humans, apes, rodents and elephants),
marsupials (such as opossums and kangaroos) and monotremes (egg-laying mammals,
such as the platypus and the echidna, a kind of Australian porcupine). In
total, the researchers worked with samples from 15 different mammals,
representing these three lineages, as well as the chicken, which they included
for comparison.
Instead of
sequencing all Y chromosomes, which would have been a " colossal task
" according to Diego Cortez, researcher at CIG and SIB and main author of
the study, the scientists " opted for a shortcut ." By comparing
genetic sequences from male and female tissues, they eliminated all sequences
common to both sexes in order to keep only those sequences corresponding to the
Y chromosome. By doing so, they established the largest gene atlas of this
" male " chromosome to date.
This study
required more than 29,500 computing hours! A gigantic task, which could not
have been performed without important technical means: the high-throughput DNA
sequencers of the genomics platform at the Center for Integrative Genomics, for
the generation of the genetic sequences, and the calculation means of Vital-IT,
SIB's high-performance computing centre, for the biological analyses.
Two
independent sex-determining genes
The study
shows that the same sex-determining gene, named SRY, in placentals and
marsupials had formed in the common ancestor of both lineages around 180
million years ago. Another gene, AMHY, is responsible for the emergence of Y
chromosomes in monotremes and appeared some 175 million years ago. Both genes,
which according to Henrik Kaessmann are "involved in testicular
development ," have thus emerged " nearly at the same time but in a
totally independent way ."
The nature
of the sex-determination system present in the common ancestor of all mammals
remains unclear, given that mammalian Y chromosomes did not yet exist at that
time -- at least not those discovered in this study. So what triggered back
then that an individual was born male or female? Was this determination linked
to other sex chromosomes, or even environmental factors such as the
temperature? The latter is not an unreasonable scenario, given that temperature
determines sex in present-day crocodiles. As far as mammals are concerned,
"the question remains open ," concludes Diego Cortez.
Story
Source:
The above
story is based on materials provided by Swiss Institute of Bioinformatics. Note:
Materials may be edited for content and length.
Journal
Reference:
- Diego Cortez, Ray Marin, Deborah Toledo-Flores, Laure Froidevaux, Angélica Liechti, Paul D. Waters, Frank Grützner, Henrik Kaessmann. Origins and functional evolution of Y chromosomes across mammals. Nature, 2014; 508 (7497): 488 DOI: 10.1038/nature13151
Cite This
Page:
Swiss Institute of Bioinformatics.
"Male or female? First sex-determining genes appeared in mammals some 180
million years ago." ScienceDaily. ScienceDaily, 23 April 2014.
<www.sciencedaily.com/releases/2014/04/140423151035.htm>.
