AMONG SEX CHROMOSOMES IN MAMMALS ARE NOT FLUKES. THEY REPRESENT INTERMEDIATE
STAGES THAT CHRONICLE THE EVOLUTION OF A DEVELOPMENTAL PROCESS.
mostly appear translucent. They unfurl their delicate strands of DNA,
the better to expose genes that direct the workings of cells. But in
the cells of female mammals, which have two X chromosomes, one sister
of the pair remains shut off in a corner, tightly bundled and dark.
The dark sister sits subdued, not directing the production
of thousands of proteins the way its voluble counterpart does--and indeed
as the other 22 pairs of human chromosomes do.
It didn't start out that way. Early in development,
when the female embryo is still a clump of undifferentiated cells, the
two X chromosomes are indistinguishable. But as the tiny female embryo
begins to develop form, fate intervenes for one of the sisters. A crucial
switch, known as the X inactivation center, turns on and a signal sweeps
down the chromosome. Genes linked with that chromosome's X inactivation
center become silent--never to be read into proteins.
As the cells of the embryo divide, the inactivated
X in each cell is duplicated like all the other chromosomes, but its
descendants remain silent even after being copied for many cell generations.
HE INACTIVE X isn't altogether silent, however. In
humans, at least 16 genes are known to escape X inactivation. These genes
are read into proteins whether they dwell on the inactive or active sister.
But many fewer genes seem to escape X inactivation on the mouse X.
Unity of Sex
when all is said and done, is a method for mixing up genetic material.
Sex is such a good idea that species ranging from ferns to frogs
have converged on near-universal methods of sexual reproduction.
Considering how diverse the different sexually
reproducing species are, how much they share in common is remarkable.
Sexually reproducing species almost invariably have two genders--males
and females, of course--which usually exist in equal proportions,
and where opposites mate. In almost all sexually reproducing species,
too, gametes come in two forms--fat eggs and tiny, mobile sperm.
Usually, an organism inherits one complete set of genes from each
parent. When the time comes to make its own gametes, the individual
packages one complete gene set, shuffled from both parents, into
each of its gametes.
Sexually reproducing species widely rely on
sex chromosomes to determine sexual fate. Some exceptions do exist--some
reptiles, for example, rely on environmental cues instead. The
alligator is one such reptile. The temperature an alligator is
exposed to during a critical window of development determines
whether it will wind up a male or a female. More commonly, however,
heredity determines sex, which seems a more reliable way of producing
even proportions of the sexes.
Like sexual reproduction, sex chromosome systems
have evolved independently but in common ways. For example, flies,
worms, birds, and mammals (to restrict the examples to animals)
all have sex chromosomes that share basic characteristics, yet
their sex chromosomes are not related at the level of molecules
(they use different genetic pathways to determine sex).
Some themes guide the evolution and logic of
sex chromosome systems. One such these is that chromosomes come
in sister pairs, one inherited from the mother and one from the
father. Sex chromosomes, for example the X and the Y in mammals,
often look starkly different, yet are thought to derive from once-identical
sister chromosomes. Generally, one sex chromosome (the X in mammals)
looks like a standard-issue, medium-sized chromosome with thousands
of genes, while the other (the Y in mammals) tends to be shrunken,
clogged with junk DNA and almost bereft of genes. For example,
the human Y may house only two dozen genes, whereas an average
human chromosome houses one hundred times that number.
A general characteristic of sex chromosomes
is that they do not recombine as other pairs of chromosomes do,
except at their tips. Recombination is a process where sister
chromosomes align, intermingle, and trade fragments of genetic
material before segregating into egg or sperm cells.
Unrelated sex chromosome systems also invented
methods of dosage compensation, which balance the dose of gene
products derived from the sex chromosomes between males and females.
Genes act in complex concert with each other, and the levels of
gene products have been fine-tuned over the course of evolution.
In many species, females (XX) inherit two X chromosomes while
males (XY) inherit just one. Fruit flies that confronted this
dilemma have solved it by elevating expression from the male's
single X chromosome. Mammals, meanwhile, inactivate the second
X chromosome in female cells--or, more precisely, they permit
only one X chromosome to remain active in a cell.
Despite differences in the specifics of how
sex and sex chromosomes have evolved, various lineages, whether
worms or flies or buffalo, have confronted similar problems and
come up with similar solutions.
What came first--inactivation, as in mice, or escape
from inactivation, as in humans? How did these genes behave in the mammalian
ancestor? Did regulation switch during the evolution of primates or rodents?
Tracing X inactivation affords a rare opportunity to reconstruct how a
fundamental process in development evolved.
Sex chromosomes, like sex, have arisen independently
many times, in different kinds of organisms, yet have evolved in parallel
ways (see sidebar).
THE SILENT X
MANY measures, the inactivated X is distinct from its sister and from
other working chromosomes. For one thing, it just looks different under
a microscope--condensed and segregated against the nuclear wall, instead
of fanned out throughout the nucleus like the other chromosomes.
Also, the inactivated X is chemically altered. Methyl
groups tacked onto it appear to cement inactivation. The methyl groups
serve as an inheritable marker, ensuring that copies made from the inactive
X are also inactive.
During development, X inactivation of a chromosome
seems an indiscriminate process. If a fragment of an X chromosome with
the inactivation center (or even the inactivation center alone) happens
to become attached to another chromosome, the inactivating force can
roll down the other chromosome too, silencing many genes with no previous
experience of X inactivation. X inactivation spreading into other chromosomes
in this way usually kills a cell.
THE view at the "forest" level. But at the "tree" level of individual
genes, X inactivation appears less thorough, at least in humans. Some
of the genes that evade inactivation are clustered at the ends of the
chromosome, but others are sprinkled throughout the chromosome's body,
among inactivated genes.
The set of genes on the mouse X chromosome matches
the set on the human X, so the activity of genes can be compared between
the two species. And the mouse X chromosome appears to be more thoroughly
X-inactivated than the human X. Nearly all the genes known to escape
X inactivation in humans succumb to X inactivation in mice; only two
genes escape X inactivation in mice as well as in humans. No additional
genes that escape inactivation are known in mice.
Again, the question arises, what came first? Were
genes that escape X inactivation in humans active in the common ancestor
of mammals as well, or were they already subject to X inactivation,
as in mice? When did regulation change? To extrapolate ancestral conditions,
our lab undertook a comparative study of X inactivation, looking at
a host of mammalian species.
Humans and mice are about as distantly related to
each other as any two placental mammals can be. The placental mammals
are thought to have begun diverging from one another about 100 million
years ago. The various Orders--primates, rodents, carnivores, rabbits
and hares, horses and their relatives, etc.--separated quickly, perhaps
over the course of 30 million years. The precise sequence of their branching
has been difficult to decode and remains unresolved. Still, surveying
X inactivation in a range of placental mammals can reveal whether inactivation
or escape is more likely to represent the ancestral condition.
Whether one or two copies of a gene are switched
on in any given female cell cannot be observed simply. Directly observing
gene activity in a wide range of species would have been impossible,
so our lab turned to DNA methylation as a way to assay X inactivation.
In every case that we and others have examined, methylation of an X-linked
gene in a region overlapping its start site correlates with X inactivation.
We studied three genes. Close relatives--all the
primates for instance, or subsets of rodents--always went together in
activating or inactivating each of the genes. So switching between X
inactivation and escape from inactivation does not appear to occur often.
But knowing the X inactivation status of one gene proved a poor predictor
of the X inactivation status of another gene in the same species or
Although X inactivation acts like a tidal wave during
development, during evolution X inactivation may have advanced much
more daintily--gene by gene or patch by patch. X inactivation may appear
as sweeping as it does only because the patchwork acquisition of inactivation
has become so complete that inactivation now appears seamless.