Contributed by: Jianzhi Zhang
The X and Y chromosomes of humans originated from a pair of
autosomes in the common ancestor of placental and marsupial mammals. In his classic book entitled “Sex Chromosomes and Sex-Linked Genes”, Susumu
Ohno (1967) (1) wrote, “During the course of evolution, an ancestor to placental mammals must
have escaped a peril resulting from the hemizygous existence of all the
X-linked genes in the male by doubling the rate of product output of each
X-linked gene,” (p. 99). This presumed
doubling of expression on X would cause X tetraploidy in females, which is
believed to be the driving force behind the evolution of the random
inactivation of one X chromosome in females.
As a result, the expressions of X-linked genes become equalized between
males and females.
So, are
expressions of X-linked genes doubled to compensate the loss of their Y
homologs, as Ohno hypothesized 45 years ago?
In the last few years, a number of groups tested Ohno’s hypothesis
indirectly by comparing the expressions of X-linked and autosomal genes in
humans or mice (2-11). These authors reached different conclusions
either supporting or refuting Ohno’s hypothesis, depending on the transcriptome
data used and the genes compared. This
controversy is now resolved by a direct comparison of the expression levels of
human X-linked genes with those of their one-to-one orthologs in chicken.
Julien et al. (12) and
Lin et al. (13) analyzed the same RNA-Seq
data published last year. They
found that the expression ratio between a human X-linked gene and its
one-to-one ortholog in the “proto-X” chromosome in chicken has a median of
~0.5. That is, the per-allele expression
level of X-linked genes is on average unchanged! This finding conclusively refutes Ohno’s
hypothesis.
Does this finding imply that a two-fold change in gene
expression has such a small fitness effect that dosage compensation is hardly
needed? The answer appears different for
different genes. First, following a
recent analysis (14), Lin et al. found
that proto-X genes that encode members of large protein complexes did
experience an on average two-fold up-regulation during sex chromosome
evolution, likely because of the high dose sensitivity of large protein complex
members. But these genes constitute only
~5% of all X-linked genes and therefore do not show up in the chromosome-wide
analysis. Second, there are X-linked
genes that have now migrated to autosomes, which may have been a strategy to
avoid a dose change. Third, a
genome-wide study showed that haploinsufficiency is rare in yeast. It is possible that the same is true in
mammals. Fourth, even for one-to-one
orthologous genes in autosomes, a two-fold expression difference between human
and chicken is not uncommon, suggesting that perhaps expression levels need not
be so finely regulated and conserved.
Together, the analyses suggest that, for most genes on the proto-X, a
50% expression reduction is quite tolerable and need not be compensated.
Because Ohno’s hypothesis is the basis of the current model
of male:female X chromosome dosage compensation, its invalidation opens the
research for a new evolutionary explanation of X inactivation in female
mammals.
References
1. Ohno S (1967) Sex Chromosomes and
Sex-Linked Genes. New York: Springer-Verlag.
2. Gupta V, Parisi M, Sturgill D, Nuttall
R, Doctolero M, et al. (2006) Global analysis of X-chromosome dosage compensation. J Biol 5: 3.
3. Nguyen DK, Disteche CM (2006) Dosage compensation of the active X chromosome in mammals. Nat Genet 38: 47-53.
4. Lin H, Gupta V, Vermilyea MD, Falciani
F, Lee JT, O'Neill LP, and Turner, BM (2007) Dosage compensation in the mouse balances up-regulation and silencing of X-linked genes. PLoS Biol 5: e326.
5. Xiong Y, Chen X, Chen Z, Wang X, Shi S, Wang X, Zhang J, and He X (2010) RNA sequencing shows no dosage compensation of the active X-chromosome. Nat Genet 42: 1043-1047.
6. Deng X, Hiatt JB, Nguyen DK, Ercan S,
Sturgill D, Hillier L, Schlesinger F, Davis C, Reinke VJ, Gingeras TR, Shendure J, Waterston RH, Oliver B, Lieb JD, and Disteche CM (2011) Evidence for compensatory upregulation of expressed X-linked genes in mammals, Caenorhabditis elegans and Drosophila melanogaster.
Nat Genet 43: 1179-1185.
7. Kharchenko PV, Xi R, Park PJ (2011)
Evidence for dosage compensation between the X chromosome and autosomes in mammals. Nat Genet 43: 1167-1169.
8. Lin H, Halsall JA, Antczak P, O'Neill
LP, Falciani F, and Turner BM (2011) Relative overexpression of X-linked genes in mouse embryonic stem cells is consistent with Ohno's hypothesis. Nat Genet 43:
1169-1170.
9. Yildirim E, Sadreyev RI, Pinter SF, Lee
JT (2012) X-chromosome hyperactivation in mammals via nonlinear relationships between chromatin states and transcription. Nat Struct Mol Biol 19: 56-61.
10. He X, Chen X, Xiong Y, Chen Z, Wang X, Shi S, Wang X, and Zhang J (2011) He et al. reply. Nat Genet 43: 1171-1172.
11. Castagne R, Rotival M, Zeller T, Wild
PS, Truong V, Tregouet DA, Munzel T, Ziegler A, Cambien F, Blankenberg S, and Tiret L (2011) The choice of the filtering method in microarrays affects the inference regarding dosage compensation of the active X-chromosome.
PLoS One 6: e23956.
12. Julien P, Brawand D, Soumillon M,
Necsulea A, Liechti A, Schutz F, Daish T, Grutzner F, and Kaessmann H (2012) Mechanisms and evolutionary patterns of mammalian and avian dosage compensation. PLoS Biol 10: e1001328.
13. Lin F, Xing K, Zhang J, He X (2012)
Expression reduction in mammalian X chromosome evolution refutes Ohno's hypothesis of dosage compensation. Proc Natl Acad Sci U S A 109: 11752-11757.
14. Pessia E, Makino T, Bailly-Bechet M,
McLysaght A, Marais GA (2012) Mammalian X chromosome inactivation evolved as a dosage-compensation mechanism for dosage-sensitive genes on the X chromosome.
Proc Natl Acad Sci U S A 109: 5346-5351.
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