Biology Reference
In-Depth Information
(i) Those that are specifically or predominantly expressed in the testis [sex
determining gene (
SRY
; Yp11.3), deleted in azoospermia (
DAZ
; Yq11),
RNA-binding motif protein 1 (
RBM1
; Yq11), testis-specific protein (
TSPY
),
chromodomain Y (
CDY
), basic proteins Y1 and Y2 (
BPY1
,
BPY2
), XK
related Y (
XKRY
), tyrosine phosphatase PTP-BL related Y (
PRY
) and testis
transcripts Y1 and Y2 (
TTY1
,
TTY2
)]. That these genes have not only been
retained on the Y chromosome, but in two cases have also been amplified,
may have been part of an evolutionary strategy to optimize male reproductive
fitness.
(ii) Those that are widely or ubiquitously expressed and which have closely
related counterparts on the X chromosome [dead box Y (
DBY
), thymosin
4
(
TB4Y
), translation initiation factor 1A (
EIF1AY
), ubiquitous TPR motif Y
(
UTY
) and
Drosophila
fat facets-related (
DFFRY
; Yq11.2),
AMELY
(Yp11.2),
RPS4Y
(Yp11.3), zinc finger protein Y (
ZFY
; Yp11.3) and
SMCY
; Lahn and
Page, 1997]. Conservation of specific X-Y gene pairs may have been associ-
ated with a requirement to maintain comparable expression levels for certain
housekeeping genes between males and females. Consistent with the predic-
tions of this postulate, the X chromosome homologues of these Y-borne genes
escape X inactivation.
The mammalian sex chromosomes are thought to be descended from a homolo-
gous pair of autosomes (reviewed by Ellis 1996; Graves
et al
., 1998a, 1998b; Wolf
et al
., 1992). This process could have been initiated with the evolutionary appear-
ance of the testis-determining
SRY
gene on the nascent Y chromosome, probably
by duplication, translocation and subsequent divergence of the X-linked
SOX3
(Xq26-q27) gene. Suppression of recombination with its homologous chromo-
some (the nascent X) then led to the gradual degeneration of the Y chromosome
owing to its inability to segregate genes carrying deleterious alleles ('Müller's
ratchet'; Charlesworth, 1978). Evidence for this degenerative process comes from
several sources. The rate of nucleotide substitution in Y chromosome genes
appears to be ~2-fold higher than the rate exhibited by X chromosomal genes
(Pamilo and Bianchi, 1993; Shimmin
et al
., 1993) although the frequency of DNA
sequence polymorphism may be lower in the sex-specific region of the Y chromo-
some than in the PARs (Allen and Oster, 1994; Whitfield
et al
., 1995). The Y chro-
mosome also exhibits a high frequency of retroviral insertion in humans,
chimpanzees and orangutans (Kjellman
et al
., 1995). Finally, there is emerging
evidence for gene loss from the Y chromosome during mammalian evolution. In
mouse and human, the ubiquitin-activating enzyme (
UBE1
) gene is located on
the X chromosome (Xp11.23-p11.3). A copy of the
Ube1
gene is also located on the
Y chromosome in the mouse, ring-tailed lemur, squirrel monkey (
Saimiri sciureus
)
and marmoset (
Callithrix jacchus
) but not in the Old World monkeys, chimpanzee
or human, indicating loss of the Y-linked gene >35 Myrs ago during the evolution
of the primates (Mitchell
et al
., 1998). Similarly, the Y-linked copy of the
EIF2S3
gene (Xp22.1-p22.2) encoding the eukaryotic translation initiation factor EIF-2
γ
was lost 35-60 Myrs ago in a common ancestor of the simian primates (Ehrmann
et al
., 1998). The gradual degeneration of the Y chromosome implies that the
retention of functional gene copies on this chromosome for a significant period of
evolutionary time could have conferred some selective advantage.
