Biology Reference
In-Depth Information
It should however be noted that the Y chromosome can also acquire genetic
material from other chromosomes. For example, the multicopy
DAZL1
gene
(Yq11.23; deleted in azoospermia) was transposed to the Y chromosome from an
autosome during primate evolution (Glaser
et al
., 1998; Saxena
et al
., 1996; Shan
et al
., 1996) as was the multicopy RNA-binding motif (
RBM1
; Yq11) gene (Chai
et al
., 1998; Delbridge
et al
., 1997). It may be that transfer to a male-specific loca-
tion provided protection against inactivation or loss. Another example of the
duplicational transposition of a gene to the Y chromosome is that of
AMELX
(Xp22.1-p22.31) and its Y-chromosome counterpart
AMELY
(Yp11.2); the latter
gene, which appears to be fully functional, is present on the Y chromosomes of
bovids and primates but not rodents thereby dating the transpositional event to at
least 40 Myrs ago (Toyosawa
et al
., 1998).
In humans, the
XG
blood group gene (Xp22.32) spans the major PAR on the X
chromosome—the first three exons are pseudoautosomal whereas the remaining
seven are X chromosome-specific (Weller
et al
., 1995). In humans and the great
apes, an
Alu
sequence is located at the boundary between the major PAR and the
Y chromosome-specific DNA (Ellis
et al
., 1990) but this sequence is not present
in Old World monkeys. The
Alu
sequence was therefore inserted into the pre-
existing boundary after the divergence of the great apes from the Old World
monkeys. Although it did not create the boundary, the
Alu
sequence does serve
to demarcate it.
Ellis
et al
. (1994) proposed a model for the formation of the boundary of the
major PAR. They hypothesized a pericentric inversion of the Y chromosome with
one breakpoint in the ancestral
XG
gene and the other breakpoint 5 kb distal to
the ancestral
SRY
gene. In a refinement of this postulate, Fukagawa
et al
. (1996)
suggested that the inversion occurred by illegitimate recombination between two
PAR boundary sequences, one in the ancestral
XG
gene and the other near the
ancestral
SRY
gene.
The PAR has undergone quite rapid change during mammalian evolution
involving both gene duplication and translocation events in the region (e.g.
STS
,
MIC2
,
XG
,
CSF2RA
,
IL3RA
,
ARSD
,
ARSE
; Meroni
et al
., 1996; Ried
et al
.,
1998) and resulting in the movement of the PAR boundary to create X-unique
regions (Perry
et al
., 1998). The evolution of the PAR and the divergence of the
mammalian X and Y chromosomes may be viewed in terms of the 'addition-attri-
tion' hypothesis (Graves, 1995; Graves
et al
., 1998a). This states that the incorpo-
ration of autosomal sequences into the PAR of either the X or Y chromosome
initially served to generate homologous regions which could pair at meiosis.
Recombination with an homologous partner could then result in PAR enlarge-
ment. Alternatively, the steadily accumulating mutations on the Y chromosome
would have served to decrease the level of homology to the X chromosome
thereby reducing PAR size. Fukagawa
et al
. (1996) proposed a further twist to this
argument in that once divergence had reached a certain level, recombination fre-
quency would have decreased thereby further increasing the rate of divergence.
Evidence in favor of the addition-attrition theory comes from the dynamic
nature of the major PAR region during mammalian evolution. Thus, the
STS
gene which is X-linked in humans and the great apes (Xp22.32) is autosomal in
prosimians as is the
ANT3
gene which is pseudoautosomal (Yp11.3) in humans
