Biology Reference
In-Depth Information
New World primates.
Like gibbons, many New World primates possess highly
rearranged genomes. All New World primates have a translocation between chro-
mosomes 8 and 18. The Cebidae possess translocations between chromosomes
10/16 and 2/16 (Richard
et al
., 1996) whilst the Atelidae are characterized by
translocations between 3/15 and 4/15. Significant synteny is nevertheless apparent
between human and the capuchin monkey,
Cebus
(Richard
et al
., 1996), between
human and the Colobus monkey (Bigoni
et al
., 1997), between human and the
howler monkey (Consigliere
et al
., 1996), between human and the black-handed
spider monkey,
Ateles geoffroyi
(Morescalchi
et al
., 1997) and between human and
the marmoset,
Callithrix
(Sherlock
et al
., 1996).
2.3.4 Evolution of the human sex chromosomes and the pseudoautosomal
regions
The human sex chromosomes are heteromorphic; the X chromosome contains
~160 Mb DNA and perhaps 3000 genes whereas the Y chromosome contains only
60 Mb DNA and probably only a handful of genes. The Y chromosome is largely
composed of constitutive heterochromatin harboring different families of repeti-
tive DNA (Wolf
et al
., 1992). Despite the size difference between the sex chromo-
somes, they are nevertheless able to pair successfully during meiosis.
Recombination, however, is largely confined to the two
pseudoautosomal regions
(PARs); a major PAR (2.6 Mb in size) at the tips of the short arms of the X and Y
chromosomes which is the site of an obligate crossover during male meiosis
(Simmler
et al
., 1995), and a minor PAR (320 kb) at the tips of the long arms of the
X and Y chromosomes (Kvaloy
et al
., 1994). Regular X-Y recombinational
exchanges have served to maintain homology between the chromosomes in the
PAR regions (Graves
et al
., 1998a).
Gene mapping studies have shown that part of the eutherian (placental)
mammalian X chromosome ('conserved region'; XCR) is shared by the X chro-
mosomes of marsupials and monotremes (reviewed in Wilcox
et al
., 1996).
Since a series of genes on the short arm of the human X are clustered in two
autosomal groups in marsupials and monotremes, these loci define a region
(XRA) that has been recently added to the X chromosome in eutherian mam-
mals. Thus, the X chromosome of the common mammalian ancestor was
smaller than that found in extant eutherians and at least two autosomal regions
have since been added to it. The location of this X-autosome fusion corre-
sponds to the border between the XCR and the XRA and lies at Xp11.23
(Wilcox
et al
., 1996;
Figure 2.8
).
The human X and Y chromosomes also exhibit substantial homology outwith
the pseudoautosomal region (Bardoni
et al
., 1992; Lambson
et al
., 1992; Vogt
et al
.,
1997). Although this homology is consistent with these chromosomes having
once constituted a homomorphic pair (Graves
et al
., 1998b), the observed homolo-
gies also reflect intra-chromosomal duplication events followed by inter-chromo-
somal translocations. For example, the minor PAR is thought to have originated
during human evolution by the translocation of 320 kb of X chromosomal
sequence to the Y chromosome, an event which may have been mediated by
recombination between LINE elements (Kvaloy
et al
., 1994).
