Biology Reference
In-Depth Information
the commonality of the end point, there seems to be endless ways, in which
the sex chromosomes are differentiated.
Evolution of
DMY
: However,
DMY
is not found in
O. celebensis,
O. mekongnensis, P. reticulata, O. niloticus, D. rerio
and
Fugu rubripes
(Kondo
et al., 2003; Tanaka et al., 2007). Notably,
O. latipes
and its sister species
O. curvinotus
and
O. luzonensis
possess an XX/XY sex determination
system. However, the sex chromosomes of
O. luzonensis
display only a weak
reduction of the recombination frequency and recombine well around the
sex determining region, in contrast to the highly restricted sex chromosome
of
O. latipes.
Hence the sex chromosomes of
O. luzonensis
must be 'younger'
than that of
O. latipes
and may be of 5 My (Tanaka et al., 2007). Similarly,
the Y chromosome in
O. dancena
is not homologous to that in
O. latipes,
although both of them have an XX/XY sex determination system. Through
progeny testing and linkage analysis of isolated sex-linked DNA markers,
Takehana et al. (2007) have shown that
O. hubbsi
, a most closely related
species to
O. dancena
has a ZZ/ZW system. Further the sex chromosome of
O. hubbsi
and
O. dancena
are not homologous, indicating different origins
of these sex chromosomes systems.
Mutations and sex reversal: Despite the estimated long history of
DMY
over
10 My, there are evidences to show that the Sex Determination System (SDS)
has not progressed to a similar level of stability, as in mammals and birds.
Firstly, the presence of homomorphic X and Y chromosomes in medaka fi sh
indicates that the molecular differentiation process of the sex chromosomes
has not progressed to a level of complete recombinational isolation to
render the visibility of X and Y chromosomes. Secondly, the sex specifi c
region in the Y chromosome is too small (cf Woram et al., 2003), spanning
to a few hundred kilobase in length. Some examples are described to show
that the SDS in medaka fi sh remains unstable rendering the spontaneous
appearences of XX males and XY females.
PCR genotyping of various strains of medaka, Nanda et al. (2003) found
a strikingly large number of males (> 10%) that did not amplify
Dmrt1bY
(Fig. 10A,B); this was also confi rmed by Southern blot analysis (Fig. 10C).
All these aberrant males had only X chromosomal alleles (Fig. 10D,E).
Incidentally, as early as in 1936, Aida recorded that of 5,000 medaka males
analyzed, seven were detected as XX males in one laboratory strain.
Shinomiya et al. (2004) surveyed 2,274 wild-caught
O. latipes
from 40
localities spread over all of Japan and 730 fi shes from 69 wild stocks from
Japan, Korea, China and Taiwan, and identifi ed 23 XY females from 11
localities (Table 14). Genetic analysis revealed that the XY females from
eight localities produced all female XY
m
, XY
Sai
or XY
wOur
progenies (carrying
Y
m
/Y
Sai
/Y
wOur
chromosome derived from XY females), while those from the
