Biology Reference
In-Depth Information
all such ambiguous fi shes has become necessary. These fi ndings implicate
that the function/expression of one or more genes in genetic cascade of sex
determination system is altered resulting in the reversal of the genetic sex
to the opposite phenotypic sex.
Possession of a genotype of one sex and phenotypic expression of the
opposite sex by an individual clearly indicates the need for sex specifi c
molecular markers in fi shes. Many molecular markers are described to
identify genetic sex of a few fi shes. A series of sex-linked molecular markers
have also been described, for instance, in many salmonids and poeciliids
(Table 9). The Y chromosome linkage maps of four salmonids
Salvelinus
alpinus, Salmo salar, S. trutta
and
Oncorhynchus mykiss
reveal the telometric
placement of the sex determining region involving a smaller fraction of
chromosome in the fi rst three species but an intercalary position for
O.
mykiss
. In each species, the Y specifi c region is very short and the major male
determining gene is located on different linkage groups in these salmonids,
indicating that their Y chromosomes have evolved independently (Woram et
al., 2003). In a creditable study on genome mapping of the sex determining
region in sex chromosomes with several different molecular makers, Artieri
et al. (2006) have identifi ed the location of the sex determining gene SEX on
the metacentric arm of chromosome 2 in
S. salar.
After genome duplication
events 360 Mya (see Pandian, 2011), many salmonids have dealt with
sex determination by deleting one copy of the sex determining locus;
for instance,
S. salar
deleted one copy SEX in females, growth hormone
pseudogene
GH-ψ
in
O. masou
and
O. tshawytscha
(Davidson et al., 2009;
see Table 9). In another remarkable investigation in
X. maculatus,
in which
three different genetically defi ned sex chromosomes X, Y and W are present,
Schultheis et al. (2006) and Bohne et al. (2008, 2009) have located the master
sex determining gene in the Y chromosome between the melanoma-inducing
oncogene
Xmrk
and its proto-oncogene counterpart
egfrb
. The known
linkage groups and sex chromosome system (Tables 7, 8, 9) in selected fi sh
(for more details see Graves and Peichel, 2010) provide many examples for
“the repeated emergence of new sex chromosomes from autosomes and
thus bringing the sex differentiation cascade under new master genes”. In
view of its importance, several projects are now undertaken to clone the
master sex determining genes identifi ed so far in stickleback, fugu, tilapia,
salmonids, guppy and platyfi sh (see Schultheis et al., 2009).
2.2 Sex determining genes
In vertebrates only two sex determining genes have thus far been identifi ed;
the mammalian
SRY
gene and
DMY
/
Dmrt1bY
gene in medaka fi shes
O.
latipes
and
O. curvinotus
(Matsuda et al., 2002; Nanda et al., 2002).
SRY
and
DMY
are not homologous genes.
SRY
encodes a transcription factor
