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other of these genes, the proliferation of PGCs is postponed for a while in
the presumptive male embryos, resulting in the production of signifi cantly
less number of PGCs in male embryos than in female embryos in the three
species, for which the relevant information is reported (Fig. 13). This is
perhaps the fi rst sign of testicular differentiation. The expression of these
genes is localized in some germ cell supporting somatic cells (GCSSCs)
that surround the PGCs. They are the pre-Sertoli cells and differentiate into
Sertoli Cells (Kobayashi et al., 2004).
In O. latipes and O. curvinotus Casp6 is a sex linked DNA marker (Kondo
et al., 2003). Males have DMY and are heterogametic in Casp6 , whereas
females are homogametic in Casp6 (Matsuda et al., 2003). Males carry
paternal genotype and females maternal genotype, indicating the DMY is
located on the Y chromosome. The cDNA sequence of DMY of O. latipes
encodes a putative protein of 280 amino acids, whereas the cDNA sequence
of Dmrt1 a protein of 276 amino acid (Fig. 9B). Many common PCR primers
can be used on DMY and Dmrt1 , as they carry similar nucleotide sequence.
A PCR primer DMY and Dmrt1 of O. latipes amplifi es DMY and Dmrt1 of
O. curvinotus (Fig. 9A).
A phylogenetic tree based on amino acid sequences of the DM- domains
of these and other DMY and Dmrt1 in database shows that Oryzias DMY
makes a clade with Oryzias Dmrt1 . The clade is divided into two lineages,
one consisting of the DMYs of O. latipes and O. curvinotus, and the other
with the Dmrt1s of these two species (Fig. 9C). Hence DMY has been derived
from Dmrt1 immediately prior to separation of O. latipes and O. curvinotus
(Matsuda et al., 2003).
Age of DMY : DMY is located on the Y chromosome, which is always present
in males of O. latipes , while Dmrt1 located on an autosome, is present both
in males and females. The hypothesis of male-driven evolution asserts that
the mutation rate is higher in males than in females due to more rounds
of cell divisions in the male germ cells (Li et al., 2002; see also Pandian,
2011). Since the duplication events (see Pandian, 2011), there are a total
of 31.6 synonymous mutations in DMY but only 18.3 in Dmrt1 branches.
Considering the nucleotide mutation rate in mammals (Kumar and
Subramanian, 2002), the duplication occurred 15.3 Mya in Oryzias . Taking
synonymous substitution rates in other fi shes into consideration, Zhang
(2004) estimated that the age of DMY is in order of 10 My.
Sex determination involves a complex hierarchy of genes. Expression
screen analysis has identifi ed hundreds of candidate genes that show
sex specifi c expression pattern. It has indeed been diffi cult to place these
genes and their regulatory function into a cascade/network of genes.
Multiple Transposable Elements (TEs) are known to have been inserted
into the Y-specifi c region on LG1 of medaka (Kondo et al., 2006). Recently
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