Differentiation in Gonochores - Genetic Sex Differentiation in Fish - page 48

II (Morinaga et al., 2004, 2007). This hotei medaka possesses an expanded
abdomen with hypertrophic gonads. Interestingly functional male-to-female
sex reversal frequently occurs in hotei mutant genetic male and 50% of hotei
homozygous males are sex reversed. Surprisingly, the amh is also involved
in sex-dependent regulation of PGC proliferation (Shiraishi et al., 2008).
In the normal XY embryos of medaka, DMY continues to express on
the zero dah and its 36 KD protein is present primarily in the nuclei of PGC
supporting somatic cells surrounding the PGCs. But after 20 dph, the Dmrt1
mRNA begins to increase more rapidly in testes than in the XX gonad.
Subsequently, the Dmrt1 mRNA is more rapidly accumulated than that of
DMY mRNA. Expression of DMY persists in Sertoli cells lineage from PGC-
supporting cells upto Sertoli cells, indicating that only DMY - positive cells
enclose PGCs. Hence the DMY -positive cells are the pre-Sertoli cells and
differentiate into Sertoli cells. In mature testes, DMY is localized in the Sertoli
cells during spermatogenesis and epithelial cells of intratesticular efferent
duct, a derivative of Sertoli cells. Dmrt1 is expressed in spermatogonium
supporting cells after testicular differentiation and its expression is much
higher than that of DMY in mature testes. In testes of sex reversed (XX)
neomales, which never expresses DMY (as XX ♀ gonad), Dmrt1 mRNA is
localized in Sertoli cells and the epithelial cells of intratesticular efferent
duct, clearly showing that DMY and Dmrt1 are co-expressed in Sertoli cells
in mature testes. This important observation of Kobayashi et al. (2004) in
O. latipes has much implication on the role played by Dmrt1 in gonadal
differentiation of O. niloticus . Briefl y DMY regulates proliferation of PGCs
and sex specifi c early embryonic differentiation, whereas Dmrt1 regulates
spermatogonial differentiation during subsequent embryonic gonadal
differentiation.
In medaka, Yokoi et al. (2002) isolated two kinds of transcripts sox9 and
soxlf . The sox9 encodes 487 amino acids and shows 70% amino acid sequence
identity with known vertebrate sox9 proteins. soxlf is a longer form of the
sox9 , which transcribes from an additional exon in 5΄ upstream region.
Nakamoto et al. (2005) cloned a novel testicular type sox9a from medaka,
which they named sox9a2 . Establishing an enhanced green fl uorescent
protein Egfp transgenic line that mimics sox9b/sox9a2 , Nakamura S et al.
(2008) found a cluster of Egfp -expressing cells in single gonadal primordium.
In this primordium, they also characterized the precursors of the supporting
somatic cell lineage common to both Sertoli cells and granulosa cells in
embryos at stage 3. In an earlier study, Nakamura et al. (2006) showed
expression of sox9b/sox9a2 gene, a homologue of mammalian Sox9 , in one of
the supporting somatic cell line of the medaka; in fact two distinct somatic
supporting cell lineages, one with sox9b/sox9a2 expressing cells and the
other with ftz-f1 expressing cells are formed prior to the formation of the