Differentiation in Gonochores - Genetic Sex Differentiation in Fish - page 45

Hd-rR HNI male sired male and female progenies, when mated to
Kesen-numa XY wKsn or Shirone XY m females. Interestingly, four females
among 18 XY wKsn F 1 progenies and 22 XY wKsn females among 35 F 2 progenies
were produced. Conversely, all the 41 F 2 progenies (bearing Y wKsn /Y wKsn
genotype) produced from a cross between F 1 XY wKsn ♀ and F 1 Y HNI Y wKsn
♂ were all males. These observations on sex ratio of F 1 and F 2 progenies
indicate that a minimum of two Y wKsn chromosomes are required for male
sex determination, a phenomenon resembling the female heterogametic
system (Matsuda, 2011). All progenies sired by Hd-rRY HNI male mated to
XY wOur female were females (Otake et al., 2006).
Otake et al. (2006) also measured the DMY expression levels in some of
these XY females. The DMY mRNA levels are signifi cantly lower in Y wKsn
Y wKsn than in XY Hd-rR and XY HNI males (Fig. 12). The same is also true of Y wOur
Y wOur . All these progenies carrying XY wKsn XY wKsn and Y wOur Y wOur genotypes
develop into females.
2.6 Model fi shes
Of four model species, in which sex differentiation is described, almost
complete, detailed information is available for O. latipes thanks to Japanese
scientists. For the others, information available is limited and fragmentary
in the order of O. niloticus < D. rerio < Silurus meridionalis . Yet these models
provide interesting diversity in sex differentiation and represent the fi rst step
in our endeavour to identify the genes responsible for induction of sexual
dimorphism; the very fi rst gene known at present to induce male differentiation
is DMY/Dmrt1bY in medaka, tDmrt1 in tilapia and amh in zebrafi sh; in the
Southern catfi sh, foxl2 may be the fi rst to induce female differentiation, as
the ovarian cavity is formed prior to the proliferation of PGCs.
2.6a Oryzias latipes
In medaka fi sh, the formation of gonadal anlage is completed at the
embryonic stage 38 (Kobayashi et al., 2004). The very fi rst sign of sexually
different trends in proliferation of PGCs becomes apparent at stages between
37 and 39, when DMY is specifi cally expressed in male XY embryos but not
in XX embryos (see also Lewis et al., 2008). Incidentally, the Dmrt1 mRNA
is not yet detectable at these stages clearly indicating that DMY begins its
expression much earlier than Dmrt1 and any morphological differentiation
such as the formation of acinus, a globular structure and the precursor of
the seminiferous tubules (see Pandian, 2010).
Figure 13 shows bisexually different trends of proliferation of PGCs in
selected fi shes. The fi rst role of Dmrt1bY is the suppression of proliferation
of PGCs through a paracrine system. The proliferation is even inhibited