Biology Reference
In-Depth Information
Fig. 17.
Genetic cascades of sex differentiation in the selected model fi shes. ST Pre = Sertolli cell
pre-cursors, E.ducts = Efferent ducts, S. genesis = Spermatogenesis, O. cavity = Ovarian cavity,
E. oocytes = Early oocytes. For the Southern catfi sh
Silurus
meridionalis
alone, genes, known to
be involved in sexual differentiation are shown, as they are not considered elsewhere
of
O. latipes, O. niloticus, D. rerio
and
S. meridionalis
. The differentiation
becomes apparent prior to hatching in
O. latipes
but in the others a few
days after hatching. In females, sexual differentiation proceeds from
meiosis in germ cells through the formation of follicles and the ovarian
cavity, and to oogenesis much earlier than the respective males. As
indicated, early oocytes and ovarian cavity are formed in all the juvenile
hermaphroditic zebrafi sh, but between the period from 15 and 29 dph,
the oocytes and cavity are degenerated by apoptosis in presumptive
males; subsequently testicular differentiation is completed. Whereas
O. latipes
and
O. niloticus
are primary gonochores, the zebrafish is a
secondary gonochore. It is not yet known whether the Southern catfi sh is
a tertiary gonochore, in which the juveniles initially possess a bipotential
gonad, which then develops directly into either an ovary or testis, as in
G. aculeatus
(Lewis et al., 2008) and
Anguilla anguilla
(Grandi et al., 2010). The
chronological events in the genetic cascades are bound to vary among these
primary and secondary gonochores. In comparison to gamety, not much is
known about which are the species that belong to the primary and secondary
gonochores. However, even within a genus, for instance,
Oncorhynchus,
one species is a primary gonochore (e.g.,
O. kisutch
) but the others like
O. masou
are secondary gonochores (see Pandian, 2011). Hence tracing the
chronological sequence of genetic cascade even among biologically and
economically important fi sh may prove to be an onerous task.
Following Morrish and Sinclair (2002), Ijiri et al. (2008), Siegfried (2010)
has made an admirable attempt to sequence the timing of gene expressions
in gonads of
O. latipes,
O niloticus
and
D. rerio
(Fig. 18). The timing and
