Biology Reference
In-Depth Information
the duplication and fate of the androgen receptor (
ar
), a nuclear receptor
known to play a key role in sex determination in vertebrates. Consistent
with an early WGD event,
ar
has been duplicated into two genes
ar-A
and
ar-B
(e.g.,
Anguilla, Heterotis
) after the split of Acipenseriformes
from the lineage leading to teleost fi shes but prior to the divergence of
Osteoglossiformes. Genomic and syntenic analyses have shown that one of
the duplicated copies,
ar-B
has been lost in several Clupeocephalids such
as Cypriniformes (fi ve species including zebrafi sh), Characiformes (one
species), Siluriformes (one species) and Salmoniformes (two species) but
not the basal Osteoglossiformes and Anguilliformes. In Percomorphs,
ar-B
has accumulated substitutions in the ligand-binding and DNA-binding
domains (LBD and DBD).
Further analysis by Douard et al. (2008) has led them to suggest the
putatitive neo-functionalization of the same duplicate in Percomorphs,
which occurred a long time after the WGD. For instance,
ar-A
and
ar-B
have
distinct expression patterns in the brain of
Astatotilapia burtoni
(Harbott et al.,
2007) with differential amplifi cation of these receptors in the maintenance
social dominance status of the male (Baumeister et al., 2007). Considering
such functional shifts observed in the Percomorph lineages, Douard et al.
(2008) have been tempted to link their functional shift and sexual lability,
in view of the fact that the Percomorphs contain 90% of all hermaphrodite
species known to date. The existence of two functionally divergent
ar-A
genes may be viewed as a permissive factor allowing plasticity (cf Le Page et
al., 2010) and evolvability of divergent sex determination in these fi shes.
2.5 Gonadal differentiation
In most teleosts, ovarian differentiation precedes and commences with the
appearance of proliferation of germ cells supporting somatic cells, arrival
of germ cells, followed by meiosis, and the formation of oogonia, early
oocytes and ovarian cavity (Guraya, 2000). For instance, the proliferation of
PGCs in embryos of chub mackerel
Scomber japonicus
is completed between
5 and 10 dph. The gonadal primordium is formed at 15 dph, followed by
ovarian differentiation between 30 and 40 dph, as indicated by the presence
of meiotic oocytes and the ovarian cavity. Perinuclear oocytes are observed
at 60 dph (Kobayashi et al., 2011). Due to delayed proliferation of germ cells,
both ovarian and testicular differentiation commences simultaneously in
Siganus guttatus
(Komatsu et al., 2006), which is in contrast to the general
norm of teleosts. However, testicular differentiation in
S. japonicus,
initiated
at 30 dph, as indicated by the formation of sperm duct primordial, develops
into spermatocytes at the chromatin nuclear stage after 90 dph. Subsequently
spermatids and spermatozoa appear at 160 dph (Kobayashi et al., 2011).
