Biology Reference
In-Depth Information
Fig. 18.
Comparative illustration of timing of gene expression in gonads of
Oryzias
latipes,Oreochromis niloticus
and
Danio rerio
during gonadal sex differentiation. The white bar
represents a time line of developmental age (dph/dpf). Hatching is indicated by a vertical slim
bar in the white bar. Land mark events in the gonadal differentiation are indicated above the
white bar within boxes. The timing and duration of expression of each gene are indicated below
the bar in horizontal rows (from Siegried 2010. In search of determinants: genes expression
during gonadal sex differentiation. J Fish Biol, 76: 1879-1902, redrawn and reproduced by
permission of The Fisheries Society of the British Isles/John Wiley & Sons Ltd)
the sequences and their sexual dimorphic expressions vary considerably
among these species. However, there are many similarities between gonadal
gene expression in these three species and in comparison to mammals they
reveal the conserved role of many genes during vertebrate gonadal sex
differentiation. The mutations inducing sex reversal at the PGC proliferation
and the results of experimental hormonal induction of sex reversal described
here clearly indicate that changes at the earlier stages of the genetic cascades
are tolerated but not at the later stages. Notably the genes at the top of
the cascade are not conserved, whereas the downstream genes have been
homologues in a much broader spectrum of species (Herpin and Schartl,
2011). The scope for wide diversity and complexity in timing and strength
of expression of one or the other genes in the cascade renders enormous
plasticity and fl exibility in dimorphic sexual differentiation in fi shes.
2.8 Gonadal ducts
The reproductive system consists of two functional units: gonads and
genital ducts. The gonads generate gametes and the genital ducts serve
to transport the gametes to the exterior. In mammals and elasmobranchs,
the genital ducts develop in close relation to the renal duct systems. The
paranephric or Mullerian duct differentiates into oviduct, uterus, cervix and
