Biology Reference
In-Depth Information
The choice between modes of reproduction depends ultimately on the role
of germ cells.
Through a series of path-breaking publications, Yoshizaki and his group
have demonstrated that PGCs can be transplanted to homologous and
heterologous alevins to generate allogenics and xenogenics, respectively.
From the point of sex determination and differentiation, PGCs have
retained the bisexual potency so long as the recipient of the transplantation
is limited to a pre-embryonic (e.g., medaka) and post-hatching stage
with undifferentiated gonad (e.g., rainbow trout, Takeuchi et al., 2003)
(see Pandian, 2011). For instance, the sex ratio was 0.55 ♀ : 0.45 ♂ for the
allogenic rainbow trout O. mykiss generated through transplantation of the
minimum required numer of (cf Saito et al., 2008) 5-10 PGCs drawn from
30 dpf old dominant orange colored trout into the blastulae of the recessive
gray colored trout (Takeuchi et al., 2003). Both male and female xenogenic
trout fry were also generated by the recipient O. masou blastulae, into
which 20-30 PGCs of O. mykiss were transplanted (Takeuchi et al., 2004),
although 50% xenogenic males and 2.6% xenogenic females alone were
fertile (Okutsu et al., 2008).
Avoiding the costs of producing sterilized testis Shimada and Takeda
(2008) have used naturally available hybrid steriles as recipients. A cross
between the Japanese medaka O. latipes ♀ and Chinese Hainan medaka
O.curvinotus ♂ produce sterile offspring; however, the gonads are sexually
differentiated in these hybrids but oogenesis and spermatogenesis are
severely impaired (Hamaguchi and Sakaizumi, 1992). Hence the use of
sterile hybrids provides an additional advantage by offering sterile ovaries
and testes. The main objective of Shimada and Takeda (2008) has been to
develop a new method for production of maternal-zygotic medaka but
some of their observations are highly relevant to allogenesis.
Orange colored mutant fgfr1 medaka labeled with rhodamine dextran
at one cell stage served as the donor source of the labeled PGCs. Blastulae,
generated by the cross between wild gray colored Kaga strain of O. latipes
females and Hainan O. curvinotus males, were the recipients. About a 100
donor cells, drawn from the deeper layer of the blastoderm, where PGCs
are known to reside (Kurokawa et al., 2006), were transplanted into the
animal pole of the recipient blastula (Fig. 34). Out of 174 transplanted
recipients, 52 (i.e., 30% ) were found to have the donor-derived PGCs that
have successfully migrated to the recipients gonad. Among the orange
colored 20 recipient females, fertility was recovered in 12, i.e., 60% (Fig. 34).
The allogenic recipient female grew and matured faster than the control
and commenced laying eggs at 5-6 weeks of age and produced about 210
eggs each. These females stopped spawning within several weeks and
once they stopped, they never produced eggs again. On the other hand,
the allogenic males failed to recover fertility, although they matured as
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