Biology Reference
In-Depth Information
screening of haploid medaka is useful to identify drastic morphogenic
mutants showing dorso-ventral polarity and antero-posterior pattern of
the Central Nervous System (CNS) but it is not suitable to identify genes
controlling regional polarity in organs and boundary formation between
tissues.
Large scale mutational screens of the medaka have identifi ed a large
number of genes that are zygotically required for normal embryonic
development (Furutani-Seki et al., 2004; Morinaga et al., 2004). These zygotic
mutants are now recognized as powerful tools for the study of the genetic
cascades involved in vertebrate development (Shimada and Takeda, 2008).
In most animals, the genomes of fertilized embryos are transcriptionally
silent until a specifi c stage of early development; the maternally supplied
products, synthesized and accumulated during oogenesis, may attenuate the
phenotype of a zygote mutant fully or partially, repairing the lost gene(s).
To understand the early functions of such mutated genes, a mutant, that
lacks both maternal and zygotic gene products, referred to as a maternal-
zygotic mutant, is required.
Taking advantage of the sexually differentiated but sterile hybrid
between
Oryzias latipes
and
O. curvinotus,
Shimada and Takeda (2008)
developed a method of using naturally available sterile ovary and testis
of these interspecifi c hybrids as recipients, in which germ-line is restored
by transplanting PGCs of heterozygous MZ mutant for
fgfr1
(lethal
heal/fi broblast growth factor) (Fig. 58). The resulting mutant reveals the
precise role of
fgfr1-
mediated signaling in the early embryos. Out of 174
transplanted recipient embryos, 30% are found to have donor PGCs that
have successfully migrated and colonized the recipients' gonad. Among
the 20 recipient females, 60% recovered their fertility and produced donor-
derived eggs. Clearly, the female hybrid sterility can be attributed to a
defect in the germ cell lineage and the somatic cells in the hybrid ovary
have retained the ability to support oocyte development. However, fertility
is recovered in the female transgenic medaka. A similar research has been
performed in the danios, in which fertility is recovered in males but not in
females (Wong et al., 2010).
There are also examples for artifi cial induction of mutants resulting
in alterations in the dorso-ventral axis. Thermal shock treatment affects
and prohibits the release of the second polar body or the fi rst cell division
(Pandian, 2011). The treatment may alter the formation of tubulin in egg
cytoplasm, which plays an essential role in segregation of chromosome
sets to daughter cells. The microtubules also regulate the early cellular
differentiation of blastomere from vegetal to the animal hemisphere of
embryos or
vice versa
. From eggs that underwent heat shock for 30 to
90 seconds at 40ºC, a high percentage of goldfi sh embryos were shown
to receive reduced signal or no signal of
gooscoid (gsc)
mRNA (Fig. 59),
