Because inadequate ultraviolet exposure may result in the sperm nucleus not
being destroyed and ordinary fertilization and genesis occurring, it is important to
examine whether the fish is indeed gynogenetic. As a first step, both parents are
thoroughly exposed to the rays. Varieties of sperm may be used that achieve
fertilization but without viability in the resulting embryo, or fertilization may
involve a male sperm with a dominant gene controlling for a trait that can be easily
distinguished outwardly and a contrasting recessive gene.
The embryos resulting when loach sperm is applied to goldfish eggs all die
particularly easily. Impregnation with loach sperm exposed to ultraviolet rays
results in a goldfish haploid that displays the characteristic deformities of the
haploid (haploid syndrome) and is incapable of survival. When multiplication is
applied to create a diploid, the result is a beautiful goldfish resembling a female.
In fish as in humans and insects, many species appear to have female homo sex
determination mechanisms (XX for females and XY for males, where X and Y are
the sex chromosomes or genes for sex determination). Males are only produced
when two forms of sperm are present with an X and Y chromosome and fertilization
takes place with the Y sperm. Because gynogenetic offspring inherit only the
genetic traits of the mother, all of them would be expected to be female. In many
cases, females have greater economic value than males as foodstuffs.
Koreans tend to prefer fish roe (such as salmon or cod) or ovaries, and growth is
typically superior for females compared to males for some varieties such as halibut.
In the cases of salmon and trout, males have a muddy-colored body as a secondary
sexual characteristic, which markedly reduced their commercial value. With
aquarium fish, the female body type is preferred, with its fleshiness and bulging
stomach. In seedling production, it is possible to have far fewer males than females,
but the typical scenario is a shortage of females.
Several means exist for controlling fish sex, but gynogenesis is applied to various species because it can be achieved through a single manipulation just after
fertilization. Gynogenetic seedlings, however, cannot be used as is for industry
purposes, as viability is low and various deformities tend to arise as a result of a
higher degree of inbreeding (as with incestuous hybridization) or the effects of
treatment. Gynogenetic fish are thus treated with hormones to make them functionally male at a point shortly after birth when their sex is indeterminate. Genetically, the resulting fish are all female and produce only X sperm. This method is
expected to be actively adopted in the future, as it is possible to achieve entirely
female seedling through normal hybridization of these males and females.
At this point, “false males” are created continuously to maintain a line from
entirely female seedlings without relying on gynogenesis. Females are always
selected from the most genetically superior for experiments in breeding outstanding
pedigrees (see Fig. 4.3) (Oshiro 1990).
In fact, examination of sex ratios for gynogenetic fish shows three categories
(Oshiro 1990): varieties in which all examples are female as expected, such as
rainbow trout, coho, silver carp, grass carp, catfish, and loach; varieties in which
roughly half or more are male, such as halibut and righteye flounders; and varieties
in which males are present at a fixed percentage or not present at all within the same
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4 Fish Breeding and Biotechnology
being destroyed and ordinary fertilization and genesis occurring, it is important to
examine whether the fish is indeed gynogenetic. As a first step, both parents are
thoroughly exposed to the rays. Varieties of sperm may be used that achieve
fertilization but without viability in the resulting embryo, or fertilization may
involve a male sperm with a dominant gene controlling for a trait that can be easily
distinguished outwardly and a contrasting recessive gene.
The embryos resulting when loach sperm is applied to goldfish eggs all die
particularly easily. Impregnation with loach sperm exposed to ultraviolet rays
results in a goldfish haploid that displays the characteristic deformities of the
haploid (haploid syndrome) and is incapable of survival. When multiplication is
applied to create a diploid, the result is a beautiful goldfish resembling a female.
In fish as in humans and insects, many species appear to have female homo sex
determination mechanisms (XX for females and XY for males, where X and Y are
the sex chromosomes or genes for sex determination). Males are only produced
when two forms of sperm are present with an X and Y chromosome and fertilization
takes place with the Y sperm. Because gynogenetic offspring inherit only the
genetic traits of the mother, all of them would be expected to be female. In many
cases, females have greater economic value than males as foodstuffs.
Koreans tend to prefer fish roe (such as salmon or cod) or ovaries, and growth is
typically superior for females compared to males for some varieties such as halibut.
In the cases of salmon and trout, males have a muddy-colored body as a secondary
sexual characteristic, which markedly reduced their commercial value. With
aquarium fish, the female body type is preferred, with its fleshiness and bulging
stomach. In seedling production, it is possible to have far fewer males than females,
but the typical scenario is a shortage of females.
Several means exist for controlling fish sex, but gynogenesis is applied to various species because it can be achieved through a single manipulation just after
fertilization. Gynogenetic seedlings, however, cannot be used as is for industry
purposes, as viability is low and various deformities tend to arise as a result of a
higher degree of inbreeding (as with incestuous hybridization) or the effects of
treatment. Gynogenetic fish are thus treated with hormones to make them functionally male at a point shortly after birth when their sex is indeterminate. Genetically, the resulting fish are all female and produce only X sperm. This method is
expected to be actively adopted in the future, as it is possible to achieve entirely
female seedling through normal hybridization of these males and females.
At this point, “false males” are created continuously to maintain a line from
entirely female seedlings without relying on gynogenesis. Females are always
selected from the most genetically superior for experiments in breeding outstanding
pedigrees (see Fig. 4.3) (Oshiro 1990).
In fact, examination of sex ratios for gynogenetic fish shows three categories
(Oshiro 1990): varieties in which all examples are female as expected, such as
rainbow trout, coho, silver carp, grass carp, catfish, and loach; varieties in which
roughly half or more are male, such as halibut and righteye flounders; and varieties
in which males are present at a fixed percentage or not present at all within the same
86
4 Fish Breeding and Biotechnology