As mentioned earlier, the sex of fish that are not differentiated into males or females at hatching can be controlled by the use of sex steroids at the time the gonads are differentiating. While direct masculanisation is frequently easier, feminisation has also been possible in some species. Methods employed for commercial production of all-female rainbow trout by using sex-reversed functional males and genetic females is summarised. An alternate method of producing monosex stock is to induce sterility, and this can be done by the administration of high doses of sex steroids or by chromosomal manipulation.
Chromosomal manipulation for inducing polyploidy, gynogenesis and androgenesis has been studied with a view to controlling sex, as well as for rapid inbreeding. Manipulation becomes feasible during the nuclear cycles of cell division, and since fertilisation in fish is external, artificial means can be employed to either gamete before fertilisation, or to the fertilised egg at any period during the formation of the zygote.
The chromosome number can be increased by subjecting the egg to a pressure or temperature shock shortly after fertilization. The normal expulsion of one set of maternal chromosomes is prevented, and so after fusion of the chromosomes from the sperm, the developing embryo contains three sets of chromosomes instead of the normal two sets. The extra set of chromosomes in the triploid individuals interferes with gonad development. Such induced triploidy is also useful for producing individuals with increased heterozygosity.
Gynogenesis, the parthenogenetic development of eggs after activation with genetically inert spermatozoa, is a very effective means of achieving relatively rapid inbreeding. It can be used to generate all-female stocks and for gene-transfer.
By exposing milt to a very high dose of atomic radiation (for example by using the radioactive isotope cobalt-60), the chromosomes of the sperm cells are destroyed. The milt is kept on ice at 0°C during radiation and can be stored thus for several days without loss of vitality. When the irradiated milt is mixed with eggs, the sperm cells penetrate the eggs but play no further part in the development of the egg. The embryo develops from the egg material only, without any male chromosomes. Since the egg is haploid (with only a single set of chromosomes), the developing embryos are also haploid. Though most of them die at or soon after hatching, there will usually be some gynogenetic diploids (with a double set of chromosomes), as a result of spontaneous diploidization of the female chromosome complex. To increase the frequency of diploidization of the female chromosome complex, temperature shock can be used. Treatment of eggs with low and high temperatures at the time of meiotic divisions results in disturbances in the process, such as disintegration of the spindle, due to which none of the chromosome sets can form the polar body, or the return of the polar body into the plasma of the ovum. The output of genetic diploids varies greatly, being high under favourable conditions according to the strength and duration of the temperature shock and the stage at which it is administered.