2.3 Androgenesis and Gynogenesis
These two genetic phenomena define the development of individuals which have all of their genetic endowment derived either from the father (androgenesis) or the mother (gynogenesis). In most vertebrate classes such individuals are either defective or non-viable due to the phenomenon of genomic imprinting, a remarkable process whereby certain genes derived either from the male or female gamete are imprinted by DNA methylation so that they are resistant to transcriptional activation and, therefore, the homologous sequence from the other parent is a necessary requirement for normal development. There is no indication of an equivalent phenomenon in fish thereby allowing the exploitation of androgenesis and gynogenesis.2.3.1 Gynogenesis
This genetic state can be induced in fish by irradiation or chemical treatment of sperm prior to fertilization (such that fertilization is replaced by sperm activation) followed by treatment of the activated eggs by pressure or heat exposure to suppress the first cleavage division (mitotic gynogenesis) or to prevent elimination of the second polar body (meiotic gynogenesis).
Gynogenesis has been successfully used in model fish systems to generate isogenic lines of fish, as with zebrafish Danio rerio and medaka Oryzias latipes. This is mitotic rather than meiotic gynogenesis. The technique has also been used in fish gene mapping to determine the proximity of known genes to their chromosomal centromeres, since all heterozygosity observed in gynogenetic progeny results from crossing over between homologous chromosomes in the egg. A third application of gynogenesis has been exploited by Thorgaard et al. (1992) to map genes to specific chromosome fragments. The procedure relies on the fact that chromosome in irradiated sperm are often broken into fragments and some of these male-derived fragments can persist in gynogenetic eggs and so contribute to gene expression in the embryos. Gynogenetic diploids could be used with species such as sturgeon to produce all-female stock for caviar production. Also inbred lines of fish could be used for production of F1 hybrids showing heterosis.
2.3.2 Androgenesis
This phenomenon is the reciprocal of gynogenesis and involves elimination of egg-derived DNA prior to fertilization. Suppression of the first cleavage division then results in the production of progeny with both chromosome sets being male derived. Since tilapia are sexually differentiated by XX female genotype and XY male genotype, YY males can result from androgenesis. Such 'supermales', when crossed with normal females, will produce all male progeny. This approach has been tried in tilapia but effectively superseded by the use of hormonal sex reversal of female fish.
