Androgenesis and conservation of fishes

Abstract

Due to non-visibility of egg nucleus, the established scheme of nuclear manipulation to clone fish may prove a difficult task. However, fishes are amenable for interspecific androgenetic cloning. A recent discovery of using cadaveric sperm to successfully generate progenies has opened the possibility of adopting a simple, widely practicable method of post-mortem preserved (at - 20°C) sperm to induce androgenesis. Inactivation of maternal genome by UV-irradiation and activation of genome-inactivated homologous or heterologous egg by a single diploid or two haploid fresh or preserved sperms are some landmark events, which have not only accelerated research activity but also focused on the importance of androgenesis in aquaculture and conservation of fish germplasm. With the absence of acrosome in the teleostean sperm, fertilization in fish is not a species-specific event. Eggs of many teleosts are amenable for heterospecific insemination. Successful heterospecific insemination results in activation or fertilization of an ovum of an alien species and is the most important strategic step for induction of interspecific androgenetic cloning. Polyspermy, especially dispermy occurs in nature and can be experimentally achieved after incubation of the milt in calcium chloride or polyethylene glycol. The paternal origin of androgenotes is verified using selected phenotypic, protein and/or molecular markers as well as karyotyping and progeny testing. Recently, reporter genes, the green fluorescent protein gene and the Tc1 transposan-specific marker have also been used. While confirming the paternity of androgenotes, progeny testing has also indicated the unexpected occurrence of females, which are, however, shown to carry XY using a single diploid, rather than two haploid sperms for activation. About 84% androgenotes succumb during embryonic development. Haploid genome regulates the time scale of developmental sequence in both homologous and heterologous eggs of Puntius spp., as effectively as that of diploid. A couple of research groups have restored a fish species using its preserved sperm and genome-inactivated eggs of another species. A comparison on the source, technique and genomes used for generation of clones of mammals and androgenetic clones of fishes indicates that from the point of conservation and aquaculture, interspecific androgenetic cloning in fishes has an edge over that of mammals.