Dispermic induction of interspecific androgenesis in the fish, Buenos Aires tetra using surrogate eggs of widow tetra

Abstract

Induction of interspecific androgenesis in fish involves introduction of one or more sperm of a species into genome-inactivated eggs of another compatible species that serve as surrogate. Incubation of milt of Buenos Aires tetra (BT), Hemigrammus caudovittatus (recessive golden strain) for a constant period of 7 min in selected concentrations of polyethylene glycol (PEG) facilitated entry of two fused haploid sperm into genome-inactivated egg of widow tetra (WT), Gymnocorymbus ternetzi (dominant black strain). Following incubation at 2.5% PEG, the milt contained 78% motile sperm, of which 37% remained single; of the 41% fused sperm, 14% alone were completely fused from head to tail subsequently activating 61% of UV-irradiated (3 min) WT eggs. Hatched F₀ androgenotes were morphologically identical to the golden BT but were distinguishable from hybrid diploids and triploids by colour and shape. Karyotype, erythrocyte measurements and sex ratio 1♀ (X²X²) : 2♂ (X²Y² or Y²Y²) (the superscript '2' indicates paternal origin) of androgenotes confirmed their diploid status. Reproductive performance of androgenetic males was equal to normal males but that of the females was inferior. Of six crosses involving the androgenetic males (X²Y² or Y²Y²) and normal females (X¹X²) (the superscript '1' indicates maternal origin), four produced 100% male progenies by sperm bearing Y² genotype; two generated 50% (X¹X²/X²X²) females and 50% (X¹Y²/X²Y²) male progenies by sperm bearing either X² or Y² genotype. Only 1.8% dispermic androgenotes survived at hatching. Causes for low survival at hatching were traced to the death of 39% inactivated eggs, 49% haploids and about 10% embryos suffering developmental incompatibility. Hatching success increased to 4.3% with availability of 19% completely fused double sperm, when sperm concentration was quadruplicated.