Production of polyploids and viable gynogens using spontaneously occurring tetraploid loach, Misgurnus anguillicaudatus

Abstract

Although a majority of the loach in Japan are diploid with 50 chromosomes, spontaneously occurring tetraploids and triploids are frequently found in the specimens collected from commercial suppliers. We made experimental crosses using mature tetraploids and manipulated chomosome-sets in order to evaluate the extent of spontaneous tetraploidy. We examined the ploidy status of the fish developed from each experimental cross, by counting chromosomes and measuring erythrocyte sizes. The cross between a tetraploid female and a tetraploid male (4n×4n) and that between the tetraploid female and diploid males (4n×2n) produced viable tetraploid (4n=100) and triploid (3n=75) offspring, respectively. Fish developed after heat-shocking a 4n×2n cross showed a pentaploid karyotype (5n=125). Crosses of diploid females with a tetraploid male (2n×4n) produced triploid offspring. Fish developed after heat-shocking fertilized eggs from a 2n×4n cross had a tetraploid karyotype (4n=100). Gynogenetic progeny were produced from the tetraploid female after fertilization with ultraviolet (UV) light-irradiated spermatozoa from normal diploid loach or fancy carp (C) Cyprinus carpio. The fish surviving from these two gynogenetic crosses had a maternally derived diploid karyotype (2n=50). Gynogen production coupled with cold shock to inhibit the second meiotic division yielded viable tetraploid offspring.