Mechanisms on Occurrence of Spontaneous Polyploid Fish and Their Application as Resources for Breeding

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The present study was made to clarify mechanisms on occurrence and inheritance manner of polyploids in loach fish. 1. Spontaneous tetraploid (4n) loach Misgurnus anguillicaudatus were not observed in the specimens obtained from river, but in those from a fish dealer. Small number of triploid (3n) loach were seen in the specimens from wild origin as well as fish farmers. 2. Gynogenetic progeny produced from eggs of normal diploid (2n) loach after fertiltzation with UV irradiated spermatozoa died before feeding, while those from eggs of 4n loach could survive. Viable gynogens were produced from the eggs of 4n race of spinous loach Cobitis biwas without diploidizing treatment. These observations conclude that 4n loach and spinous loach are true tetraploidy having four sets of homologous chromosomes. 3. Viable 3n loach were produced from reciprocal hybridizations between spontaneous 4n and 2n. 5n loach were produced by inhibiting the 2nd polar body extrusion in 4n*x2n* eggs. Both 3n and 5n females had mature eggs, While no male had functional spermatozoa. Offsprings developed from hybridizations (3n*x2n*, 5n*x2n*) and UV-induced gynogenesis were normal and survived. 4. Two types of eggs, large and small, were observed in 3n female. Frequency of large eggs was different among females. 5. Fertility in 3n and 5n females and viability of offsprings from 3n and 5n suggest presence of unusual gametogenesis to prevent formation of aneuploidy eggs and spermatozoa.