Genetics of the loach, Misgurnus anguillicaudatus: recent progress and perspective.

The loach Misgurnus anguillicaudatus is an interesting fish from the viewpoint of polyploidy and its related atypical reproduction. Among Japanese populations, bisexual diploid individuals are most common, but natural triploids have been recorded in some localities. Experimental crosses including induced gynogenesis and inter-specific hybridization showed that triploid loaches arose from unreduced eggs formed by sympatric diploids. Two types of unreduced eggs produced by these diploid females, bisexual and unisexual reproduction, have been identified by molecular genetic analysis. The first one required fertilization with normal sperm and produced triploids. The other one normally initiated spontaneous gynogenesis as clone, but sometimes incorporated sperm genome to generate triploid progeny. Natural tetraploids found in the specimens from commercial sources were considered as exotic fish. Progenies of tetraploid loaches such as triploids, pentaploids, and other polyploids exhibited various atypical modes of reproduction. Allozyme genetics suggested possible involvement of two or more cryptic species or races on natural polyploidization through their hybridizations and further studies using more sensitive markers are required. Recent progress in molecular techniques makes it possible to map DNA markers on the loach chromosomes. Such approaches will identify genomic synteny among cobitids. Integration of chromosome manipulation, transgenics, germ-line chimera and nuclear transplantation will open new possibilities of loach as a new experimental model, not only for basic biology, but also for aquaculture biotechnology.