High-speed system to generate congenic strains in medaka

The congenic strain, an inbred strain containing a small genomic region from another strain, is a powerful tool to assess the phenotypic effect of polymorphisms and/or mutations in the substituted genomic region. Recent substantial progress in the genetic studies of complex traits increases the necessity of congenic strains and, therefore, a quick breeding system for congenic strains has become increasingly important in model organisms such as mouse and medaka. Traditionally, more than ten generations are necessary to produce a congenic strain. In contrast, a quick method has been reported previously for the mouse, in which the use of genetic markers reduces the required number of backcross generations to about a half that of the traditional method, so that it would take around six generations to obtain a congenic strain. Here, we present an even quicker congenic production system, which takes only about four generations. The system can produce medaka congenic strains having part of the HNI-II (an inbred medaka strain derived from the northern Japanese population, Oryzias sakaizumii) genome in the HdrR-II1 (another inbred strain from the southern Japanese population, O. latipes) background. In this system, the availability of frozen sperm and genotype data of the BC₁ male population makes it possible to start marker-assisted congenic production after obtaining the BC₂ population. Our evaluation revealed that the system could work well to increase the percentage of recipient genome as expected, so that a congenic strain may be obtained in about one year.