Creating New Medaka Varieties — Crossbreeding to Strain Fixation Roadmap

Medaka (Japanese rice fish) selective breeding has developed as a uniquely Japanese hobby, with numerous new varieties created by enthusiasts every year. The dream of creating your own original variety is arguably the ultimate pleasure of medaka keeping. This article explains a practical roadmap from crossbreeding planning to variety fixation.

Basic Breeding Strategy

To create a new variety, first establish a clear goal of "what kind of medaka do you want to create?" For example, "a medaka with lame (glittering scales) and red-white coloration" or "a medaka with external light and hikari body type." Then select parent fish varieties that might carry the target traits. Cross-variety hybridization is the fundamental technique for new variety creation. There are two approaches to crossbreeding: one is "adding trait B to medaka A," such as crossing a non-lame variety with a lame variety to create a new body color with lame. The other is "aiming for a new intermediate trait between A and B." In either case, basic knowledge of medaka genetics significantly affects planning precision.

F1 Generation Management and Observation

The first generation obtained from crossing different varieties is called F1. The phenotype (appearance) of F1 fish, in most cases, takes on an intermediate form resembling neither parent. This is because dominant traits tend to show on the surface while recessive traits remain hidden. F1 fish often appear disappointingly plain, but this is a normal part of the process. What's important at the F1 stage is not to carelessly cull all individuals. Since F1 carry recessive genes in heterozygous form, crossing F1 with each other can produce the target traits in F2. Secure a minimum of 30-50 F1 individuals and raise them to good health. Selecting individuals with good body shape at this stage helps prevent body form deterioration in later generations.

Selection and Direction Setting in the F2 Generation

The F2 generation, obtained by crossing F1 fish with each other, is the most critical generation in selective breeding. Following Mendelian inheritance, F2 produces various combinations of phenotypes. This is the generation where individuals with traits matching your goal may appear. The more F2 fish you raise, the higher the probability of finding individuals with your target traits, so raise as many fry as possible. Ideally, aim for 100-200 or more. From the F2, rigorously select the individuals closest to your goal and keep them as parent fish for the next generation. This selection is the most important task that determines breeding success or failure. Set selection criteria with priorities based on your goals, such as body color, body shape, fin shape, lame intensity, and light strength. Selection is not a one-time event but should be performed multiple times as fish grow -- at 1 month, 2 months, 3 months, and so on.