Introduction to Medaka Selective Breeding

Medaka selective breeding is one of the most fascinating hobbies for enthusiasts. The joy of creating your own varieties with beautiful body colors, elegant fin shapes, and unique body forms is unparalleled. Today, there are said to be over 500 varieties of improved medaka, with new breeds continuing to emerge every year.

This article explains the basic knowledge that those interested in medaka selective breeding should know first.

Characteristics That Can Be Modified Through Medaka Selective Breeding

In medaka selective breeding, the following characteristics can primarily be modified:

Body color: Medaka body color is determined by a combination of multiple chromatophores. - Black chromatophores (melanophores): contain black pigment - Yellow chromatophores (xanthophores): contain yellow to orange pigment - White chromatophores (leucophores): reflect white pigment - Iridescent chromatophores (iridophores): reflect light to produce iridescent and metallic luster

By genetically manipulating the presence or proportion of these chromatophores, diverse varieties are born such as Yōkihi (orange), Mikuki (bluish-white luster), Kōtei (red), and Orochi (deep black).

Body shape: In addition to normal body type, there are variations such as Daruma (short body type), Hikari body type (light enters the back), long fins, and Swallow (elongated fins).

Fin shape: There are breeds with variations in fin length and shape, such as Matsui long fins, Tennyo no Mai, and Mela.

Basic Knowledge of Genetics

To conduct selective breeding, it is necessary to understand the basic mechanisms of heredity.

Dominant and recessive traits: Most medaka traits follow the laws of dominant and recessive inheritance. For example, normal body color is dominant over Yōkihi body color. When crossing an individual with dominant traits and an individual with recessive traits, all F1 (first generation) individuals will display the dominant trait, but in F2 (second generation), the recessive trait will reappear with a probability of one in four.

Homozygous and heterozygous: When an individual possesses both copies of a gene in the same form, it is called homozygous; when possessing different forms, it is called heterozygous. Since recessive traits require homozygosity to be expressed, "fixing homozygosity" becomes an important point in selective breeding.

Polygenic inheritance: Traits involving multiple genes, such as the intensity of body color and fin length, cannot be explained by simple Mendelian inheritance. These are gradually enhanced through repeated selective crossings.

Basic Process of Selective Breeding

Step 1: Set your goal

First, clarify "what kind of medaka do you want to create?" For example, "I want to add Mikuki's luster to Yōkihi body color" or "I want to create a Daruma body type medaka with black body color." Having a clear goal makes the selection criteria more consistent.

Step 2: Choose parent fish

Select a pair with traits close to your goal. By using individuals from lineages with stable traits, you can reduce the number of generations needed to reach your goal. Evaluate body color, body shape, and fin form comprehensively, and pair individuals with few drawbacks.

Step 3: Breeding and selection

Medaka begin spawning when water temperature reaches 20°C or higher. Collect eggs and hatch them, then conduct selection once the fry reach 1-2cm in length. In selection, keep individuals close to your target traits and separate others into different containers.

Perform selection a minimum of two times. - First time (body length 1-2cm): Remove individuals whose body shape or color clearly deviate from the goal - Second time (body length 2-3cm, near adult size): Discern final traits and select candidates for the next generation's parents

Step 4: Accumulate generations

Re-pair selected individuals and accumulate generations to fix the traits. Generally, F3 to F5 generations (3-5 generations) are needed for new crossings' traits to stabilize.

Tips for Increasing Fixation Rate

Fixation rate refers to the proportion of offspring that inherit the same traits as the parents. The higher the fixation rate, the more stable the variety.

• Perform strict selection: If you compromise and keep mediocre individuals, the fixation rate will decrease. It is important to select only individuals close to the goal
• Secure sufficient numbers: Offspring from just one pair provide too small a sample for selection. The ideal approach is to raise 30 or more fry and select the top individuals from them
• Balance inbreeding and outcrossing: Inbreeding (crossing siblings) is effective for raising fixation rate, but excessive inbreeding increases deformities and weak constitutions. Every few generations, introduce a different lineage (a different strain of the same variety) to refresh the line
• Keep records: Recording the crossing content of each generation, selection results, changes in body color, and so on allows you to apply this information to future crossing plans

Obtain parent medaka for improvement at Brichoku

The success of selective breeding is greatly influenced by the quality of the parent fish that serve as the starting point. Obtaining high-quality parent fish with stable traits is the first step toward reaching your goal via the shortest route.

At Brichoku, you can purchase directly from specialized medaka breeders, allowing you to ask in detail about lineage information, fixation rate, and crossing history. The ability to get direct answers to questions like "What is the fixation rate of this variety?" and "What crossings produced this trait?" is a major advantage unique to direct-from-breeder sales.