Medaka genetics: how do new colour variants arise?

Als Medaka-liefhebber ben je ongetwijfeld gefascineerd door de enorme variatie aan kleuren en patronen die deze kleine rijstvissen kunnen hebben. Van diep oranje tot metallic blauw, en van effen wit tot gespikkelde patronen – de diversiteit is werkelijk verbluffend. Maar heb je je ooit afgevraagd hoe al deze prachtige colour variants really come about? In this article we dive into the fascinating world of Medaka genetics and discover how new colours develop, both in nature and in breeding. Whether you are a beginner hobbyist or an experienced breeder, this article helps you better understand the genetic basis behind your favourite fish.

How do new colours arise in Medaka?

New colour variants in Medaka arise mainly through mutations in the DNA. A mutation is simply a change in the genetic material passed from parent to offspring. These mutations can occur spontaneously during reproduction, when a small error is made while the DNA is copied. Although most mutations have no visible effect, some can affect the pigment cells (chromatophores) in the fish's skin, resulting in a new colour or pattern variation. In nature this process is relatively slow, because only fish with colours that help them survive (for example good camouflage) have a greater chance of reproducing. With bred Medaka it works differently. Breeders can selectively pair fish that show interesting colour mutations, even if those colours would be disadvantageous in the wild. This is exactly how many of the spectacular colour variants we know today, such as the popular Miyuki and Hikari varieties, came about and were further developed. An interesting aspect is that some new colours arise from combining existing genetic variants. When you cross two different colour variants, the offspring can sometimes show entirely new colour combinations that you did not see in the parents. This is what makes breeding Medaka so exciting: you never know exactly what surprises the next generation will bring! Many breeders therefore keep detailed notes on their crosses so they can recognise patterns in inheritance.

The role of genetics in colour development

The colour of a Medaka is determined by several types of pigment cells in the skin: melanophores (black/brown), xanthophores (yellow), erythrophores (red), leucophores (white) and iridophores (reflective/metallic). Each pigment cell type is controlled by specific genes that determine whether the cell is present, how much pigment it produces and how the cells are distributed over the body. A mutation in one of these genes can have a dramatic effect on the fish's appearance. For example, a mutation that switches off melanophores can result in a much lighter or even white fish. What makes it even more interesting is that colour genes can be inherited in different ways. Some traits are dominant, meaning you only need one copy of the gene to see the effect. Others are recessive, and you need them from both parents before they become visible. This explains why you sometimes have to skip a generation before a certain colour reappears. In Medaka, sex-linked traits are also known, where certain colours occur only or mainly in males or females. For breeders it is important to understand that genetics is not always predictable. Besides the main genes that determine colour, there are also modifier genes that have subtle influences. Environmental factors such as water temperature, food and light can also affect the expression of colour genes. A Medaka with the genetic potential for bright colours will, for example, be less intensely coloured if its food is suboptimal. That is why good care is essential for the genetic splendour of your fish to fully show! Understanding Medaka genetics opens up a whole new world of possibilities for enthusiasts of these special fish. Whether you simply want to understand why your fish look the way they do, or you want to develop new colour variants yourself, knowledge of the genetic basis is indispensable. Keep in mind that the magnificent colour variants we admire today are the result of years of careful selection and sometimes also lucky coincidences. If you are interested in keeping Medaka, start small, keep good records and above all be patient. The genetics of colour is complex, but that very complexity is what makes it so fascinating. Who knows, you might discover the next spectacular colour variant that conquers the Medaka world!

Further reading

• The black bowl effect: why medaka change colour
• Medaka the perfect pond fish for the Netherlands
• Medaka in Japan: tradition and culture
• How Medaka rice fish conquered Europe