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Breeding Strategies for the Development and Improvement of Goldfish Lines by Rob Crosby

Updated: Mar 6, 2023

**There is a brief glossary of genetic terms at the end of this article.

If you have ever wondered how goldfish breeders go about pairing and breeding their fish to consistently produce show quality specimens, then please read on, because this article is intended for you. I would like to explain some important concepts to consider when devising a breeding strategy. We have all visited the popular online goldfish forums where it is common to see the question posted; “What will I get if I cross goldfish A with goldfish B?” The goldfish are typically the posters’ favorite individual fish that have been chasing around their tank lately, but are almost always unrelated fish, often of different colors or scale types, and even different breeds altogether. I need to stress here that such pairings of unrelated fish are not always a bad idea, and can often yield very satisfactory (and original) results, however, the breeder should have realistic expectations and must understand that the resulting fish are “hybrids”, and the development of a true strain will take many more generations of careful selection and breeding. If you have not yet begun your breeding program, and you are wondering where to source your breeders, you should first consider and prioritize your objectives and options.

Objectives: Are you looking for a common, easy to keep breed, or would you prefer a fancier, less common type possibly with special care considerations? Do you plan to show your fish and is there already a TGC breed standard, and a presence of this type fish at regional goldfish shows? Do you hope to find a market for the offspring that you produce? Is there a sizable following for this breed type? Do you hope to produce a unique variety by incorporating specific traits from two different strains?

Options: Are there any local/domestic breeders of this type from whom you might be able to purchase line-bred fish? Are there multiple breeders of this type from whom you might be able to purchase similar fish, but from different strains? Do you have access to high quality imported fish from a reputable dealer, and can the dealer offer any assurances that their fish are from a line bred strain, and that fish of both sexes are available?

If you choose to work with a breed that is already popular and for which there are already established breeders, then by all means, you should start with the best breeding stock that you can find that is within your budget. Individual fish chosen as breeders, or “seed fish” do not necessarily need to be the best overall specimens. While it is important not to use seed fish that might pass on a strongly heritable defect, some fish that have excellent expression of one particular trait but might be weak in another area may still make excellent seeds if paired with a fish that complements it well in the areas where it is weaker. Similarly, breeding two fish that have no serious defects, but also no particular strengths either, amounts to breeding toward mediocrity. While it may be tempting to purchase a single excellent fish, usually for top dollar, at a goldfish show, or from an online auction, you should first consider what you plan to breed this fish to. If the sex and breeding history of the individual fish is unknown, and especially if there is no option to purchase others from the same line, then your money might be better spent acquiring between 4-8 juvenile fish from a line bred strain with the probability that you will have both males and females, and with the assurance that these fish are genetically compatible. On the other hand, if the quality of an individual fish (for which there is no line-bred mate) is far superior to any other available options, then it may be perfectly reasonable to purchase that individual with the intention of making it a “foundation fish” upon which you would set out to establish your own line. Such an endeavor may prove frustrating and may involve many years of carefully backcrossing the best offspring to the foundation parent, but at times, this may be the best path forward.

Let’s first assume that you decide to go with a more common goldfish type for which there are plentiful sources of breeder fish. For example, side view or top view Ranchu, Ryukin, Orandas, or Telescopes. While it is always preferable to start with some breeder fish from the same line, it is also true that for some breeds, many of the existing lines have a common background, and may actually be very compatible. As an example of this, consider the various lines of Japanese Ranchu (TVR) that are being kept in the U.S. today. U.S. breeders of Japanese Ranchu routinely do outcrosses to other Japanese Ranchu lines, which are typically then described as hyphenated lineages or a series of initials such as: Matsuyama-Oishi, SxK, or TOKS. The first generation offspring from such crosses are often of very good quality and may exhibit some degree of “hybrid vigor” seen as an increased growth rate, greater disease resistance and improved fertility. However, many breeders have also found that while the first generation (F1) offspring are of a very uniform quality, the second generation (F2) usually includes a much broader array of different types, both excellent and those exhibiting a variety of undesirable traits. For Ranchu, some of the common problematic traits are poor tail angle, soft tails or stiff tails, undivided tails, inserted tail cores, pointed heads, dorsal spines, thin peduncles, rounded bodies, and many others. We will review the reasons for these occurrences later in this article. I would also like to stress here that these types of breed-specific faults can appear even when crossing closely related lines. On the other hand, if you were to cross two very different lines, such as a Chinese side view “ranchu” with a Japanese top view Ranchu, you should not expect very good results in either the F1 or F2 generations. With such crosses, it is common to see full dorsal fins reappear, as well as single tails and a general reversion back to wild type goldfish. Such a cross would basically destroy the work of many generations of careful breeding by earlier breeders. The path back to a reasonably stable line would be very long and would likely require years of careful selection and backcrosses to the preferred foundation fish. A general rule of thumb is the more specialized a breed type is, the more features must be fixed in the genetics, and the more features will potentially be lost as a result of the outcross. Consider the degree of specialization of the body type, finnage, eye shape, scale type and color. The more variables, the harder it is to fix and maintain a stable line.

The term “inbreeding” often connotes a path to weakness and deformities in the minds of novice breeders but a deeper understanding of the genetic principles will help to alleviate these concerns. Inbreeding in and of itself is not necessarily detrimental to the constitution of a line, and if done correctly, it can greatly improve the quality and stability of a line. What inbreeding does is make matched or “homozygous” pairs of alleles at loci (chromosome positions) where two or more gene variants (polymorphisms) exist in a breeding population. These gene variants normally occur at very low frequencies in large, wild breeding populations, however, when closely related individuals are mated, some of the resulting offspring may inherit two copies of the same mutant allele. What was once a hidden recessive gene, whose effect was masked by the dominant allele, will now be unmasked and seen as a changed physical characteristic (phenotype). This process occurs without regard for the survival fitness or breed conformation of the affected individual. It is important to consider here that all fancy goldfish are by definition “inbred” and it is due to this process that we can appreciate the many breed variants that currently exist. Such recessive mutations have given us telescope eyes, twin tails, pearlscales and many other traits. It can be argued that all of these examples do negatively affect the wild survival fitness of goldfish, however, in the manmade aquarium world, expression of these traits actually confers a survival advantage, since the breeder is selecting for these traits and excluding fish that do not possess the desired traits.

It is also important to consider the opposite effect of the generation of desired matched gene pairs, which is the exclusion of the unwanted alternate allele(s) from the population. When alleles are made homozygous in a breeding population, the expression of those alleles is then fixed in that population, and the genetic variability at that locus is lost. By carefully selecting for some positive traits (or the absence of a negative trait), the breeder may also exclude undesired or deleterious alleles from the gene pool. For example, if a breeder notices a large percentage of goldfish fry with a certain defect, such as pinched peduncles in ranchu, careful selection for breeders without this defect paired with successive inbreeding may eventually permanently exclude the responsible, undesired, alleles from the breeding population. The take home message here is not to give up on a breeding line simply because some percentage of fry possess an unwanted trait. As long as the line produces some fish without the unwanted trait, careful selection and breeding of these fish should improve results in the future generations.

Now that I have explained all the wonderful advantages of inbreeding for the development of goldfish lines, I also need to expose the very real pitfalls to this practice if not done carefully. While the goldfish breeder may be selecting for and breeding fish that exhibit a single or a few desired traits, it is important to understand that all variable loci (not just the targeted breed specific trait loci), are also affected by inbreeding. All individuals harbor a few deleterious gene mutations that may be insignificant when paired with a normal allele, however, when made homozygous by inbreeding, these formerly unnoticed alleles may have unintended negative consequences. Not all significant mutations can be observed as obvious phenotypic changes; and some may negatively affect either internal anatomy or physiology in unseen ways. The accumulation of such negative mutations in a population might manifest as a gradual loss of vigor, smaller size, lowered disease resistance and loss of fertility. Avoidance of such issues is partly the responsibility of the breeder to identify individual fish that may be exhibiting such weaknesses and remove them from the breeding population. It is also key to identify relatively stronger fish with good spawning history and longevity and keep these individuals in the breeding population. Those fish that live to be Oya and have overwintered outdoors in cold climates are good prospects. Backcrosses to stronger fish can help to restore the genetic variability needed to maintain the health of the line.

The practice of breeding sibling fish for many consecutive generations is the most severe form of inbreeding and although this is the fastest way to fix desired alleles in a line, it is also most likely to eventually cause weakness, also known as inbreeding depression. Realistically though, most novice breeders are far too concerned about inbreeding depression in their goldfish lines, as the correct use of inbreeding is just as capable of improving the line as weakening it. Since goldfish spawns number in the hundreds or even thousands of fry, and stringent culling is an absolute necessity, the appearance of undesirable mutants in a spawn is not such a huge concern, as those fry can be easily culled at an early stage. With consistently good culling and breeder selection, many undesired traits can eventually be eliminated from the population. After a cold outcross, or mating of two distantly related fish, it may be appropriate to breed siblings for four to five generations to help stabilize the line and achieve some uniformity of the desired type. However, the importance of culling inferior fish cannot be overemphasized. If only a couple of fry bearing the same deleterious, homozygous alleles enter the breeding population, and all future generations are descended from these two fish, then the genetic variability at that locus is lost, and no amount of careful selection and breeding (within this closed population) will restore the normal or preferred allele.

The best way to avoid inbreeding depression is to keep a sufficiently large breeding population such that there are always several closely matched fish which are all presumably homozygous at many of the loci governing the breed-specific traits, but which are largely heterozygous at other variable loci. One way to accomplish this is to keep parallel lines (line A and line B) of related fish, and then cross the two lines every fifth generation or so. For example, if a group of prospective breeders are selected from a spawn, and two different but closely matched breeding pairs are identified, then those pairs and their progeny can be inbred as separate but parallel lines for four generations or so. It would be important to use the same selection criteria when culling both lines such that the eventual crossing of the two lines does not simply undo the progress made so far. In theory, the crossing of the two lines would restore some vigor without disrupting the progress made toward fixing the desirable breed-type traits. For those hobbyists fortunate enough to have other competent breeders interested in the same lines, this practice can be simplified by exchanging related breeder fish on occasion.

Another practice often used by goldfish breeders is called backcrossing. Backcrossing is the pairing of offspring with a parent or older generation fish. The two main reasons for backcrossing are to increase the genetic contribution of an outstanding fish or to restore some genetic diversity to a line that is beginning to show signs of inbreeding depression. In the first case, consider a scenario where a single excellent specimen exists, perhaps purchased as an individual fish, and is mated to an unrelated fish of the same breed. If the resulting fry are not of the same or better caliber as the excellent foundation fish, then the best of those fry can be crossed back to that individual. This can be done indefinitely, and with proper selection, the effect will be to gradually eliminate the genetic contribution of the inferior parent fish while enriching for those genes contributed by the desired foundation fish. The fry resulting from these backcrosses will never be more inbred than this foundation fish, as any resulting new homozygous alleles would be reverted back to a heterozygous state at the same rate. In the case where a stable line exists and already produces consistently excellent quality fish, there is no good reason to continue a strict sibling-sibling type inbreeding program. Such continual inbreeding would hasten the onset of inbreeding depression, whereas backcrosses to parents or outstanding aunts and uncles would be better for preserving genetic diversity and the overall quality of the line.

The breeding strategies presented in this article are not intended to be used as either a strict set of “rules”, or a guaranteed formula for success, but simply a framework upon which breeders can make more informed decisions about how to structure their own breeding programs. Previous books and articles on this topic have gone so far as to diagram very detailed breeding plans with the exact pairings that should be done year after year. I would suggest that such “templates” only be regarded as examples of sound breeding practices (for the maintenance of established lines), and not rules for success. Any serious breeding program must also include an ongoing evaluation of the results, and decisions about successive pairings should be based upon what is needed for the continued improvement of the line. Imagine a funnel with your foundation breeder fish perched at the rim; now imagine each successively inbred generation occupying a spot fractionally down into the funnel. Once the generational position is within the neck of the funnel, the genetic diversity of the line is very limited. If the line is producing excellent fish, then the lack of diversity is a good thing, however, if there is still much room for improvement, then you made need to “back up” and get out of the funnel neck in order to make improvements to the line. Breeding goldfish is as much an art as it is science, and the breeder’s own experience and insights into the genetics of their fish should dictate when backcrossing, outcrossing, or further inbreeding is warranted.

The images below illustrate the results of a pairing of two unrelated, and dissimilar fish: a Broadtail Black Moor and an Azumanishiki. In the first generation (F1), the fry were relatively uniform, and predominantly of the normal eyed, sakura (nacreous red and white), broadtail type. In the following year, a sibling x sibling cross was done, and the F2 generation produced a larger variety of types, with both normal and telescope eyes, long and short fins, wild type metallic, pinky mattes, nacreous blacks and both blue-base and orange calico varieties. From such a wide array of types, a breeder can select the fish exhibiting the desired traits and can develop his/her own line. It may take several generations of careful selection and inbreeding, but eventually, a uniform line-bred strain can be produced.

Glossary of terms: Chromosome: a threadlike structure of nucleic acids (DNA) and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. Goldfish have 94 chromosomes or 47 pairs (diploid number) of chromosomes. This number is roughly double the number of chromosomes of most cyprinid fish species. At some point in its evolution the entire genome of some ancestral species was duplicated resulting in a doubling of the total number of chromosomes. While initially many chromosomes occurred in sets of 4 (tetraploidy), over long periods of time, the chromosomes begin to differentiate and are no longer identical quartets, this is called “returning to diploidy”. Gene: (in informal use) a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring. Genes are encoded in the DNA and occur in a specific location (locus) on a chromosome. Different versions of the same gene occur naturally due to small differences in the DNA sequence. These different versions of the same gene are referred to as alleles. Locus: A locus (plural loci) in genetics is the position of a gene on a chromosome. Each chromosome carries many genes. Backcross: Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent (aunt/uncle), in order to achieve offspring with a genetic identity which is closer to that of the parent. The effect of backcrossing is to enrich the next generation offspring for the genetic contribution of a specific parent. Homozygous: Having two copies of the same allele at a particular locus. In a diploid organism, both copies of a particular chromosome in a chromosome pair would contain the same version (allele) of a particular gene. Heterozygous: Having two different versions of the same gene at a particular locus. In a diploid organism, each copy of a particular chromosome in a chromosome pair would contain a different version (allele) of a particular gene. For example, one chromosome may contain the normal “wild type” form of the gene, while the other chromosome may contain a mutated version of this gene.

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