Basic Genetics

I have many friends who make very detailed recordings of their breeding results yet have achieved very little from their decades of hard work. Why is it so? In order to decipher the encoded messages in the piles of data, a good understanding of genetics is essential. Even though the subject is difficult for a layman to understand, after spending enough effort to study it, the breeder will come to realise every single discus transmits its genes in a certain specific way and all those seemingly unrelated results begin to make sense. Understanding the inheritance of one's brookstocks is the only way to cut the Gordian knot so that one can produce the desirable discus every time he/she breeds.

Mendelian Inheritance

Modern principles of inheritance came from an Austrian monk, Gregor Mendel. In the late 1850s he conducted many breeding experiments on the garden pea (Pisum sativum) in his monastery. After working for eight years, Mendel published his results in 1865 which included two radically new ideas.

He notices different traits in the pea plants are inherited separately. The Law of Independent Assortment and Segregation is his first principle of inheritance. Before Mendel's paper, biologists of the 19th century believed offspring receive a blending of traits from each of the parents.

Mendel observes certain traits always appear in the offspring while others disappear. He then proposes his second principle—Law of Dominance. He calls the heritable substance "elementen", a gene in modern genetics. Every "elementen" has two forms. The form that appears in the offspring is dominant, the hidden one is recessive.

He also records the number of different types of offspring (phenotypes) which appear from crossing of parents that are different (heterozygous) in one and two traits. 3: 1 is the monohybrid ratio and 9: 3: 3: 1 is the dihybrid ratio.

Mendel's work was not recognized in his lifetime. It was only in the early 20th century when scientists rediscovered his thesis. Almost immediately, Archibald Garrod applied Mendel's principles to his study of alkaptonuria in the human populations and thus kick-started the research of genetics.

Modern Genetics

Gregor Mendel was lucky to have chosen the garden pea to study inheritance. When biologists begin to work with other organisms, they soon realize their genetics are a very complicated matter and Mendel's Laws do not always apply. Phenomena such as linked genes,  sex linked genes, multiple alleles, incomplete dominance, polygenic inheritance, and epistasis were all discovered after Mendel had died.

DNA

It was discovered in the 20th century that deoxyribonucleic acid (abbreviated as DNA) and ribonucleic acid (abbreviated as RNA) are the materials used by organisms to store genetic information. The double helix structure of the DNA molecule was discovered by Watson and Crick in 1953.

Chromosomes & Mitochondria

Most of the DNA is found inside long structures called chromosomes which are in pairs and are contained in the cell's nucleus in eukaryotic organisms. However, there is a small amount of DNA inside the mitochondria which are membrane bounded organelles found in the cytoplasm of the cell.

 
The somatic cells of nearly all the sexually reproducing organisms are diploid (2N) which means the nucleus contains two complete sets of chromosomes, one from each parent. Organisms having more than two sets of chromosomes are polyploids.

The model chromosome number for South American cichlid species is 2N=48 which should include the genus Symphysodon.

Gene: multiple alleles, homozygous & heterozygous organism, dominant & recessive gene, incomplete dominance)

The unit of inheritance is the gene which is a segment of the DNA molecule that contains enough DNA to code one protein.

A gene has at least two forms called alleles. In higher organisms like human beings, a gene can have more than two alleles in a population. That condition is known as multiple alleles.

An organism possessing two like alleles for a trail is homozygous while those that have unlike alleles are heterozygous.

Inside the DNA of every organism, besides expressed dominant alleles are many recessive alleles and hidden genes. These are the organism's genetic reserve which can only be expressed by sexual reproduction.

In some organisms, the dominant allele does not completely mask the effects of the recessive allele so that the resulting organism's appearance shows a blending of both alleles. This phenomenon is called incomplete dominance.

Genotype

The total genetic materials of an organism, latent or expressed, constitute its genotype.

Phenotype

 

The sum of the expressed traits is its phenotype.

Karotype

​

Karyotype is an individual's collection of chromosomes.

Genome

 

Genome is the term used to express an organism's complete set of DNA.

Genetic Variation

Genetic variation in an organism is either brought about by reassortment of genes during sexual reproduction or through mutation.

Gene Recombination

During sexual reproduction of a diploid (2N) organism, the chromosomes align randomly and split into two halves during the formation of the haploid (N) gametes (eggs and sperms) in the process of meiosis.

When the zygote (fertilized egg) is formed, genetic materials of the father are combined with those of the mother to provide a chance for alleles to recombine. The diploid (2N) zygote then develops back into the entire organism by cell division and specialization. If one or both parents are heterozygous to many traits, a lot of gene recombinations occur. This reshuffling of genes produces new genotypes and that are expressed in the offspring as diversity of appearances (phenotypes) and also as increases in vigor called heterosis in genetic.

Another source of genetic variation occurs during meiosis when the DNA molecule overlaps to result in a change in the gene sequence. The phenomenon is known as meiotic crossing over.

 

Mutation

Mutation is a sudden, random change in the gene. It can be spontaneous or induced. The double helix structure of the DNA molecule is very strong and stable and that is why mutation is very rare in nature. There is usually a big change in the organism when it does happen. Most of the mutated organisms die as a result. In my 25 years breeding discus, I have never seen a mutation in the fish. There might be one but the discus invariably died in the embryonic stage.

Mutations can also be induced by subjecting the organism to extreme environmental conditions such as high heat and deep cold; high pressure or a vacuum and radiations such as X-rays, UV lights and chemicals. Anything capable of changing DNA structure is a mutagenic agent.

I have tried to induce mutation without success. In the mid 1990s Manfred Göbel accidentally sterilized all his male discus when he sanitized the eggs for artificial raising with formalin and Clorox®. Many years ago, Dr. Eduard Schmidt-Focke subjected discus to X-ray but nothing interesting came out of his experiments.

Polygenic Inheritance

In most vertebrates, including the discus, the simple Mendel Laws do not apply in a lot of cases. The reason is most traits are governed by polygenic inheritance  meaning that there are more than one gene controlling a trait. In the mating of heterozygous parents, if the trait is controlled by a few genes, all the F1 generation will look intermediate between the parents and different classes will only appear in the F2 inbred generation. When the trait is controlled by many genes, say 10 or more, the classes will be so numerous yet so close in appearance that they form a continuous series of variations. If more than one trait is selected simultaneously in a cross, the variations are immense and the chance all the desirable traits are found in one fish is very remote because all relevant genes have to be expressed together.

Linked Genes

Linked genes are those that are inherited together. Most of them are physically close to each other on the same chromosome. It is a common phenomenon in many organisms such as fruit fly (Drosophila melanogaster) and yeast (Saccharomyces cerevisiae). The bonding of linked genes is broken by meiotic crossover.

Sex Linked Genes

Genes for sex are carried inside a special pair of chromosomes called sex chromosome. On the sex chromosome are also genes  controlling other traits, many of them we treasure very much in our discus. These sex linked genes are inherited together with the organism's sex, hence, males and females transmit their genes differently.

 

Epigenetics

Decades ago, geneticists believed the expression of genes was controlled only by genetics. It has been demonstrated the environment plays an important role in the expression of a lot of genes in recent years. The science of epigenetics is the study of the interaction between the environment and gene expression. Therefore, the phenotype of an organism is controlled by both its genetics and the environment. 

Breeding Techniques: Heterosis (hybrid vigor), Polyploidy, Inbreeding, Backcrossing, Reciprocal Cross

Heterosis (hybrid vigor)

There are two ways to obtain superior organisms. The first method is the hybridization (outcrossing) of unrelated organisms. The great reshuffling of genes brings back vigor into the offspring.

Heterosis or hybrid vigor has been used since ancient times to produce superior plants and animals for various purposes.

Polyploidy

Another way is to use polyploid breeding stocks. A normal, diploid (2N) somatic cell has two sets of chromosomes. In nature, organisms with four complete sets of chromosomes are sometimes found. These are tetraploids (4N) varieties. Polyploids are far superior to diploid individuals. Tetraploids have also been used extensively in hybridization to improve the species. Although polyploid organisms carry out meiosis in a great variety of ways that is different from species to species, however, it is sometimes possible to mate tetraploid with diploid to produce triploid (3N) offspring. Triploids are also very strong and vigorous but are mostly sterile.

There are now superior polyploid varieties of trout, salmon and tilapia. Mono sex and sterile offspring can also be obtained by polyploidy. A lot of the most important crop plants are also polyploids. 

I do not know polyploidy exists in discus but I have produced gigantic discus reaching 23 cm in length and 350 grams in weight. Could this be the result of polyploidy?   

Inbreeding

Inbreeding is the breeding of closely related individuals such as brother and sister or father with daughter. With each generation of inbreeding the alleles of the organism become more and more homozygous. Consequently, they look and behave more and more alike as the process proceeds.

 
Inbreeding very often gives us unexpected surprises when recessive alleles are expressed since most of them control traits which are unfavorable for survival in nature but are highly desirable to us. For example, a high fin, red eye discus with bright turquoise and red colors has a much higher chance to be killed by predators in nature before sexual reproduction when compared to a drab, brown discus with short fins. This means these desirable traits will have a very high chance to be lost in the F1 generation hybrid when we outcross discus. Inbreeding is the only way to retrieve them.

The changes in the physiology brought about by the expression of recessive allele and alleles also make the organism weak. There is also a rapid reduction in fertility as inbreeding continues and that is especially serious in discus.

 

How many generations of inbreeding is possible with discus?

 
In 1979 the Hirose Company of Japan made the boldest claim in the whole history of discus hybridization: Jack Wattley has inbred his turquoise discus for 15 generations. The six specimens of this strain that I purchased from Jack in 1983 all looked different which means this claim is only a clever but unscrupulous propaganda to promote Jack Wattley's discus in Japan.

Klaus Eckert had bred his Brilliant Turquoise for 10 generations before he gave up in 1985. Due to a poor selection of broodstocks in the last two generations, his strain was highly degenerated to become pale, silvery fish at the end. Dr. Schmidt-Focke also claimed to have inbred his Red Turquoise strains for 11 generations.   

I was able to inbreed the WW19LS Red spotted Green Leopard Skin and the WB22 Blue Diamond strains for only five generations in a period of 16 years but the difference is that I made a very careful selection when choosing the broodstocks in every generation. There was a reduction in size and fertility in the later generations but no regression in color.   

I must clarify the exact meaning of an inbred generation. In genetics, the parents constitute the P or parental generation and their offspring the first filial generation (abbreviated as F1 generation). By breeding brother and sister together the F2, F3, F4...generations are produced. When an outcross is made, even with another line of the same strain, the offspring become a F1 generation hybrid. Therefore, if I cross a F2 generation WB2 Cobalt Blue with a F2 generation WB3 Electronic Blue, the offspring are the F1 generation hybrid of (WB2 X WB3), not the F3 generation of WB2 or WB3.

Most people do not realize that the captive bred F1 offspring of wild discus are actually hybrids. The worldwide preference for round or high body form, red eyes, high fins, brilliant colors, and a large size is forcing wild discus and their offspring to evolve in the opposite direction in our aquarium than what has been going on in nature since ancient times.

Backcrossing

The mating of fish from two or three generations is called backcrossing. For example, mating the father with daughter is a first generation backcross (BC1 backcross) and mating with granddaughter is a BC2 backcross. Backcrossing is a form of inbreeding which is a very useful tool to maximize the genetic contribution of an exceptional fish to a strain. A top quality fertile specimen of any inbred strain is extremely valuable. When one occurs, it should be used repeatedly to backcross with discus from younger generations.

Reciprocal Cross

The reciprocal cross is a pair of crosses between the male of one strain and a female of another and vice versa. It is useful to test the role of parental sex on a given inheritance pattern. In the genus Symphysodon, its usefulness is greatly diminished because there is no true breeding discus.

Summary

Hybridization results in the creation of new varieties. Inbreeding is the only way to preserve a strain. Any valuable strain should be inbred until it becomes impossible. Outcrossing with another line or a different strain will bring in variation and the reshuffling of genes also restores vigor into the offspring. A few generations of inbreeding coupled with rigorous selection are necessary to restore the strain. The work of a breeder is a continuous series of outcrossing; inbreeding; backcrossing, and reciprocal crosses at the appropriate times.

A breeder who acquires a pair of discus, breeds it and then sells both the parents and offspring never can learn: nearly all the desirable traits are controlled by recessive allele or alleles which can only be expressed by more than a generation of inbreeding.

 

The Scientific Way of Discus Breeding (Inbreeding & Line Breeding)

The main problem facing every breeder is how to select the two fishes to form a pair that will produce the desired offspring. There are two very useful guidelines to help us. Firstly, we should select parents with as many desirable traits as possible. Secondly, only use fish that have a known inheritance pattern.

 

Breeding fishes with unknown genetics is a big gamble because the variations in the offspring are unpredictable.

 

In any breeding program, continue to breed the original pair as long as possible and keep all the offspring so that we can assess all the variations in the crucial F1 generation and to retain the very rare (1% or less) exceptional specimen or specimens. 

The F1 generation should be divided into groups according to their appearance when reaching sexual maturity. Use only the group containing the highest percentage of desirable traits and subdivided it further into at least two smaller groups. These are the breeding lines which should be inbred separately and vigorous selection must be made in every generation. Use only the best fish for breeding. I must emphasize that inbreeding without selection cannot produce the desired fish.

 
To slow down the bad effects of inbreeding, use multiple pairs to breed in the form of reciprocal crosses for each line.

Please remember there are significant differences between individuals and their genetic contributions are very significant in the first and second generations of a strain. The breeding lines are to be crossed only when inbreeding becomes impossible.

Breeding Programs

One can begin a breeding program by the following methods:

1) crossbreed hybrid,

2) crossbreed hybrid with wild discus,

3) breed with pure wild discus.

Crossbreed Hybrid

The use of hybrid discus as parents is the safest way to obtain colorful offspring. The reason is simple. Tank raised strains have been specially selected for desirable traits such as turquoise stripes, red eyes, long finnage or solid turquoise. With respect to these traits, the gene or genes controlling such traits are a lot more homozygous than wild discus.

Crossbreed Hybrid with Wild Discus

When we crossbreed a hybrid with wild discus, a knowledge of the parents' inheritance is a must to avoid destroying color in the offspring. If we bring two genotypes together that are very different is going to result in only the dominant traits, such as short fins and an elongated, dull brown body to appear in the offspring.

Breeding Wild Discus

Breeding wild discus is the last option that can be very interesting or very frustrating. All wild discus is heterozygous to main traits and that means breeders should always inbreed wild discus to ensure colorful offspring are produced. It is pertinent to select the two fishes of a pair from the same population, i.e., discus from the same lagoon or igapó. How to select such breeders is a real challenge. One must observe wild discus very closely. Every population always has a distinguishing trait or traits, no matter how subtle it or they may be. To be completely sure, one can join the Brazilian exporters on their collecting trips to catch broodstocks in the Rio Amazonas.