By Rui Carneiro-Martins, Biology Teacher, Portugal
One of the most important concerns of the intermediate/advanced breeder of killifish is how to keep the stain he has in good condition, generation after generation. This article will try to give a few hints on how to properly maintain (or at least to not excessively degrade) the genetic background of a strain. One should bear in mind, however, that in order to keep this article fairly simple, some basic genetic concepts have been trimmed in a manner that might not be considered satisfactory to the initiated.
Usually the first thing you do when introduced to a new species is to keep the fish alive. You search all possible sources for information on the fish and if they last more than a month you become pretty confident that they are safe for the time being. However, after a while most species begin to breed. Then comes the problem: what should you do to keep the population/strain strong and viable?
You have to remember that an organism is the result of the information that it carries on its genome. The genome is organized into packets of DNA, known as chromosomes. Most organisms have duplicates of each chromosome on its karyotype (the number of chromosomes a cell possesses), usually depicted as 2n; those organisms are said to be diploid, as opposed to haploid, which show only one set of chromosomes (n).
Since killifish are diploid, their chromosomes are paired, each of them carrying a single gene for the same characteristic. Each inherited characteristic is determined (at least in part) by two genes, with each parent contributing one from the pair he or she has. Assuming that both parents carry two identical genes (homozygosity), it’s obvious that the progeny will also have that characteristic.
However, if the genes of a parent are different (heterozygosity), the offspring will receive either the characteristic shown by the parent (the dominant one) or a concealed characteristic that we know little about (the recessive one). Recessive genes can be wholly harmless; they can be undesirable; or they can be absolutely deleterious, resulting in visible or invisible deformities, and even death.
If both parents are heterozygous, the progeny may become homozygous over time, expressing the undesirable characteristics and passing them on to the following generations. The risk of this happening increases immensely when we intercross generations, which is commonly done when trying to “fix” a strain.
None of the above takes into consideration phenomena like the mutational rate, multiple allele genes, genetic drift, etc. However, these concepts may be important if you want to have a better insight on the reasons subjacent to the following guidelines.
Optimally, you should choose the best examples of a strain for breeding purposes. The selection criteria, however, are difficult to establish, if only that most of the time we don’t know what those should be. The simplest and most obvious gauges are the pecking order within a group of fish (the dominant ones are usually, but not always, the most fit) and their physical beauty, although this depends immensely on personal taste. If you can, choose at least two or three males: the leader, which should be the top breeder, and then one or two others you can use to help keep up the genetic diversity of the strain.
Another important thing to keep in mind is that all fish, however healthy and sound they may appear to be, may be carriers of undesirable genes. We’re not talking here about those responsible for such obvious problems as physical deformities and such; we’re talking about the less conspicuous problem genes which gradually accumulate over time in a population and contribute to the depletion of genetic diversity.
However, we can take a few simple measures to avoid the rapid genetic decay of captive populations (especially due to inbreeding), that homebred strains are subject to. The following information is not meant as a panacea, but will hopefully improve your chances of maintaining a sound population for many years.
Never cross fish from different populations of the same species, or from different species. This is something the average breeder already knows, but that bears repeating. Resist the temptation to cross populations; they may look alike, but they may be genetically so far apart that they produce infertile offspring. In other cases it may take a few generations before problems occur; eventually the fish may be unable to produce viable eggs and fry, and the entire strain is lost. As killikeepers we need to strive to maintain genetic diversity, not contribute to its destruction.
Keep a minimum number of individuals for breeding from each generation. There is no set rule for this, however some studies have pointed out that the risk of inbreeding grows exponentially when the breeding stock number less than seven. That number may vary with the species and may be affected by a number of exogenous factors-aggressive behavior, size, feeding habits, tank size, etc. The effort of keeping more than ten breeding individuals may not be compensated for by the genetic contribution received from the extra fish.
Don’t intercross generations. In the natural setups often favored for killifish, it is easy to let the fry grow to adulthood, freely breeding with their parents. This practice can quickly result in serious inbreeding, since genetic malformations increase exponentially. Remember that each parent shares half of its genome with its progeny (F1); most siblings share much less common genetic material (and sometimes none at all), making inbreeding problems less problematic. If you use a natural setup, it’s best to move the parents before the fry reach breeding age.
Try to breed more females than males. Males are usually more aggressive than females and tend to establish more strict hierarchies, although this varies considerably among species and among individuals. A consequence of this is that extra males will have less chance to breed, and may even become a nuisance to the breeders. Perhaps the best setup would be a tank full of females with a single top ranking male selected from a bachelor tank. This male should occasionally be rotated out and replaced for a time by a second or third rank male. Another possibility would be the use of two tanks, one solely for females and the other for males. Simply select the best individuals from each tank and pair them. Again, there are many choices. The emphasis on females is not mandatory to a successful breeding program, just practical.
Don’t look for “sports”. Some aquarists breed fish solely to find and select for that particular morph (mutation) that makes a difference. In the context of genetic diversity, that’s the wrong approach. Selection and breeding to “fix” a particular characteristic requires much inbreeding and artificial selection, making the possibility of strain depletion a real possibility. Even if the resulting “sport” is sound, it may be so different from the original wild form that the strain/population has become completely decharacterized.
Be very careful when crossing fish from different sources. This may sound like heresy, but is in fact simply a precaution. The reason for frequent exchanges between two or more strains of the same population is obvious – improvement in overall genetic quality, as a direct result of the introduction of new genes. However, new genes may also be a synonym for trouble-sometimes the new strain carries a deleterious gene that only becomes apparent later. The end result of this may be the destruction and loss of both previously sound strains.
In addition, a great deal of misidentification exists within the killifish hobby; you might inadvertently end up with a disaster. Unless you are completely confident about the genetic integrity of the second source, it’s better to play safe for the first generation. Keep a number of specimens from both sources apart from the rest, just in case the crosses don’t go the way you thought they should. Be alert for signs of problems: an increasing number of fungused eggs, anomalies in fry or juveniles, unhatched eggs, etc.
Never use a fungicide. That sounds harsh, and flies against a rule of thumb, but it’s paramount. Most viable eggs have some natural capacity to elude fungus attack. That defensive capacity is genetically determined (whether enhanced by environmental conditions or not). We should look for that important characteristic and try to preserve it. If you artificially protect eggs that lack this natural defense, you are in fact weakening the strain by selecting fish that will pass this lack on to the next generation. In time you may end up with a strain that is unable to produce fungus resistant eggs. In itself, that is an indicator of poor genetics, since in nature such fish (or eggs) would not have survived. Fungicides may have other undesirable effects as well, particularly mutagenic ones. Fungicides also are known to harden the outer layer of the egg (the chorion), making hatching difficult.
There are a few more rules that could be added. However, the above list is fairly simple, the rules are fairly easy to follow, and they are effective.
Finally, you must not forget that before applying any guidelines to maintain genetic diversity, you must first keep the fish alive and healthy (good husbandry), and correctly identified (don’t mislabel, misspell, or modify the scientific names and codes). Good luck!
— GCKA Newsletter, August 1998