Inbreeding

When fish are removed from the natural environment and placed in the aquarium environment, different selective forces act upon fish in the aquarium environment compared with the natural environment. This can reduce their genetic variability through both selective processes and random genetic drift. Genetic drift arises from chance fluctuations in gene frequencies from one generation to the next. Even if individuals mate randomly within populations, changes in gene frequency will occur with each generation. Not all genes will be present in the next generation, because not all individuals will successfully reproduce, and overall diversity will decline with each successive generation.

Captive populations of rainbowfishes are at high risk of loss of genetic diversity through drift and inbreeding. Inbreeding can be a major concern in captive populations of rainbowfishes, particularly if populations are small and there is little or no exchange among populations. Inbreeding depression and reduced levels of genetic diversity are expected to be more prevalent in small populations where breeding between close relatives is more likely and the effects of genetic drift more pronounced. If there are no mechanisms to prevent mating with close relatives (i.e., kin recognition), inbreeding would accelerate loss of genetic variability and could result in expression of lethal recessive genes leading to lower probabilities of population persistence. Levels of inbreeding will accrue in captive populations with high probability, so considerable attention has to be devoted to captive breeding programs.

Both inbreeding depression and the impacts of reduced genetic diversity have a sound basis in theory and have been well documented by animal and plant breeders over many decades. There are many studies that describe the deleterious effects of inbreeding in fish, generally in captive conditions or in association with artificial selection. The selection of small numbers of parents can reduce genetic variability. Equally serious is the fact that brood stock may be continually selected from closely related, perhaps full sibling individuals. This leads to generation after generation of inbreeding of closely related individuals. The limited gene pool caused by continued inbreeding means that deleterious recessive genes inherited from both parents become homozygous. The condition can manifest as reduced fertility, growth and disease resistance, lower hatching rates and survival, behavioural changes and high occurrence of abnormalities to name but a few.

Inbreeding depression is probably the most serious consequence of small population size. Expression of a trait is determined at the gene level by information contributed by each parent, and a predictable percentage of offspring will display these traits. If one parent’s gene is recessive, then the trait it codes for will be expressed by a predictably small number of the offspring. Others will possess the gene, but won't express it. The population is said to be heterozygous. However, in any strategy involving inbreeding it is necessary to take effective steps to insure against excessive fixation of deleterious alleles.

Inbreeding depression resulting from increased homozygosity is well documented in fish. The majority of inbreeding experiments on fish have been done in aquaculture and laboratory-type environments. One study showed that a 10% increase in the inbreeding coefficient in rainbow trout can result in a 10% decline in hatchability, and a 24% decline in viability of fingerlings. It should be noted that such independent effects are multiplicative in their impact on total, absolute survival and reproduction. Thus the unavoidable conclusion is that relatively small amounts of inbreeding can do tremendous damage to the reproductive potential and productivity of a fish stock. Unfortunately, once it has occurred, inbreeding depression is not reversible except by intraspecific hybridisation, or cross-breeding. Cross-breeding is accomplished by using parental stocks representing different strains of the same species. Cross-breeding can produce strains of superior performance by introducing greater genetic variability.

While the actual inbreeding depression varies widely between fish species and inbreeding levels, significant levels of inbreeding depression have been found in many aquacultured brood stocks after only one generation of brother-sister mating. Therefore, the high level of abnormal fish especially spinal malformation appearing in hatchery fish is a major problem in many aquaculture farms. Malformation often is associated with growth depression, leading to high mortality rates at early fry stage.

Inbreeding, along with selection and cross-breeding has been traditionally used to create new varieties and colour forms in many aquarium fish species. With rainbowfishes, however, I would hope that most enthusiasts are trying to breed a species as close as possible to its wild form. Unintentional domestication of rainbowfishes may be unavoidable, but it is possible that it can be minimised by the introduction of new broodstock. It is possible for rainbowfishes derived from the same source to have different genes when separated for any length of time. It’s very important to properly select the breeding stock (related or not) and also to properly cull the fry for obvious defects.

One way to avoid inbreeding is to outbreed. The opposite of inbreeding depression is outbreeding enhancement, which is often referred to as heterosis or hybrid vigour. Individuals from different populations are not likely to be homozygous for the same recessive alleles. Thus, outbreeding among individuals from different populations (wild or captive) can lead to masking of different deleterious recessive alleles present in different populations. If offspring from outbred matings subsequently contribute reproductively in future generations, and if the deleterious recessive alleles are present in low frequency, then these alleles are likely to be randomly lost from the population after several generations due to simple Mendelian segregation and genetic drift. The fitness of individuals and the long-term viability of an outbred population can be higher than that of either parental population due to the reduced frequency of these deleterious recessive alleles.

In wild populations, outbreeding may also result in a reduction of fitness, because populations can become adapted to living in particular areas with a particular climate, diseases, and so forth. If individuals from other populations interbreed with the adapted population, new alleles are introduced. These alleles may not be as well adapted to the local conditions and may reduce the fitness of the population. For example, two populations of fish may have evolved a particular colour pattern that is advantageous in the environments in which they live. If the two populations were to interbreed, they may produce an intermediate form of the pattern that is not advantageous in either of their environments, reducing the fitness of the overall population (Attiwill and Wilson, 2003). Natural selection (evolution) causes changes in wild populations over many generations. This often results in many subspecies originating from the founder population. Domestication enhances and speeds up the process via artificial selection dictated by rapid change.

These days most rainbowfishes are either obtained from commercial sources or bred by individual hobbyists using a limited number of broodstock fish. In both cases the genetic background and the degree of inbreeding of the fish is generally unknown. Aquarists must give careful consideration to the choice of brood stock if genetic ‘pollution’ of the aquarium stock is to be prevented. To avoid genetic problems it is best to start with as many fish as possible (minimum of five pairs) or get your fish from at least two different sources, or at different times. If you get all the fish from the same source or at the same time, there is a good chance that the fish will be related, especially if they have been bred in captivity.

Diversifying your sources for the fish and expanding their genetic base will help enhance their genetic variability, and may reduce problems resulting from inbreeding depression. In this strategy, individuals from another population (hopefully with greater genetic variability) are introduced to your population in an attempt to recover genetic diversity and reduce inbreeding. Breed every fish in the group, using random selection to determine pairing. Separate the fry from each pairing and select 4-6 fry from each spawn for breeding the next generation. With problem species it is advisable to raise the fry from each spawn separately until they sex out, then select a male and a female from each for breeding. In small populations it takes as little a one individual per generation to maintain genetic diversity. Ideally, a number of individuals should be sourced from the wild population every so often.

It is apparent that numerous New Guinea rainbowfish species may be in danger of being lost both in the wild and in captivity. There are only very small populations of some species now kept in captivity. A number of the early New Guinea species have all but disappeared from the hobby (I think some have disappeared). Therefore, saving the existing aquarium species and the integrity of each individual species becomes vital as there may in the future be no wild fish to restore captive stocks or genetic variability.

To maximise genetic diversity captive populations need to be perpetually managed so an adequate number of separate brood stocks are maintained with occasional intercrosses between them to reduce the probability of fixing deleterious genes. Zoos engaged in captive breeding programs are aware of this need to outcross their own stock to animals from other collections. Captive populations are at risk from inbreeding since relatively few mates are available to the animals, hence zoos must borrow animals from each other in order to maintain the genetic diversity of offspring. A practical solution for this problem is for rainbowfish enthusiasts to collaboratively maintain a much larger gene pool collectively, than they would as individuals.

A successful breeding program, in essence, depends on the successful manipulation of inbreeding. A well designed long term breeding program should be directed at preserving the basic genetic diversity of the population, and must be based on detailed information about the species characteristics and the origin and genetic history of the population. The main strategy for reducing inbreeding is to maintain a large population of broodstock fish, and ensure that a large proportion of them get a chance to breed and contribute to the next generation. At least a few fry from all of these broodstock should be retained and grown up for use as the next generation of broodstock, before the previous generation gets too old and is discarded. This sometimes requires a lot of small tanks or raising tubs, and good record keeping to record breeding results, both successes and failures. Good record keeping is essential in solving problems with inbreeding and is paramount in improving survival rates. Records of fry growth and development should be made regularly. This activity is often carried out by the serious hobbyists.

In wild populations, inbreeding is avoided because individuals prevent themselves from mating with relatives. Evidence for this “behavioural avoidance of inbreeding” has been found in a wide variety of animals, including fish. A number of different species have demonstrated the ability to recognise and discriminate in favour of familiar conspecifics in laboratory trials. Further research has found that this preference for certain individuals may persist for relatively long periods, even after a 2-month period of separation. The duration of time over which it persists is likely to vary between species. The ability to recognise related kin is based on visual and/or chemical cues. In other words, individuals are less likely to interact sexually with others with whom they were intimately familiar during development, namely siblings and parents. Females were found to avoid breeding with siblings and parents more actively than males. This may be why we have the occasional breeding failures with some species of rainbowfishes we maintain in our aquariums. Maybe it might just be that the females are reluctant to breed with their siblings. This could also explain male aggression toward uncooperative (possibly related) females.

One study (Arnold, 2000) found females rainbowfishes were able to differentiate between levels of relatedness at a relatively fine scale. Full-siblings could be distinguished from halfsiblings, and half-siblings from non-relatives. In terms of shoaling preferences, the sex of shoal-mates was also important. Females always preferred to associate with those individuals of the same sex with which they were most closely related. In contrast, when given the choice of associating with males of different levels of relatedness, females always spent significantly longer with non-relatives than with fullor halfbrothers. Thus, females appeared to avoid associating with potential mates that were close relatives.

However, other researchers have noted that females can still reduce the probability of inbreeding by seeking multiple mates, regardless of the ability to recognise kin, because by so doing they increase their chance of producing at least some outbred young. This would be especially important in situations where females cannot avoid breeding with close relatives as may be the case in captivity. The group spawning of rainbowfish species can also decrease the harmful effect of inbreeding due to the intensive mixing of gametes and high number of progenies.

In another study (Gleeson et al., 2000) two species of rainbowfishes from three locations in Australia were experimentally infected with the parasite Ichthyophthirius multifiliis. One of the species (M. eachamensis) was much more susceptible to the parasite than the other species (M. splendida). M. splendida served as a control for a follow-up hybridisation experiment which involved crossing M. eachamensis from the original population with another population of the same species located some distance away. The population hybrids had significantly higher resistance than the single-population fish. It was tentatively suggested that there may be a link between the heterozygosity of populations of rainbowfish and their initial ability to resist infection by Ichthyophthirius multifiliis. Therefore inbreeding rainbowfishes in captivity may reduce their natural disease resistance.

A broad genetic basis has been suggested as a key element for parasite resistance, with heterozygous individuals assumed to detect and present a wider range of pathogenderived antigens due to a larger number of different major histocompatability complex (MHC) molecules. MHC plays an important role in the immune system, autoimmunity, and reproductive success. Other studies have reported similar results and have found the most homozygous populations display greater infection and mortality compared to heterozygotes. Further, inbred populations showed lower survival and higher infection compared to outbred populations, leaving the authors to conclude that low genetic variation in general, or for the important MHC genes, and populations with a history of inbreeding are more likely to suffer detrimental effects from parasitic infection.

Visible symptoms of inbreeding: Deformed fins. Absence of abdominal fins. Thinning and deformation of hard rays of fins. Deformed gill cover operculum. Skeletal deformities.