"Inbreeding is deleterious because it increases overall genetic homozygosity, and thus the expression of recessive deleterious mutations in offspring." (Arnold, 2000)
Although not generally well know, Australian rainbowfishes have been maintained in home aquaria at least since the beginning of the last century. On the other hand, New Guinea rainbowfishes have only been available since the mid 1950's. They were being maintained by only a handful of Australian enthusiasts and were virtually unknown to the international hobby. During the 1960's and 70's a small trickle continued to arrive in Australia from New Guinea. The importation of New Guinea rainbowfishes into Australia during this period did not have any significant restrictions and a number of different species were brought into the country by private collectors, which were subsequently distributed in the hobby. However, the publication in 1982 of Rainbowfishes of Australia and Papua New Guinea by Gerald Allen and Norbert Cross, greatly increased the popularity of keeping rainbowfishes and the desire for the newly discovered New Guinea species, turned that trickle into a flood.
Unfortunately, the initial number of wild-caught fishes that came in to the hobby was very small. This resulted in a very small genetic base from which to establish larger aquarium populations. The problem with having such a small genetic base is that most of the aquarium populations are closely related. Since the initial population was small, they are likely to have a higher prevalence of recessive genetic disorders, as the parents are likely to share many genes. In addition, a small population base may not be truly representative of the original wild population. For the most part, aquarium breeding has effectively resulted in domesticated strains of rainbowfishes that may be well adapted to life in captivity, but are far removed from their life in the wild.
This can be clearly seen in a number of New Guinea species where the quality and colouration has diminished greatly, as well as increases in breeding failure. This also applies to Australian species collected from remote locations where the initial numbers of wild-caught specimens was low. Almost every day I see photos of rainbowfishes posted on Internet forums asking for identification. Most replies given are in the negative, suggesting that they are hybrids. This may well be true; however, what is happening for the most part is that continuous inbreeding or selective breeding has effectively resulted in domesticated strains of rainbowfishes that no longer resemble their congeners from their natural state. 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 and reduces their genetic variability through both selective processes and random genetic drift. There are many studies that describe the deleterious effects of inbreeding in fish, generally in captive conditions or in association with artificial selection.
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. Selective breeding can produce similar effects.
Selective breeding is a breeding technique in which the breeder chooses the next generation's broodstock, based on some predetermined criteria. The process by which crosses are accomplished between the parental stocks representing different strains of the same species is referred to as crossbreeding, and can also be called intraspecific hybridisation. Crossbreeding can produce strains of superior characteristics by introducing greater genetic variability.
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 sub-species originating from the founder population. Domestication enhances and speeds up the process via artificial selection dictated by rapid change.
For the most part, aquarists maintain small populations of rainbowfish species. When a new species is obtained, the number of fish acquired is usually low. In many cases the fish that are acquired come from one or two spawnings, or more often, offspring that have been produced by a single spawning. In these situations, it is more likely that related individuals will breed, simply due to the lack of alternative mates. Breeding closely related rainbowfishes in captivity increases the likelihood of the population suffering from inbreeding problems. In aquarium populations the effect of inbreeding may be severe and should be a concern to breeders. While inbreeding can be used to improve a population when it is planned and directed, unplanned and uncontrolled inbreeding can ruin a population through a process known as 'homozygosity' (inbreeding depression), i.e., less variation in genes. Greater homozygosity may be desirable in some cases, such as line-breeding, where an outstanding individual is mated with a descendant to increase that individual's characteristics (e.g., better colour, size, etc.) to the gene pool.
Tave (1986) reported that inbreeding populations can still produce good offspring even though inbreeding depression occurs. The depression has been found from various genotype and population means, but outstanding individuals are still produced. These outstanding specimens could be kept for selective breeding purposes because these individuals contain more desirable alleles and are free of degenerative alleles.
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.
The problem with breeding related individuals is that over time you remove the heterozygosity from the population and create a population homozygous at all genes (i.e., both genes code for the same trait expression). This can increase the occurrence of traits which are detrimental to a species' fecundity, disease resistance, fertility, and growth. 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.
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 the 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.
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). Saving the existing aquarium species and the integrity of each individual species should mean more to rainbowfish keepers than their just being a passing aquarium adornment. Therefore, insuring genetic diversity 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 captive stocks. Records of fry growth and development should be made regularly. This activity is often carried out by the serious hobbyist.
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 half-siblings, 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 full- or half-brothers. 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 recognize 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.
Visible symptoms of inbreeding:
- Deformed fins.
- Absence of abdominal fins.
- Thinning and deformation of hard rays of fins.
- Deformed gill cover operculum.
- Skeletal deformities.
Literature
Arnold, K.E. (2000). Kin recognition in rainbowfish (Melanotaenia eachamensis): sex, sibs and shoaling. Behavioral Ecology and Sociobiology 48 (5): 385-391.
Attiwill, P. and Wilson, B. (2003) Ecology: an Australian Perspective, pp. 1-633, Oxford University Press, Australia.
Gleeson, D.J., H.I. McCallum and I.P.F. Owens (2000). Differences in initial and acquired resistance to Ichthyophthirius multifiliis between populations of rainbowfish. Journal of Fish Biology 57 (2): 466-475.
Tave, D. (1986) Genetics for Fish Hatchery Managers. AVI Publishing, Westport, Connecticut, 299 pp.
Tave, D. (1999). Inbreeding and brood stock management. Fisheries Technical Paper No. 392. Rome, FAO. 122 pp.
© Copyright Adrian R. Tappin
Updated January, 2009
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