Line Breeding and Inbreeding Depression in Aquarium Fish

What line breeding and the inbreeding coefficient mean, what fish studies show about fertility, survival and deformities, and how outcrossing and breeding records limit the damage.

Definitions

Inbreeding is the mating of relatives. It raises the share of homozygous genotypes, where both copies of a gene are identical, and lowers the share of heterozygous ones. It is measured by the inbreeding coefficient F. A fish with F = 12.5% is expected to have 12.5% more homozygous loci than the average fish in its population. When the common ancestor is not itself inbred, F is the sum of 0.5 raised to the number of fish in each path that links the two parents through that ancestor. One generation of half-sib mating gives F = 12.5%. One, two, three and four generations of brother-sister mating give 25%, 37.5%, 50% and 59.4%. An inbred fish looks no different from one that is not inbred (FAO Fisheries Technical Paper 392).

Linebreeding is a form of inbreeding in which an animal is bred back to its descendants, or in which mates share a common ancestor without being closely related. It is used to increase an outstanding animal's contribution to the population. It is less likely than close inbreeding to cause problems in the first generation, but over longer periods it can reduce genetic diversity and lead to similar problems.

Inbreeding depression is the loss of fitness or productivity, such as growth rate and fecundity, that comes with rising inbreeding. The main explanation is that harmful recessive alleles, normally masked in heterozygotes, become exposed in homozygotes. A second is overdominance, where some alleles are advantageous only in the heterozygous state. Each individual is estimated to carry dozens of mutant alleles that lower viability. Most of them reduce fitness only slightly, but several can be lethal. Two carriers of the same recessive allele produce an affected offspring with a probability of 25%, and relatives share many such alleles.

Evidence from fish

Rainbow trout (Oncorhynchus mykiss): in the most complete long-term study, which tested six levels of inbreeding, F = 12.5% already reduced hatching and survival but increased 77- to 150-day weight. Levels of 25% and above severely depressed fecundity, growth and survival.
Fish in general: studies mostly found lower growth, fecundity and survival and more deformed offspring, although results were mixed between species. Inbreeding reduced the return rate of Atlantic salmon (Salmo salar) and the growth and viability of Mozambique tilapia (Oreochromis mossambicus); in channel catfish (Ictalurus punctatus) it raised growth in one study and lowered it in another. Most work tested only 25–60%, and no study had examined levels below 12.5%.
Guppy (Poecilia reticulata): after full-sib matings, inbreeding depression ranged from −1.0% to 24.6% across six traits and was significant for survival to day 120 and salinity tolerance. Crosses between genetically different strains raised the same traits by up to 42.2% (Nakadate et al. 2003).
Guppy: inbred fish have shown lower survival and lower sperm counts. Inbred guppies (f = 0.25) were less bold than outbred ones, and inbreeding lowered inhibitory control in fish raised at 30 °C but not at 26 °C (Vinogradov et al. 2024).
Zebrafish (Danio rerio): one strain kept by full-sib mating for 16 generations laid few eggs and had a short lifespan. A second strain, inbred for more than 20 generations, needed almost no special care (Sadamitsu et al. 2024).

Effects can also be delayed. In guppy populations founded by full siblings, those descended from two earlier generations of sib mating at first grew faster than outbred-founded ones. The authors explained this by earlier purging of harmful recessives, followed by inbreeding depression once the outbred-founded lines began to inbreed (Zajitschek et al. 2009). Not every deformity is genetic either. In a captive-bred wild line of Poecilia wingei, 22 of 600 fish developed scoliosis, lordosis or kyphosis, and the authors could not separate trauma, nutrition and genetic causes (Arbuatti et al. 2013).

Outcrossing strategies

Mate unrelated fish. Two fish that are each inbred but unrelated to each other produce offspring with F = 0%.
Add unrelated breeders. In the guppy study above, adding two outbred males to inbred populations gave significantly faster population growth than adding none, and adding females gave an intermediate result. In wild Trinidadian guppies, gene flow from introduced fish produced long-term increases in population size (Fitzpatrick et al. 2016).
Relax selection for one generation. If inbreeding depression becomes significant in a selected line, the FAO manual suggests mating the selected breeders with outstanding fish from an unrelated strain.
Use rotational mating. Keep breeders in separate groups (cohorts) and mate females of group 1 with males of group 2, group 2 with group 3, and so on. This prevents or slows the build-up of inbreeding.
Keep the effective breeding number (Ne) high. Ne falls when few fish actually breed, when the sex ratio is skewed and when family sizes vary widely. For example, 53 females and 25 males give an Ne of about 68, not 78. A single generation with very few breeders lowers the long-term average sharply.

Outcrossing is not risk-free. Outbreeding depression, the loss of local adaptation after crossing distinct populations, is the opposing concern. In the Trinidadian guppy, however, genetic rescue produced hybrids with better fitness rather than swamping local adaptation.

Record keeping

F can only be calculated from a pedigree. Fish are hard to mark, so separate tanks or clearly labelled cohorts take the place of tags. Pedigrees are usually traced back four generations, because each ancestor's genetic contribution halves with every generation: 50% from a parent, 25% from a grandparent and 12.5% from a great-grandparent. A covariance table built from the pedigree can predict the F of any planned pairing before it is made.

Identity of each breeding fish or tank, with its sire and dam
Dates of birth and death, and sizes or weights at set ages
Which pairs or groups produced each brood
Brood size, survival and any deformities seen
The source and date of any fish brought in from outside the line