Inbreeding

[dspeers]

I did a search on this forum and was surprised by the relative lack of discussion on the impact of inbreeding. From what I have read this is a very common practice.
I am not a geneticist but have a fairly good theoretical understanding of the potential negative effects of inbreeding having been a practicing physician for over 30 years.
I would expect negative effects in a variety of areas including reduced fertility, higher mortality in the juvenile population, visible physical deformities, immunological issues, shortened lifespan, etc. etc. Not necessarily in fish but essentially any species using a sexual reproductive method with diploid chromosomes.
Given that, it surprises me that 1st generation discus crosses are exceedingly common without frequent deleterious obvious effects (which I would expect). Frankly I am at a loss to explain why there are not more problems. After all, we all saw Deliverance, right? Seriously, zoo breeding programs go to great lengths to avoid inbreeding. As well, inbreeding has contributed to species extinction when the gene pool becomes too small.
I understand the risk imposed by mixing two fish from diverse backgrounds, but has anyone tried such to see if the breeding outcomes are improved?

I occasionally read about a "breeding" pair that repeatedly produces eggs but never hatch, and wonder if that might represent one generation too many of sibling pairing....

[Filip]

Very interesting and pretty uncommon subject in discus hobby. I also often wondered about why discus are kind of "exception" of this natural "law " and why inbreeding ussualy ends up with nice looking , healthy discus , instead of deformities and other health related problems.
I'll surely be following this interesting thread .

[smsimcik]

I don't have any insight on why discus don't seem to be as negatively affected by inbreeding as other species, but I do recall John Nicholson once saying he has successfully bred discus siblings for 4-5 generations with few problems. I believe it was his red turq line he had several years ago.

[LizStreithorst]

Steve, it's the same in Angelfish. After that you start having problems. My feeling is that the simpler the organism the less inbreeding becomes problematic.

[brewmaster15]

I have a few thoughts on the subject I can throw out there.

One thing to keep in mind is we really don't know how variable the genetics of a batch of 100-200 fry is. Just because the fry are from the same mother and father, doesn't mean they are all twins and clones.. we know thats not true at all by the variety we can get in a cross. The term F1 implies they are all siblings from the cross but it doesn't mean they are genetically identical/

My next thought is that excessive inbreeding discus is actually very uncommon. Most keepers rarely breed their fish with a plan at all...it just happens.At most a more experienced hobbyist will breed to F1 and then maybe carry on to the next F1x F1. Why? because it takes a ton of tank space and time...dedication thats hard for most hobby breeders to commit to. You might ask what about the commercial Discus Breeders? Theres alot more stock being crossed there than you would think Asian breeders diversity of discus colors, patterns, strains and forms is a testament to it. What about a hatchery like Stendker? That hatchery is known for maintaining a very tight control on their breeding stock, they don't typically bring fish in according to most sources. I would expect you to see inbreeding defects there, yet we don't. I think in that case its because of the shear size of facility... They have enough stock so it doesn't occur.

Lets consider the stock as well... with so many people from so many geographic areas, breeding discus and the ability to ship stock, theres a pretty diverse gene pool out there.

Discus in captivity don't really need as strong a gene pool as wilds would simply because we control multiple aspects that could adversely affect them and that genetic variability would be needed if we didn't. The water is clean with low pathogen loads, the food is plentiful, the temp is stable, there are no predators, many discus are kept in monocultures.. Its not exactly the kind of existence that would require Iron man Genetics.

Last note.. and probably the most important one. We Cull. If you had a small population of fish that kept inbreeding and one developed a deformity like a missing fin, That fish could pass on the gene to the rest of the small pool of fish.. By culling we remove that defect from the population. We cull for various traits that are physically visible but we also are culling out weak traits by culling fry that don't grow as the others do.

I am not saying that it not possible to inbreed too much and get defects in the offspring.. but we have had discussions here before on how many generations it takes to actually set a trait and create a strain.. though no hard data, the conclusion was "many' In most cases we just aren't going to see defects from inbreeding as theres just not excessive inbreeding being done.


Just my thoughts on the subject.

al