Canis lupus hominis

The popular sire is not limited to dogs or even common domestic animals. It is a problem that plagues a wide array of species that are bred in captivity.

Domestic dogs have a wealth of genetic diversity, and although much of it is sequestered in various breeds and strains, it is generally fairly easy to produce healthy puppies.

Dogs are the oldest domestic animal, and they descend from a wild ancestor that was quite widespread. That means that as a species, dogs would (and should, in theory) always have enough genetic diversity to produce viable offspring.

But what about species that have only recently been bred in captivity?

What about those species that have a much narrower range and only a very small founding population?

Such is the case when the Centre for Fortean Zoology began breeding a Guatemalan livebearer, Scolichthys greenwayi.

Unlike wolves, these fish are native to only the Rio Chixoy or Rio Salinas river system in Guatemala. That means that attempts to produce “domestic strains” are dependent upon imports, and imports are necessarily quite small populations in relative terms.

To make things even more complicated, these fish are typically harem-bred, with one male servicing six females.

And that’s where the popular sire effect takes place.

When the Centre for Fortean Zoology purchased two pairs of these fish at an auction, it looked like they were going to have the start of a successful breeding program.

As their name suggests, livebearers give birth to live young that hatch from eggs that are retained within the female’s body.

The particular livebearers produced offspring within two weeks of being introduced to their new tanks, and it was expected that these little fry would do well.

The Centre’s tanks are set up with weeds for the fry to hide in, and this allows a certain amount of natural selection to go on. The fry hide within the weeds to avoid being eaten by their parents or other fish within aquarium. Fry that cannot do this very well are eaten.

(This is very different from the way I remember seeing swordtails bred. The “pregnant” females are typically placed in a breeding trap, which is placed in a nursery tank. The female is placed in that trap, and when she gives birth to her fry, they escape through tiny holes on the floor of the breeding trap, escaping their mother’s jaws. It allows the breeder a chance to select which offspring survive.)

The Guatemalan livebearers produced several “clouds” of offspring throughout the summer, and hopes were high that the tank would soon be full of fish.

However, months passed, and no small fish came swimming out of the weeds.

Upon inspection of the weeds, no fry were found, but one horribly deformed juvenile was discovered . The spine was twisted in a “S” shape. Upon more careful inspection, a few more juveniles were found– all with this deformity.

Careful checks of the tank chemistry found no abnormalities.

The problem could only be genetic, as Oll Lewis writes:

The problem stems from too much inbreeding leading to a small gene pool which means genetic problems can become magnified within the population. Small fish are often prolific breeders and will produce lots of young, which is fine when they live in a large river and it is likely that there will be lots of other fish with different genetic lineages just down the river to breed with but in captivity often closely related fish will breed with each other meaning the available genes are reduced. Often the fish that look outwardly fine are selected for further breeding or sale and the ‘rejects’ euthanised, but a fish can easily be carrying the genes that could contribute to deformities in their offspring even if the appear to be fine. Other things exacerbate the problem and can speed up the shrinking of gene pools such as selling fish to other collectors with a much larger number of one sex than the other. For example if you were to attempt to breed using one male and six females you’re going to reduce your gene pool very fast indeed. I have actually seen evidence of a few scolichthys greenwayi being sold in similar ratios online so it is easy to see how the captive genetic stock can be reduced quite fast. In order to counter this many of the more responsible breeders will buy in adults from other breeders to attempt to counter this, but often unless you know the full lineage of the fish you might be just adding inbred stock. Zoos and conservationists have been aware of the problems inbreeding causes in a population for a long time and accurate stud books are kept for most animals to ensure that, when breeding, genetic diversity is maintained. Some fish breeders will keep stud books but often hobbyists and some commercial breeders will not and because of this it often becomes near impossible to keep a studbook only a few generations after a particular species has been introduced to the pet trade.

In other words, popular sire effect.

Now this problem could be solved using two methods.

1. Import new stock from Guatemala, if this is indeed possible.
2. Use breeding stock from different breeders.
3. Breed from more males.  Keep a higher ratio of males to females in a breeding population

Of course, people who breed livebearers are always trying to breed “pretty.” I’m sure that using just a few males as breeding stock is a good way to ensure that only the most attractive fish produce fry. No one wants to buy homely fish.

It’s the same thing that happens in domestic dogs, whether they be show or trial stock.

Of course, with dogs, we have the ability fix things rather quickly. It is merely cultural reasons that we don’t work to solve our problems with genetic diversity.

With these fish, one is reduced to working with what one has. I’m not sure if these fish are currently being collected in the wild, so it may be impossible to get new blood.

And the vast majority of these fish breeders are in the mass production business. No one really traces the bloodlines. It’s about producing fish at volume, regardless of the health of the gene pools– or the long-term health of  the fish.

These fish have very short generations and are prolific breeders, so it would be hard to set up really clear bloodlines.

But is clear that the popular sire effect causes problems in fish.

What happened with these livebearers should be a warning to us about what could happen if we aren’t careful about genetic diversity.

It can mean severe consequences.

The story of these fish should put everything in perspective.