Author: Glynis Giddings
From Issue 36 Jan 2001
I read with interest the article "When is a Campbell not a Campbell?" in the NHC and BHA newsletters. The article, by Melissa Chamberlain, was criticising the production of hybrids between Campbell and Winter White hamsters. I also read the responses sent to the NHC email group, suggesting ignorance rather than malice as the main cause for the production of hybrid dwarfs. I have no wish to offend anyone, especially as I'm a relatively new member of the "hamster world". I do wonder though if we have considered this issue in such a rational and well informed way. Having not any had experience of producing such hamster hybrids I am reluctant to come down hard on either side of the argument. However I hope some of my general thoughts will help add understanding, or at least provoke others to think further. To illustrate some points I have strayed into examples from other, non-hamster species. I hope you will forgive me this in the interests of increased clarity, and might anyway find the examples interesting in their own right.
Melissa seems to have tried to put too specific a definition on the word hybrid, when in reality even biologists use the word quite broadly. Although the definition of a hybrid is not central to this argument it probably helps that we all understand what a hybrid really is. I looked the word up in several genetics textbooks. In the context of this discussion the definitions all more or less say that a hybrid is "an individual resulting from a cross between two genetically dissimilar parents". The parents could be of the same species (e.g. different breeds of dogs) or of a different species (e.g. mules and hinnies result from crossing horses with donkeys). Hybrids could have hybrid offspring (e.g. mongrels resulting from a cross between two different cross-breeds).
The existing evidence suggests that Campbell's and Winter Whites are very closely related species (or perhaps sub-species) that have only recently diverged from a common ancestral species. As Melissa rightly says the two species "are slightly different genetically", though I doubt that it is known how different. The extent of genetic differences could be tested using modern techniques but I expect it hasn't been (I can't imagine that it will be done without private funding). Despite the differences between Campbell's and Winter Whites I wonder about Melissa's assertion that in the wild if they did meet they "would ignore each other". Without actual evidence I'd be reluctant to accept this. The reason for this is that it is often not true for other species in similar situations.
Closely related wild species often interbreed in places where their home ranges overlap (forming "hybrid zones"), or if one species is accidentally or intentionally moved into the range of the other (e.g. by humans). Examples include feral dogs breeding with African wild dogs, coyotes and grey wolves interbreeding in the US, and several examples of fish that either naturally interbreed, or that interbreed with feral fish escaped from captivity. It is usually interbreeding between introduced and native species, rather than between related native species, that causes ecological problems. An example is that of the ruddy duck, introduced into Europe as an ornamental species from America. There are now conservation programmes to protect related European white-headed ducks, following the escape and naturalisation of ruddy ducks. One problem is that the ruddy ducks compete for food, nest sites and other resources. Another is that they hybridise with the white-headed ducks. The phenomenon where one species is reduced by hybridisation, being replaced by a hybrid population, is called genetic assimilation. I do not know of any cases where genetic assimilation has caused species extinction, but it is a theoretical possibility. Possibly genetic assimilation lead to the demise of wild red wolves in the USA, but it seems at least equally likely that red wolves were in fact always hybrids of grey wolves and coyotes, and not a distinct species after-all.
The territories of Campbell's and Winter Whites must have overlapped at some time in the recent past, otherwise the species wouldn't be so similar now. I don't know if they still overlap, or even if this is known for sure. However if they do one might expect some natural hybridisation, most probably with no real damage to either species. Eventually other barriers to reproduction may evolve to separate the species, e.g. differences in lifecycle, or in reproductive or behavioural biology.
The statement that "If you have a cross-breed or mongrel you will not, by selective breeding, come back to a pure bred animal." may, in fact, not be strictly accurate for all cases, surprising though this might seem. Consider the offspring of a cross between a Winter White and Campbell's. If you were to cross these to a Winter White their offspring would, on average,
Editor's note: At this point in the article, the original fraction characters have been lost, and it isn't clear what ratio/percentage Glynis stated. On further research, this is more complex than following the normal inheritance pattern of a simple dominant or recessive colour inheritance gene or genes. To that end, I haven't attempted to guess the ratios, but have left '?' placeholders.
be ? Winter White, ? Campbell's. The "on average" is important as some particular puppies would be more like Winter Whites, and others more like Campbell's. Now suppose these offspring were crossed again to Winter Whites, the resulting puppies would be, on average, 7/8 Winter White, 1/8 Campbell's. (When I say "be" I am really referring to the proportions of genes descended from each species.) Again some of these babies would have fewer than 1/8 of their genes of Campbell origin, some more. After another generation of crossing to Winter Whites the offspring would have only about 1/16 of their genes of Campbell origin. This kind of breeding programme is known as a backcross programme, the Winter White species is called the recurrent species (because it is the parental species that is always crossed to the hybrid offspring).
If such a backcross programme were continued the proportion of Campbell genes carried by the hybrid offspring would be halved at each backcross generation - being (on average) 1/32, 1/64, 1/128, 1/256 etc. After only 7 backcross generations the hybrids would, on average, carry less than 0.4% of Campbell's genes - some may even carry no Campbell's genes at all. If one employed selection for Winter White types then it is reasonable to suppose that the drop in the percentage of Campbell's genes would occur at a greater rate than that expected from the breeding programme without selection - so that selective breeding is the best way to "breed out the Campbell's". Of course if Campbell's were used as the recurrent parent then it would be the Winter White genes that would be eliminated from the hybrid stock.
There would be no point in performing such a backcross programme unless one wanted to transfer some character from one species to the other. In this case the most obvious character is colour. Then the breeder would select to keep the desired colour genes, but select against other "undesirable" features. In theory it should be possible to produce Campbell coloured Winter Whites that carry few or no other Campbell characters (or genes). In reality hybrids would most probably be of lower initial fertility, which might make the programme more difficult to get going than the above suggests. However evidence from other backcross programmes suggest that fertility would increase with each succeeding generation, as fertility is automatically being selected for. Such a programme might well be quicker than waiting for natural mutations to "show" themselves by chance (by being inherited in homozygous form), or to occur by chance (if they are dominant mutations). Backcross breeding would also have some advantages over the alternative method of "finding" natural mutations by using an inbreeding programme.
It ought to be possible for a breeder to avoid introducing health problems from one species to the other. Furthermore it is the much more common practice of inbreeding that is "flushing up health problems". I could fill the journal with an explanation of why the merging of populations does in fact tend to lead to a reduction in health defects due to recessive mutations, however this is the case (anyone really interested should look up "isolate breaking" or the Wahlund effect in a genetics or evolution textbook). By carefully selecting stock and not breeding from individuals if problems do arise the risk of transfer can be minimized. By using unrelated recurrent parents, and minimizing inbreeding after the backcross-breeding programme, one could substantially reduce the risks of problems such as glaucoma and diabetes.
I hope you see that I'm not advocating the uncontrolled production of hybrids. Mainly this is to do with the possible problems of small females giving birth to large headed young, which Melissa very rightly points out. But there are various ways of avoiding this problem, e.g. if Campbell's females produced the initial hybrids, Winter White males might be used as recurrent parents. Like you I would not want to condone practices that are likely to lead to suffering. (And of course, if others have evidence of other problems of hybridisation then they should offer this information into the debate.) But I suppose also that "commercial breeders", so criticised in Melissa's article, do not really make much (if any) money from their venture. I suspect that, like the rest of us, they are just people who love keeping and breeding hamsters. Couldn't we engage them in debate rather than alienating them completely?
Suggested further reading