Tuesday, May 04, 2010

Haldane and Hybrid Bears

Haldane's* Rule:
When in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex.
This rule is almost universally true (see table**) and is supported by the observations that among hybrids such as ligers, zonkeys and mules the males (XY) are all sterile while females (XX) may be fertile (though often poorly so, due to mismatched chromosome numbers, discussed briefly here). The reverse observation - sterile females - is true in species where the male is the homogametic sex such as birds and butterflies. Hypotheses explaining the genetic basis of Haldane's Rule and its challenges are explained nicely here.

Recently, a new fertile hybrid was discovered. The grizzly-polar bear hybrid has been seen before, as discussed here, and with changing habitats and more grizzly-polar bear encounters we'll likely see more. The Toronto Star is reporting the first offspring of a hybrid bear.
Researchers in the Northwest Territories say they may have found the first recorded offspring of a hybrid female polar-grizzly bear in the wild. [...] Officials with the territorial government say those test showed that the dead bear was the offspring of another hybrid bear — a female polar-grizzly mix who had mated with a male grizzly.
Given Haldane's Rule, that the hybrid parent was female isn't surprising, but it remains to be seen whether the males are sterile or if this becomes the first mammalian exception.

*In addition to being one of the founders of population genetics, Haldane also wrote poetry about rectal cancer, a disease he succumbed to in 1964.

**Table source: Nature 355, 511-515 (1992). Ref. 41 refers to Coyne, J.A. and Orr, H.A. in Speciation and its Consequences (eds Ott, D. and Endler, J.) 180-207 (Sinauer, Sunderland, Massachusetts. 1989)


rob said...

Does the existence of the pozzly bear eliminate the need for conservation, if the genes of the pozzly mix with the grizzly sufficiently so that they are preserved and the polar bear niche (what is left of it) is filled by the grizzly and pozzly?
If not, isn't that a bit racist? (I thought that they were different species but I guess Haldane would argue otherwise). Would you argue that pure white bears are better?
On a side note, a bear ravaged my garbage just last night. The garbage container said that it was 'animal proof'. Granted it had a picture of a raccoon on it, not a bear.

Kamel said...

I don't think this is considered a new species yet; I think this is considered an early step in speciation. See here. A lot of the current literature re: Haldane has to do with speciation. Since the hybrid can mate with a parental species and produce offspring, it might not be considered a separate species yet.

rob said...

It is somewhat surprising that, in addition to being somewhat genetically compatible, they have mating behavior that is still compatible in the wild.

Kamel said...

And since polar bears and grizzlies also have the same number of chromosomes it will be interesting to see how successful the matings, and their offspring, are.

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