|Steel-blue Whydah, Seronera, Dec 2011.|
|Greater Honeyguide, Tarangire, Sep 2011|
|Male Greater Honeyguide, Tarangire, Sep 2011|
They show very nicely that Greater Honeyguides have two main groups of host species - birds that nest in tree holes (African Hoopoe, Green Wood-hoopoe, etc.), and those that nest in earth holes (Little Bee-eater, Striped Kingfisher, etc.). The former have larger and longer eggs, the latter smaller, rounder ones. And so two forms of female greater honeyguides seem to have evolved - one specialising in the tree nesters, one in the ground nesters and as expected the females of each group lay appropriately shaped and sized eggs. So how do they do it? Well, one of the important theories that was developed as long ago as 1933 is based on another fundamental difference between birds and mammals that's important to know. In both mammals and birds the sex of a developing embryo is determined by chromosomes, the DNA containing structures that control inheritance. In mammals, everyone has one 'X' chromosome we inherit from our mothers, but from our fathers we can either inherit another 'X' chromosome (which would make us female), or - like our father - we could inherit a 'Y' chromosome, which would make us male. The 'Y' chromosome is therefore inhereted father to son, to grandson, etc., without ever finding itself in a female, and it's this pattern of inheritance that makes us male or female. Now what differs in birds is that instead of the X and Y combination making us male, it would make a bird female. Male birds have two of the same type of chromosomes, females are the ones with the different pair, and to make this distinction easier we don't use the X and Y terminology, but talk of W and Z chromosomes instead. So, unlike in mammals, it's the females of birds who have a unique chromosome that is passed one through mother to daughter to grand-daughter, without ever passing through a male. So if the information for how to colour your egg is stored on this chromosome, no information about it will ever come from a male. A neat solution to how the species as a whole can be unified by the males, but females can differ (possibly substantially) in the genes they have on their unique chromosome.Hope that's clear...
Now, Claire and her group went one step further and decided to look for differences in a special sort of DNA called mitochondrial DNA that is also only inherited from mother to daughter, and compare the degree of difference between the two groups of tree and ground parasitising females in the mitochondrial DNA with the difference in the DNA in the main part of the cell that comes from both male and females. They expected - and rather neatly demonstrated - that there might be substantial differentiation between the females in mitochondrial DNA, but that the males would mean there's little difference in the main 'nuclear' DNA. And the degree of difference in the mitochondrial DNA between the tree and ground nesters was so much that their ancestors started breeding in these two different way millions of years ago! That's pretty remarkable, and rather different from the more recent splits reported for cuckoos, probably brought on by relatively recent host changes. Why this difference? Well, they speculate that it's thanks to the greater staility of the African climate compared to the Northern one where most of the work on other brood parasites has been undertaken, but I'm not yet convinced - if we could compare similar patterns for a few local cuckoos too, that might be very interesting!
Anyway, all very impressive and a great lesson not only in the complexities of brood parasitism that is fascinating to me, but a bit on sex determination too - a subject we're sure to return to in the future...
Spottiswoode, C., Stryjewski, K., Quader, S., Colebrook-Robjent, J., & Sorenson, M. (2011). Ancient host specificity within a single species of brood parasitic bird Proceedings of the National Academy of Sciences, 108 (43), 17738-17742 DOI: 10.1073/pnas.1109630108