Sunday 11 January 2015

Hybrids showing traits that do not appear in either parent species

Gadwall x Northern Shoveler hybrid, South Platte River, Adams County (Colorado, USA), 26th February 2012 - copyright Steve Mlodinow
(photo ID: 1318)

I think many of us tend to expect hybids to show either a combination of traits that are normally present in one parent and those that are normally present in the other parent, or to show traits that are intermediate between those shown by the two parent species - or perhaps a mixture of both combined traits and intermediate traits.  It might be interesting to look into which of these types occurs most often, and if that varies between different types of hybrid (perhaps the subject of another page if someone wants to look into that...?) but this page is about those hybrids that throw up traits that are not associated with either parent species.

An assumption that hybrids inherit their traits from their parents might be intuitive, but it's not always the case.  In this page we will give a few examples of hybrids that show traits that are not normally present in either parent species and which are not intermediate between traits shown be the parent species.

One such example is the pale cheek shown by a number of duck hybrids, especially ones involving Gadwall, like the Gadwall x Northern Shoveler shown at the top of this page.  Other examples of this are some Gadwall x Mallard hybrids and some Gadwall x Northern Pintail hybrids, like these:

Gadwall x Northern Pintail hybrid, Jumbo Reservoir (Colorado, USA), 25th March 2012 - copyright Steve Mlodinow
(photo ID: 1309)

Gadwall x Mallard hybrid, St Vrain State Park, Weld County (Colorado, USA), 10th April 2011 - copyright Steve Mlodinow
(photo ID: 1431)

It's worth remembering that any trait expressed in a bird (such as a pale buff-brown cheek) is not caused by a single gene but by many genes (I am no geneticist, so correct me if I go wrong).  In the Gadwall x Mallard hybrid you could maybe (I think it would be a stretch) argue that Gadwall has a pale cheek covered in dark speckling (making it look darker) and that Mallard has a dark cheek with no speckling, so the hybrid has inherited the pale cheek from the Gadwall and it has inherited the absence of dark speckling to make it look darker from the Mallard.  If that's not already too much of a stretch, it certainly would be for the Gadwall x Shoveler above which has a far paler cheek than a Gadwall, with or without speckling, but maybe the principle could still be part of the explanation?  Traits as we observe them could be made up of overlaying characteristics.  One set of genes might give rise to a whitish-brown element to the cheek, another set might give rise to a darker grey-brown element while another gives rise to the speckling.  If the hybrid only inherits the first then it could appear pale-cheeked.  I'm sure that's a gross over-simplification and may not be the explanation for this particular hybrid at all, but the principle that a trait as we observe it is made up of a number of underlying traits, only some of which may be inherited by a hybrid, is the point I am really trying to make.  If someone can explain it better, please do... or indeed if anyone can confirm (or refute) that this is indeed one cause for the phenonemon of hybrid showing traits not shared by either parent, then please shout!

Another possible cause (and again I invite you to support or refute this) is that the set of genes required for a bird to exhibit a trait may, if present, be either expressed (in which case we see the trait) or dormant (in which case we don't see the trait).  An example where I suspect this may be the case is the green headband of a Eurasian Wigeon.  Actually in this case it is sometimes expressed in pure Eurasian Wigeon (I wrote about that here) but it isn't most of the time, at least not much.  The fact that it is sometimes shown makes me suspect the set of genes responsible for it are always present but only sometimes expressed.  If that is the case, we might perhaps find that the set of genes are inherited by a Eurasian Wigeon hybrid.  Althouh it's not terribly distinct, that appears to be the case with this bird:

Gadwall x Eurasian Wigeon hybrid, Whitlingham Country Park (Norfolk, UK), 6th March 2011 - copyright Dave Appleton
(photo ID: 0311)

Perhaps related to this theory is the phenonemon whereby some adult hybrid geese show characters that are not normally shown by either adult parent, but which may be shown by juveniles of the parental species.  Many examples of adult hybrids between geese in the genus Anser and geese in the genus Branta show dusky marked undertail-coverts, yet no adult species with the genera (only Emperor Geese and some blue phase Snow Geese) show such markings.  On the other hand juveniles of some species may show darker markings on the undertail-coverts.

Greylag Goose x Canada Goose hybrid, Swanton Morley (Norfolk, UK), 6th April 2009 - copyright Dave Appleton
(photo ID: 0060)

Greylag Goose x Canada Goose hybrid, Swanton Morley (Norfolk, UK), 16th April 2011 - copyright Dave Appleton (photo ID: 0099)

Greylag Goose x Canada Goose hybrid, Skövde (Sweden), 19th September 2008 - copyright Carl Gunnar Gustavsson (photo ID: 1048)

Greylag Goose x Barnacle Goose hybrids, Malmoe (Sweden), 28th June 2008 - copyright Carl Gunnar Gustavsson
(photo IDs: 1060-1061)

probable Lesser White-fronted Goose x Barnacle Goose hybrid, Swanton Morley (Norfolk, UK), 27th October 2007 - copyright Dave Appleton
(photo ID: 0127)

I suspect a different explanation altogether may be responsible for this phenonemon in some other instances.  I'm not able to show a photo that illustrates this yet, but it appears that most hybrids between Ring-necked Duck and Tufted Duck ordinarily show a green head sheen.  Both parent species normally show a purple head sheen (though it may appear green in some lights) so this is not to be expected.  In this case the colour of the sheen is dependent on the structure of the feathers, not the pigment of the feathers.  I wonder if the fact that both parents share the same head colour means that they necessarily share the same feather structure?  Does anyone know that?  Or could it be that the structure is different, with both structures coincidentally resulting in a purple sheen, whereas the intermediate structure results in a green sheen?  That's all speculation, but I throw it out there in case anyone can shed any more light on it.

Specific identification dangers

In November 1988 a duck was discovered in Norfolk (UK) that was moulting out of eclipse plumage.  It showed a complex face pattern consisting of two large buff patches on the lower cheeks which were separated by a black vertical line, and a greenish tint behind the eye.  This, together with several other features, led to its identification as a Baikal Teal, an extremely rare vagrant to western Europe. Several hundred birders came to see this bird and went away happy that they had seen a Baikal Teal.  In January 1989 the bird was rediscovered at the same location.  By now it had moulted out of eclipse plumage and was clearly not a Baikal Teal!  Apparently it showed clear features of both Eurasian Teal and Northern Pintail so was considered to be a hybrid between these two species (source: Hybrid Duck resembling Baikal Teal by Steve Gantlett, Birding World 1.12: 426-427).

We now know that a number of dabbling duck hybrids show a bimaculated face pattern which may, more so in some than others, resemble that of Baikal Teal despite neither parent species showing such a bimaculated face pattern.

Oversize or undersize hybrids

Most hybrids are either the same size as one or both parent species or intermediate in size.  The example of Red-crested Cardinal x Yellow Cardinal hybrids referred to below is one where the hybrid can be larger than either parent.  I believe I have read of a gull hybrid that was larger than either parent, though cannot find the reference now.  If you have any  examples please let us know.

Heterosis and negative heterosis

In the "Handbook of Avian Hybrids of the World" (Oxford University Press 2006) Eugene M McCarthy describes the expression of parental traits in hybrids as being intermediate or combined, or sometimes heterotic.  In the example cited for heterotic traits, hybrids between Red-crested Cardinal and Yellow Cardinal can be larger and stronger than either parent.  He describes this as an example of positive heterosis, and mentions that negative heterosis may also occur where the hybrid's trait falls below the range of potential variation for either parent species.

I hesitate to copy McCarthy in using the term heterosis (and its associated adjective heterotic) for hybrids traits that are not shared by either parent species.  The reason for my caution is that heterosis seems to be widely understood (see various online dictionaries/Wikipedia for example) to describe traits that are beneficial or give rise to increased vigour (or decreased in the case of negative heterosis).  That might be assumed to be the case for a larger stronger hybrid cardinal, but it seems to me that many of the traits we see expressed in hybrids that are different from either parents are neither beneficial nor harmful but neutral.  Indeed for those hybrids that are infertile the benefit of specific traits is entirely academic anyway.  If traits arising from hybridisation are neutral, is the term heterosis still applicable?

Having said that, heterosity (is that a word?  I mean the extent to which a hybrid may be more or less vigorous than its parent species) is certainly of interest to the student of hybrids when it comes to those hybrids that are fertile.  The relative vigour of fertile hybrids when compared to either or both parent species may be of some academic interest but also of conservation significance.  Perhaps that should be the topic of another page...

Species mentioned:
Lesser White-fronted Goose Anser erythropus
Greylag Goose Anser anser
Barnacle Goose Branta leucopsis
Canada Goose Branta canadensis
Gadwall Anas strepera
Eurasian Wigeon Anas penelope
Mallard Anas platyrhynchos
Northern Shoveler Anas clypeata
Northern Pintail Anas acuta
Baikal Teal Anas formosa
Eurasian Teal Anas crecca
Ring-necked Duck Aythya collaris
Tufted Duck Aythya fuligula
Red-crested Cardinal Paroaria coronata
Yellow Cardinal Gubernatrix cristata

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