A note on the white spot on the diving duck's chin

A black spot in our knowledge about the relationship of diving ducks?

Personal observations show that adult males of a number of diving ducks (genus Aythya) show a white spot under their chin, just at the base of their bill. Although not much is written about these spots in literature, museum skins show the same white spots in the same species. Canvasback, common pochard, ferruginous duck, Baer's pochard and Australian white-eyed pochard show this spot Other closely related ducks such as redhead and Madagascar pochard do not show this spot. 

As this intrigues me, next questions came to my mind are:

• Can the presence (or absence) of the white spot be lead back to only one Aythya-species?
• If yes, how did it end up in the other species?   
• Is there a correlation between the morphologic character and the genetic relationship between species within the genus of Aythya. 

Does anyone know more about this topic and can you recommend sources to me?

Please let me know. 

From my personal observations/ skin studies: 

I have no valid data yet on the white spot in scaups (Aythya novaeseelandiae, Aythya affinis, Aythya marila), and the closely related tufted duck (Aythya fuligula) and ring-necked duck (Aythya collaris). 

Above: male Baer's pochard (Aythya baeri) shows the white spot on its chin. 

Specimen photographed at Natural History Museum at Tring, UK (Harteman, 2019). 

Above: these two males Madagascar pochard (Aythya innotata) do not show the white spot on their chin. 
Specimens photographed at Natural History Museum at Tring, UK (Harteman, 2019). 

Above: Redhead (Aythya americana) at top and common pochard (Aythya ferina) at bottom. 
The redhead does not show the white spot, but the common pochard does.

Canvasbacks show the spot as well. 

Specimens photographed at Natural History Museum at Tring, UK (Harteman, 2019). 

Above: Baer's pochard (Aythya baeri) at top and ferruginous duck (Aythya nyroca) at bottom; both show the white spot. 

Specimens photographed at Natural History Museum at Tring, UK (Harteman, 2019). 

Above: adult male tufted duck (Aythya fuligula) does not show the white spot. 

Photo taken at Aeres MBO Barneveld (Harteman, 2019). 

Above: adult Baer's pochard drake display, just when it "throws its head in its neck".

The drake shows it white spot to the females nearby.